GREY WATER HEAT RECOVERY SYSTEM

Abstract

A grey water to clean water heat exchanger and methods of use. A heat exchanger having a copper coil carrying clean water inside a grey water discharge pipe is described. The heat exchanger can use counterflow geometry. The heat exchanger has a controller that directs grey water through a bypass pipe whenever the grey water has too low a temperature to provide useful thermal energy. When the grey water is hot enough, the controller directs it through the heat exchanger to heat the clean water. A filter is provided to eliminate fouling of the interior of the heat exchanger. The controller provides an indication when the filter needs to be cleaned or replaced.

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to and the benefit of co-pending U.S. provisional patent application Ser. No. 61/266,275, filed Dec. 3, 2009, which application is incorporated herein by reference in its entirety.

FIELD OF THE INVENTION

The invention relates to heat recovery systems in general and particularly to a system that recovers heat from waste water.

BACKGROUND OF THE INVENTION

There is a growing need for a simple system that recovers energy from hot commercial wastewater. The equivalent of 235 billion kWh worth of hot wastewater is discarded annually in the United States according to the U.S. Department of Energy. A significant amount of the energy from this wasted hot water could be recovered. Desirable features of such a system would include easy installation, minimal impact on existing plumbing and low maintenance. Over time a wastewater recovery system will be able to pay for itself through savings earned from energy conservation. Energy prices have increased exponentially over the past few years.

Each day thousands of gallons of hot wastewater (also known as grey water) go down the drain, taking valuable heat along with it. A large amount of this lost energy could be used to preheat incoming cold water. There are several devices currently on the market that can take the wastewater from one household shower and transfer its energy to the cold water going to the shower. Heat energy is also lost in other waste water systems used in residential and commercial environments. This loss of energy is especially prevalent in businesses or commercial locations such as residence halls, barracks, gym locker rooms and laundromats where large amounts of hot water are being used continuously throughout the day.

Heat exchangers are common devices used to transfer heat from one medium to another. This transfer of heat can be used to recover potentially wasted energy from hot grey water.

As energy conservation becomes an ever pressing issue on the minds of people, businesses and governments around the world, new methods of conserving energy are continuously under development. The rising cost of natural gas, oil and electricity helps to solidify the importance of energy conservation in modern society. One area where energy is wasted on a regular basis is the hot water being sent down drain after showering, washing hands, and doing the laundry. This wasted water is known as grey water and is fast becoming a new area of research and change in the initiative to “go green”. Today there are four devices on the market that are available to assist in capturing this wasted energy. None of these devices are rated to capture more than fifty percent of the energy within the grey water going down the drain or to handle the capacity of more than two full bathrooms. The four systems are described in the following articles: “How the Power Pipe Works,” RenewABILITY Energy Inc., 2006; “Lo-Copper GFX,” Heat-Xchangers and Water Heater Boosters, WaterFilm Energy Inc., 2002; “ReTherm Heat Recovery Technology,” Discover Heat Recycling, ReTherm Energy Systems Inc., 2004-2008; and “Drain Water Heal Recovery,” WaterCycles, 2007.

Other examples of heat exchangers for such systems that are described in the prior art include the following United States patents.

U.S. Pat. No. 1,703,655, “Heat Exchanger for Waste Process Water,” teaches a very large and advanced system; the system utilizes storage tanks for the incoming heated wastewater which is of concern due to sanitary conditions. The device uses multiple pipe passes to increase the surface area for heat transfer in a tube bundle design.

U.S. Pat. No. 4,256,170, “Heat Exchanger,” teaches a liquid-to-liquid heat exchanger that transfers heat from hot wastewater to the cold supply water. This is a pipe in a pipe design that is composed of an inner pipe and outer shell in which the outer shell is sub-divided into a serpentine flow path for the cold water. The inner pipe is sized so that flow of the hot water does not need to completely fill the pipe, this design allows for less than full pipe Howl.

U.S. Pat. No. 4,291,423, “Heat Reclamation for Shower Baths,” teaches a system intended for a bath or shower unit, the design utilizes hot drain water that is leaving the shower and uses it to heat the incoming cold water. The piping is designed to be placed underneath a tub or shower. The coils are located in a basin at the bottom of the tub which allows the lines to stay submerged under the wastewater while the shower is in use.

U.S. Pat. No. 4,304,292, “Shower,” teaches a system designed to be installed in a tub or shower unit and includes two different possible configurations. Both configurations use the warm shower wastewater to heat the incoming cold water supply. The first configuration is a normal plumbing drain trap made of copper with a copper helical pipe wrapped around the outside. The second configuration is a standard plumbing drain tap with a copper helical pipe placed on the inside of the drain trap.

U.S. Pat. No. 4,341,263, “Waste Water Heat Recovery Apparatus,” teaches a wastewater heat exchanger that is designed to capture the grey water from multiple household or commercial drains and then pass it through the exchanger. This system uses counter flowing pipes and a liquid heat exchanger medium to better accommodate the transfer of heat between hot and cold flows.

U.S. Pat. No. 4,372,372, “Shower bath Economizer,” teaches a helical pipe in a pipe heat exchanger design. This design is intended to conduct heat from spent shower wastewater and transfer it to the incoming wastewater. The heat exchanger has at least two concentric helical coils through which the incoming cold water flows and over which the spent wastewater flows. A central core directs the spent water between the helical coils for good heat transfer.

U.S. Pat. No. 4,542,546, “Heat Recuperator Adapted to a Shower-Cabin,” teaches a system that utilizes a hot water basin placed under the shower which stores the hot wastewater. This warm stored water is then used to pre-heat incoming cold water. The cold water pipe passes through the shower tank on the way to the shower head, heating as the flow passes through the tank.

U.S. Pat. No. 4,619,311, “Equal Volume, Contra Flow Heat Exchanger,” teaches a system that utilizes the hot wastewater leaving a household shower and sends it through a heat exchanger which passes the incoming cold water across it. With this design the water travels extremely fast through the system due to the fact it travels straight down the pipe with no obstructions, this would require the pipe to be extremely long to recover a beneficial amount of heat. This is due to the fact the amount of time the water has to fully transmit the heat to the incoming cold water is significantly decreased in a pipe in pipe design that is a short length.

U.S. Pat. No. 4,821,793, “Tub and Shower Floor Heat Exchanger,” teaches a heat exchanger that is located in the floor area of a bathtub or shower. The device extracts heat from spent shower or bath water and uses it to heat the incoming cold water to reduce the amount of preheated boiler water needed to be sent to the shower.

U.S. Pat. No. 5,143,149, “Wastewater Heat Recovery Apparatus,” teaches a design that includes a valve that controls the hot and cold waters entrance and exit from the heat exchanger with pipe in pipe heat exchanger configuration. The cold water enters the pipe flowing in the opposite direction of the heated wastewater causing a counter-flow condition.

U.S. Pat. No. 5,301,745, “Installation for Heat Recovery,” teaches a facility producing warm drain water and consists of a distributor tank, many pipelines, and a heat exchanger. The distributor tank reads the temperature which is then conveyed to a mixing valve which adds warm water or cold water back into the storage tank. This allows the storage tank to maintain a set temperature.

U.S. Pat. No. 5,740,857, “Heat Recovery and Storage System,” describes a water system that recovers heat from hot wastewater and transfer it to a cooler water supply. The heated water is then stored in a tank until needed for use. The patent teaches a high thermal conductivity pipe enclosed by a low thermal conductivity pipe. These pipes consist of an inlet on one end and an outlet coupling on the other. There are also endplates that are adapted to accommodate other household piping allowing the device to take water from more devices then just a shower.

U.S. Pat. No. 5,791,401, “Heat Recovery for Showers,” teaches a system that is intended to be placed right in the wall of the shower. This design also uses the hot grey water to heat the incoming cold water going to the shower head. This is a copper pipe on a copper drain pipe design which has been discussed previously, the increased spacing in this design decreases the contact area the cold water flow has to exchange heat.

U.S. Pat. No. 7,322,404, “Helical Coil-on-Tube Heat Exchanger,” teaches a device that consists of copper tubing wrapped around the outside of a copper drain pipe. This design is used in all of the competing devices currently on the market. This patent uses multiple parallel helical tubes to limit liquid pressure losses while providing similar performance to other coil and tube designs. Two or more copper tubes are wrapped around a pipe in a helical shape allowing the heat to be transferred in a counter-flow fashion. This system contains a header or manifold to allow more than one helical pipe to be connected.

There is a need for a commercial device that can accommodate the flow of multiple showers or other large volumes of grey water.

SUMMARY OF THE INVENTION

According to one aspect, the invention features a grey water to clean water heat exchanger. The grey water to clean water heat exchanger comprises a discharge pipe having an exterior surface, an interior diameter, a grey water inlet and a grey water outlet, the discharge pipe configured to convey grey water from a location of use to a discharge location; a helical coil having a clean water inlet and a clean water outlet, the helical coil situated within the interior diameter of the discharge pipe with the clean water inlet and the clean water outlet configured to be accessible from locations outside the exterior surface of the discharge pipe, the helical coil configured to carry clean water from a water source to location of use of the clean water, the discharge pipe and the helical coil configured to allow heat energy to flow from the grey water to the clean water; a filter configured to be removably positioned within the grey water flow, the filter configured to filter material from the grey water; a bypass valve and a bypass pipe configured to allow grey water to controllably bypass the discharge pipe and the helical coil and to reach the discharge location; and a controller configured to activate and deactivate the bypass valve in response to a non-volatile control signal.

In one embodiment, the discharge pipe and the helical coil are configured to have the grey water and the clean water flowing in counterflow to each other.

In another embodiment, the controller is a thermal switch.

In yet another embodiment, the thermal switch is a bimetallic element. In still another embodiment, the controller is an electronic microprocessor-based controller configured to operate under instructions provided on a machine-readable medium.

In a further embodiment, the controller further comprises a thermal sensing element.

In yet a further embodiment, the thermal sensing element is a thermocouple.

In an additional embodiment, the controller further comprises a flow sensing element.

In one more embodiment, the helical coil comprises copper.

In still a further embodiment, the filter comprises a nylon filter element.

According to another aspect, the invention relates to a thermal energy recovery method. The thermal energy recovery method comprises the steps of: providing an grey water to clean water heat exchanger, comprising a grey water discharge pipe configured to carry grey water and a helical coil situated within the discharge pipe, the helical coil configured to carry clean water, a removable filter configured to filter material from the grey water, a bypass valve and a bypass pipe configured to allow grey water to controllably bypass the discharge pipe, and a controller configured to activate and deactivate the bypass valve in response to a non-volatile control signal; providing a source of grey water and a source of clean water; setting the bypass valve to convey grey water through the bypass pipe; sensing whether a flow of grey water having a temperature above a predetermined value is present in the bypass pipe; in response to sensing that a flow of grey water having a temperature above a predetermined value is present in the bypass pipe, causing the controller to operate the bypass valve to flow the grey water through the discharge pipe; and recovering thermal energy from the grey water and heating clean water in the helical coil with the recovered energy.

In one embodiment, the controller controls the bypass valve to cause grey water to flow through the bypass pipe when the filter becomes clogged.

In another embodiment, the controller indicates that the filter requires cleaning or replacement when the filter becomes clogged.

The foregoing and other objects, aspects, features, and advantages of the invention will become more apparent from the following description and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects and features of the invention can be better understood with reference to the drawings described below, and the claims. The drawings are not necessarily to scale, emphasis instead generally being placed upon illustrating the principles of the invention. In the drawings, like numerals are used to indicate like parts throughout the various views.

FIG. 1A is a diagram in axial view of the exterior pipe and interior pipe in a pipe heat exchanger.

FIG. 1B is a diagram in cross-sectional view of the exterior pipe and interior pipe in a pipe heat exchanger, in which counter flow is employed.

FIG. 2 is a perspective view of a helical coil in a pipe heat exchanger.

FIG. 3 is a diagram in axial view of a helical coil in a pipe cross flow heat exchanger.

FIG. 4 is a diagram in vertical cross-sectional view of the helical coil in a pipe counter flow heat exchanger.

FIG. 5 is a diagram of a copper helical coil used in the heat exchanger of FIG. 3 and FIG. 4.

FIG. 6 illustrates a heat exchanger location under a shower room.

FIG. 7 is a schematic diagram illustrating a heat flux representation of one design.

FIG. 8 is a graph showing the outlet temperatures of water flows in a heat exchanger as a function of length.

FIG. 9 is a graph showing the prototype cost (straight line) and yearly energy cost savings (curved line) as a function of heat exchanger length in feet.

FIG. 10 is a graph showing the net saving as a function of heat exchanger length in feet for the example illustrated in FIG. 9.

FIG. 11 is an illustration of a test set-up for a heat exchanger constructed according to the principles of the invention.

FIG. 12 is an illustration of a restricted flow test setup for a heat exchanger constructed according to the principles of the invention.

FIG. 13 is a graph showing the results of restricted flow test 1.

FIG. 14 is a graph showing the results of restricted flow test 2.

FIG. 15 is a graph showing the results of restricted flow test 3.

FIG. 16 is an illustration of an unrestricted flow test setup for a heat exchanger constructed according to the principles of the invention.

FIG. 17 is a graph showing the results of unrestricted flow test 1.

FIG. 18 is a graph showing the results of unrestricted flow test 2.

FIG. 19 is a graph showing the results of unrestricted flow test 3.

FIG. 20 is a perspective diagram of one embodiment of a helical coil counterflow heat exchanger in assembled form.

FIG. 21 is a perspective diagram of one embodiment of a filter module built and used according to principles of the invention.

FIG. 22 is an exploded diagram of one embodiment of a helical coil counterflow heat exchanger that illustrates the parts of the invention.

FIG. 23 illustrates one embodiment of the system, which includes a temperature valve 2301 allows cold water to bypass the heat exchanger. This valve also allows for backed up water to go through the bypass pipe 2303. The filter 2302 is provided to prevent dirt in the grey water from clogging the interior of the heat exchanger.

FIG. 24 illustrates another embodiment of the system, which includes a closed off pipe through the middle of the heat exchanger to distribute the flow and ensure water flow over the helical pipe.

FIG. 25A illustrates a housing for a filter configuration.

FIG. 25B illustrates a plug that is inserted into the housing for the filter configuration. The plug and the housing are threaded and can be threadedly connected.

FIG. 26 illustrates a filter configuration having a baffle built into the filter mechanism and can be used any of the embodiments.

FIG. 27A illustrates a filter handle that mates with a filter plug illustrated in FIG. 27B. The handle and plug fit together with a tight fit holding them in place. There is a built in baffle and there are distributor tubes within the filtering mechanism.

FIG. 28A illustrates a filter handle and plug that mates with a filter plug illustrated in FIG. 28B. The handle and plug fit together with a tight fit holding them in place. This embodiment comprises the first filtering mechanism described herein.

FIG. 29 illustrates a manufacturing improvement to allow this product to be fully assembled without welding. Welding is a distinct problem when fitting the entire assembly together. By having thread ends with an outside piece being the male or female threading, depending on the coils male or female threading, the two mate allowing for a water tight fit. The other end of the outside pipe can have a standard joint.

DETAILED DESCRIPTION

We describe an effective wastewater heat recovery system through the use of a simple heat exchanger that can accommodate multiple grey water producing devices. An efficient heat exchanger using a copper helical pipe design has been shown to provide heat recovery of approximately 40.5° F. and a cost savings of approximately $1500 per year. Through the use of this design businesses could significantly reduce overhead operating costs.

A design that would meet the needs of commercial businesses could be installed in a building's mechanical room or basement. The device would fit within an 8 inch by 8 inch by 8 foot box, these dimensions allow the device to be retrofitted easily into a typical building mechanical room or basement and allow for the existing drains lines to enter and exit the device. The device would use gravity and water main pressure whenever possible to conserve energy. The device should be easily installed in current plumbing and require minimal maintenance. The device should also be able raise the temperature of the incoming cold water by 30° F. to remain competitive with devices currently on the market. Currently there is no single device on the market which will take in grey water from large numbers of showers and capture the wasted energy. The design of a system which could efficiently capture this wasted energy and use it to pre-heat the incoming cold water could drastically lower the energy usage in these commercial areas.

The design that was chosen is a helical pipe in a pipe design. This design improves upon the shortcomings of other heat exchanger designs. The design calls for the hot wastewater to flow over the helical copper pipe which contains the incoming cold water that is being sent to the showers. This design allows for gravity to drive the hot wastewater through the system and out of the heat exchanger.

The helical pipe design maximizes the area of contact because it is placed directly in the hot grey water flow. This set-up captures the largest amount of energy from the water. Due to the nature of the helical pipe, debris build up can be prevented by using an appropriate spacing between the coils. The outer pipe size was determined by equating the unmodified drainage pipe size to a larger pipe with the helical pipe inside. This allows for the same flow rates that a typical unmodified drain pipe can accommodate as well as preventing additional clogging.

The design specifications were taken into consideration when determining the necessary pipe dimensions, spacing and length for the heat exchanger. The length of the copper tubing determines how much heat transfer will take place. The longer the pipe, the more heat will be transferred until an equilibrium is reached. The design has been reduced to practice and has been tested successfully.

Pipe in a Pipe Design

Depicted below in FIG. 1A and FIG. 1B is the pipe in a pipe heat exchanger, the most basic heat exchanger possible, which was analyzed first. D_i represents the inner diameter of the inside copper pipe where the incoming cold water would flow and D_o represents the inner diameter of the outside PVC pipe where the hot grey water would pass over the cold water pipe.

The conditions used in the analysis were ideal to achieve a best case scenario. Assumptions included an adiabatic system, negligible changes in potential and kinetic energy, and constant fluid properties. It also did not consider the thermal resistance of the pipes or fouling factors. The flow of the cold water was found to be turbulent in the inner pipe, while the hot water flow in the annulus was found to be laminar. After completing this analysis for a group of ten showers, the length of pipe needed to raise the domestic cold water 5° C. with incoming cold water at 12° C. and grey warm water leaving at 27° C. is 120.29 meters. These values were obtained using methods described later in section 6.2. Using the length found in the ideal case, this number was determined to be the minimum pipe length needed. This case would not be a practical design and cannot be implemented at reasonable cost, but it can be used as a basis for comparison and insight into other designs.

Shell and Tube Design

The shell and tube counter flow heat exchanger design was the next design under consideration. This design would consist of conducting numerous passes of the grey water over the cold water pipes. It was determined that the shell and tube design would not meet the design specifications due to the amount of space needed to pass the grey water pipes by the cold water pipes. The design would require a similar length of pipe as was found for the case of the pipe in a pipe design, weaved into a closed space.

Helical Coil in a Pipe Design

The current design was conceived to eliminate the disadvantages of the previous designs. This design includes increasing the contact area of the copper coil to the hot grey water flow in a shorter length of vertical pipe, therefore increasing contact time between the two flows and allowing for the capture of more recoverable energy. The capture of the greatest amount of energy possible is also aided by increasing the turbulent flow within the pipes. By placing a helical coil in the middle of the pipe and providing an appropriate flow rate into the heat exchanger, the turbulent flow can be increased. The advantages of the previous two designs are also still present in this design; these advantages include the ease of installation and maintenance.

The helical copper coil inserted into a vertical PVC pipe provides several advantages. The design exhibits cross flow conditions which are optimal conditions for liquid heat exchangers. The helical copper coil contains the cold (clean) incoming water and the outer PVC pipe contains the hot grey water. The cold incoming water is fed in through the bottom of the heat exchanger and is warmed by the hot grey water flowing down over it. The heated clean water exits via a copper pipe near the top of the PVC pipe. The grey water exits through the bottom of the PVC pipe. This will be enclosed with additional insulation around the outside of the outer PVC pipe, providing for adiabatic conditions around the outside of the heat exchanger. FIG. 2 is a perspective view of a helical coil in a pipe heat exchanger without insulation.

In FIG. 3, the hot grey water is found in the white areas which are defined by D_o which is the diameter of the outer PVC pipe and x which is the inner diameter of the copper coil. The variable d is the diameter of the helical coil containing the cold incoming water. These hot and cold flows should never come into contact with one another, to prevent the contamination of the incoming clean cold water.

FIG. 4 is a vertical cross-section of the design, the same variables hold true. The warm grey water is denoted by the red arrows and seen going through the center dimension as well as down the sides and finally out into of the heat exchanger. The cold clean incoming water is denoted by the blue arrows going up through the helical pipe.

The major challenge with the helical in pipe design was the potential for clogging. This potential for clogging is diminished by increasing the spacing between coils in the helical pipe. This spacing is 0.125″ and can be seen in FIG. 5. The spacing allows for the grey water to pass from one side oldie coil to the other. By allowing this passage, it flushes out any possible debris that could potentially clog and therefore reduce the efficiency of the heat exchanger.

The location and intended use of the heat exchanger can be seen in FIG. 6. The heat exchanger will pre-heat cold incoming water destined for the showers. This water combines in a mixing valve at the shower which regulates the temperature of the water coming out of the shower head. This allows for the boiler to heat less water and use less energy because the desired temperature can be obtained with less hot water. The design also provides for the use of gravity, as shown in FIG. 6, to move the flow of the hot grey water down the heat exchanger and ultimately to the sewer. The water main pressure allows for the cold clean incoming water to make it to the top of the heat exchanger with a slight pressure drop of 0.618 psi, determined through calculations. This pressure drop shows that the design does not need a pump to assist in moving the water through the heat exchanger and on its way through the rest of the plumbing system.

Helical Coil in a Pipe Analysis

To begin analysis on the chosen full scale design, the flow conditions in the heat exchanger needed to be determined. These conditions include the flow rates of both the cold water and the hot grey water as well as the temperatures each of these would be entering the heat exchanger. Flow rates were determined from the widespread use of low-flow shower heads mandated across the United States. These low-flow showerheads allow 2.5 gallons per minute (gpm) of water to exit, down from 5 gpm that older showerheads allowed. Since the hot shower water coming out of the showerhead is the same water going down the drain towards the heat exchanger, this 2.5 gpm became the flow rate of warm grey water per shower for all analyses. Shower water is a mixture of cold water coming from a well or town supply and hot water heated in a boiler. Approximately 0.5 gpm of the 2.5 gpm shower Water flow comes from the cold water pipeline. The 0.5 gpm of cold water being mixed is the same that flows through the heat exchanger prior to entering the shower mixing valve and is therefore the flow rate used in analysis. The heat exchanger was analyzed for ten showers happening at the same time, so flow rates of 25 gpm grey water and 5 gpm cold water were used. Through research and testing it was determined that the average temperature of the cold water entering from the street, T_ci, and thus the heat exchanger, is 48° F. and the temperature of grey shower water going down a drain, T_hi, is 104° F. Projected exit temperatures were used to determine an average temperature for each of the water types, which were then used to find the specific heat, thermal conductivity, viscosity, and Prandtl Number for each flow.

In order to calculate the dimensions of the full scale heat exchanger, a series of optimizations were performed. The first sets of dimensions to be analyzed were the cross-sectional dimensions. These include the diameter of the outer PVC pipe, D_o, the diameter and thickness of the copper coiled pipe, d and t respectively, and the diameter of the coil itself, X. Values for D_o and t were chosen from a list of available sizes from McMaster-Carr. The wall thickness of the copper coil depends directly on the diameter of the pipe and was not treated as an independent variable. The idea behind the optimization was to provide the maximum heat transfer while minimizing costs and maintaining design integrity. One design feature that was addressed in this optimization was making sure the heat exchanger did not impede the normal flow of drain water. This was accomplished by equating the area of a standard four inch diameter drain pipe to the area the grey water was allowed to flow through in the chosen design. This relationship is seen below in Equation 1. Since Do and t had a finite set of values, the equation was solved for X, the modified equation is shown in Equation 2. From this equation an optimization table was populated that yielded maximum values of X for certain combinations of Do and d. The inequality expressed in Equation 3 was then used to determine which values of X were feasible values that did not have the copper coil existing outside of the PVC pipe. The nine values of X that satisfied the inequality became design dimension considerations. The optimization table can be seen below in Table 1, with the design consideration values italicized in red text. These design considerations were then analyzed in terms of the heat transfer they produced in a six foot tall heat exchanger as well as the cost to produce a one foot length of a heat exchanger with the given dimensions. The method used in determining both of those values will be discussed below.

$\begin{array}{cc}\frac{1}{4}{\pi \left(4\right)}^2=\frac{1}{4}\pi \left(D_0^2-{\left(X+2d+4t\right)}^2+X^2\right)& \left(1\right)\\ X=\frac{D_0^2-{\left(2d+4t\right)}^2-16}{2\left(2d+4t\right)}& \left(2\right)\end{array}$
X+2d+4t<D₀²  (3)

TABLE I
Cross Section Dimensional Optimization
	Do
	d	t	5.761	6.065	7.625	7.981
	0.527	0.049	6.250	7.688	16.231	18.453
	0.666	0.042	4.979	6.178	13.296	15.148
	0.995	0.065	2.694	3.497	8.239	9.474
	1.245	0.065	1.750	2.403	6.286	7.297
	1.481	0.072	1.019	1.572	4.858	5.712
	2.009	0.058	−0.102	0.320	2.832	3.486

Cross flow heat exchangers transfer heat between liquids that are flowing roughly perpendicular to each other. The coiled copper pipe inside of the PVC outer pipe closely resembles cross flow conditions, where the cold water is considered unmixed and the warm grey water is considered mixed. As such, the chosen design will be analyzed as a cross flow heat exchanger.

To begin cross flow heat exchanger analysis the heat capacity, C, of each liquid has to be determined. This is found by multiplying the mass flow rate of the liquid, m, with its specific heat, c_p, as shown in Equation 4. The heat capacity is calculated for both the hot grey water and cold water flows and the flow with the smaller heat capacity is denoted as C_min. For this design, the cold water flow was found to be C_min and the hot grey water flow was found to be C_max. The value C_r is the ratio of C_min to C_max and can be seen in Equation 5.

C={dot over (m)}×c_p  (4)

$\begin{array}{cc}{C}_r=\frac{C_{\min }}{C_{\max }}& \left(5\right)\end{array}$

The next step in the analysis is to calculate the Reynolds' Numbers for both the cold water flow and the hot grey water flow. The cold water Reynolds' Number is found through Equation 6 where μ_c is the viscosity of the cold water. The hot grey water flow depends on the hydraulic diameter, D_h, wetted perimeter, P_w, and contact area, A, of the design. The relationships between the three variables are shown below in Equations 7, 8 and 9. The Reynolds' Number of the hot grey water flow is calculated in Equation 10.

$\begin{array}{cc}{\mathrm{Re}}_C=\frac{4{\stackrel{.}{m}}_C}{{\mu }_Cd\pi }& \left(6\right)\\ D_h=\frac{4A}{P_W}& \left(7\right)\\ A=\frac{1}{4}\pi \left(D_0^2-{\left(X+2d+4t\right)}^2+X^2\right)& \left(8\right)\end{array}$
P_W=π(D₀+(X+2d+4t)+X  (9)

$\begin{array}{cc}{\mathrm{Re}}_h=\frac{4{\stackrel{.}{m}}_h}{{\mu }_hP_W}& \left(10\right)\end{array}$

Both flows exhibit turbulent flow, as can be seen by the Reynolds' Numbers of 14,715 and 8,515 as well as the complexity of the design. The Nusselt's Number for both turbulent flows is calculated with Equation 11, and is then used to calculate the convection coefficients, h_c and h_h. The cold water convection coefficient is found using Equation 12 and the hot grey water convection coefficient is found using Equation 13.

Nu=0.023Re^0.8Pr^0.4  (11)

$\begin{array}{cc}{h}_C={\mathrm{Nu}}_C\frac{k_{\mathrm{fC}}}{d}& \left(12\right)\\ h_h={\mathrm{Nu}}_h\frac{k_{\mathrm{fh}}}{D_h}& \left(13\right)\end{array}$

The stretched out length of the copper coil is calculated using Equation 14 where H is the desired height of the heat exchanger and s_c is the spacing between consecutive coils. This length is used to determine the surface area of the copper coil, which is also the heat transfer area used in the heat exchanger. The surface area is broken up into two calculations; cold side area, A_c, and hot side area, A_h. The difference between the two areas is that the hot side area takes the copper pipe's thickness into consideration. Calculations for both areas are shown in Equations 15 and 16. Using the areas and convection coefficients calculated above, along with the copper pipe thickness and thermal conductivity, k; the overall convection coefficient across the area of the heat exchanger, UA_c, was calculated. A graphical representation of the heat flux is shown in FIG. 7 and the equation derived from that is shown in Equation 17.

$\begin{array}{cc}L=\frac{\pi \left(X+d\right)\left(H-d\right)}{\left(d+s_c\right)}& \left(14\right)\end{array}$
A_C=πdL  (15)

A_h=π(d+2t)L  (16)

$\begin{array}{cc}{\mathrm{UA}}_C=\frac{1}{\frac{1}{h_CA_C}+\frac{t}{kA_C}+\frac{1}{h_hA_h}}& \left(17\right)\end{array}$

Once the overall convection coefficient is determined, the number of transfer units, NTU, and the efficiency of the heat exchanger, ε, can be calculated. The maximum heat transfer, q_max, and the actual heat transfer, q, can also be calculated. The equations that relate all of these values are shown below in Equations 18, 19, 20 and 21.

$\begin{array}{cc}\mathrm{NTU}=\frac{{\mathrm{UA}}_C}{C_{\min }}& \left(18\right)\\ \varepsilon =\left(\frac{1}{C_r}\right)\left(1-{}^{\left(-C_r\left(1-{}^{-\mathrm{NTU}}\right)\right)}\right)& \left(19\right)\end{array}$
q_MAX=C_min(T_hi−T_ci)  (20)

q=ε·q_MAX  (21)

Using the actual heat transferred one can find the outlet hot and cold temperatures, T_ho and T_co respectively. The formulas to do so are shown in Equations 22 and 23.

$\begin{array}{cc}{T}_{\mathrm{ho}}=T_{\mathrm{hi}}-\frac{q}{{\stackrel{.}{m}}_h{\mathrm{cp}}_h}& \left(22\right)\\ T_{\mathrm{Co}}=T_{\mathrm{Ci}}+\frac{q}{{\stackrel{.}{m}}_c{\mathrm{cp}}_c}& \left(23\right)\end{array}$

Now that the dimensions of the heat exchanger can be related to a temperature increase in the cold water, the prototype cost per foot will also be related to the cross-sectional dimensions. This will allow for the optimization started above to narrow down the possible design dimension considerations. To obtain a cost per foot for each of the potential design dimensions, prices for each of the PVC and copper pipe sizes were obtained. One foot of heat exchanger includes one foot of PVC pipe and the length, L, of copper pipe that can be coiled inside. This length was found earlier in Equation 14. Table 2 was then constructed, displaying the dimensions of each consideration along with the pricing per foot to prototype. Designs 2, 3, 6, and 7 were not considered for cost analysis since the maximum temperature they could put out was far less than the other designs.

Designs 5, 8, and 9 were then removed from consideration due to costing more and producing lower temperature results than the other designs. Designs 1 and 4 were then modified to nominal dimensions of X and acceptable values for the spacing between the outside of the coil and the inside PVC wall. The final design chosen was the modified version of Design 1 which has a PVC diameter of 5.709″, a copper coil diameter of 0.995″, and a value of 2.7″ for X. This design yielded a change in temperature of the cold water equal to 31.4° F. over six feet of heat exchanger.

TABLE 2
Heat Transfer and Prototype Cost Optimizations
					LH=1 ft	ΔT
Design	D0 (in)	d (in)	t (in)	X (in)	(in)	(° F.)	$/Foot
1	5.761	0.995	0.065	2.695	104.402	30.957	$30.96
2	5.761	1.45	0.065	1.750	69.547	24.991	—
3	5.761	1.481	0.072	1.019	49.754	20.080	—
4	6.065	0.995	0.065	3.492	126.238	34.563	$34.56
5	6.065	1.245	0.065	2.404	84.093	28.490	$28.49
6	6.065	1.481	0.072	1.573	59.555	23.233	—
7	6.065	2.009	0.058	0.320	33.715	15.196	—
8	7.625	2.009	0.058	2.833	68.358	26.280	$26.28
9	7.981	2.009	0.058	3.486	77.370	28.528	$28.53

The most common way to heat water in commercial applications is with a natural gas boiler. As such, all cost savings analysis will be performed assuming a natural gas boiler is being used. Cost savings were determined for an individual shower operating under the conditions stated in the design specifications, primarily that ten showers are running at once. This was accomplished by performing the analysis at a ten shower flow load and then dividing by ten to get an individual heat transfer value, in Joules per second. To transfer this into energy, the heat transfer was multiplied by the length of an average shower, ten minutes. This energy, Q, is the energy that the boiler or water heater no longer needs to provide to the water for shower use, and is therefore saved. To convert this to cost savings, the energy saved is divided by the boiler efficiency, 0.8 in the case of natural gas boilers, then divided by the energy produced per cubic foot of natural gas, then multiplied by the price of natural gas per cubic foot. The equation that represents this conversion is shown in Equation 24. To change this into a yearly savings, a scenario of a gym that services 200 showers a day, under ten showers at a time conditions, is assumed.

$\begin{array}{cc}\frac{Q}{0.8}*\left(\frac{1{\mathrm{ft}}^3}{1.80776*{10}^6J}\right)*\left(\frac{\mathrm{\$15}\mathrm{.43}}{1000{\mathrm{ft}}^3}\right)=\$/\mathrm{Shower}& \left(24\right)\end{array}$

The next dimension of the heat exchanger to optimize was the overall length. A graph of the outlet temperatures of the chosen heat exchanger dimension vs. the length was created and shown in FIG. 8. The amount of heat transfer gained by adding another foot of length gets smaller as the heat exchanger gets larger. Since the price of the heat exchanger per foot remains constant, and the cost savings associated with the heat exchanger depend on how much heat is transferred, an optimal length can be determined. FIG. 9 shows the cost savings per year as well as the cost to prototype as a function of length. FIG. 10 shows the difference between the cost savings and the prototype cost. As seen in FIG. 10, the optimal length of a heat exchanger with the chosen dimensions is between six and eight feet long. A length of six feet was selected for the final design to fit within the design specifications. At this length, the expected cost savings for a 10 shower gym are $600 for the first year and $1050 a year for subsequent years.

The complete analysis of the chosen design and optimized dimensions can be seen in Table 3.

Test Set-Up

In order to validate the theoretical calculations a prototype device was created. A scaled down model of the final design was chosen as the prototype because of manufacturability and limitations of the testing conditions available such as water flow rates. The scaled down model reduced the PVC pipe diameter to 4″, the copper pipe diameter to 0.31″, the diameter of the coil to 2″ and the vertical height of the copper coil to 3′. Due to manufacturability, the spacing between the coils was increased to 0.3″. The flow rate available in the testing lab was a maximum of 12.5 gpm, with maximum hot water temperature of 120° F. and a cold water temperature of 53° F. It was decided to assume a 5 shower load which would require a hot water flow of 12.5 gpm and a cold water flow of 2.5 gpm. The dimensions of the scaled model were placed into the calculations spreadsheet and the expected outlet temperature of the cold water was found to be 67.82° F., an increase in temperature of 14.49° F. The analysis for the scaled model can be seen in Table 4. Test results that prove that the scaled down model works would also confirm the full scale model's behavior.

Temperatures during the test were measured through the use of 4 K-type thermocouples. The thermocouples were placed inside the pipes at the inlets and outlets of both the hot water and the cold water. Epoxy was used to keep the thermocouples in place and sealant was used to prevent leaks at connection points on the device. A LabVIEW VI was created to acquire and record data from the tests.

Water was brought to the heat exchanger by 0.75″ garden hoses and several connections that attached to either the copper tubing or the PVC pipe. Ball valves were included on all water lines to control flow and as an added safety measure. Once the set-up was fully attached, flow rates of the water through the heat exchanger were obtained using a 5 gallon bucket and a stopwatch. The set-up of the test apparatus can be seen in FIG. 11.

Six tests were conducted to verify the operation of the heat exchanger. The design was tested under two different flow conditions. During the first configuration the PVC pipe was half filled with water due to the use of a restriction on the hot water exiting the heat exchanger. The second configuration tested was without a restriction on the hot water flow outlet which did not allow for the hot water to back up inside the PVC pipe.

The tests performed with the restriction can be seen in action in FIG. 12. This figure demonstrates how the hot water half-filled the PVC pipe which created a very turbulent flow in that section. FIG. 13, FIG. 14 and FIG. 15 show the temperature plots of the testing when the restriction was applied. As can be seen in FIG. 13 through FIG. 15, the change in temperature and cold water outlet temperature follow the fluctuations of the incoming hot water.

The tests performed without the restriction can be seen in action in FIG. 16. This figure shows the water going straight through the PVC pipe without any hot water backing up into the pipe. FIG. 17, FIG. 18 and FIG. 19 show the temperature plots for the testing without the restriction in place. As can be seen in FIG. 17, FIG. 18 and FIG. 19, the change in temperature and cold water outlet temperature follow the fluctuations of the incoming hot water, which also occurred when the restriction was in place. This configuration would most accurately replicate conditions seen in the full scale device.

The testing results show that the heal exchanger performed slightly better with no restriction on the drain pipe. The change in temperature of the cold water was 38.5° F. for the restricted flow, which corresponds to an outlet temperature of 90.5° F. The change in cold water temperature for the unrestricted flow was 42° F. which corresponds to an outlet temperature of 95° F.

Data acquired during testing shows the scaled prototype performing much better than expected. The reasoning behind this is the hot water flow is much more complex than modeled in the calculations spreadsheet. The effect of placing the copper coil in the middle of the grey water flow and the use of spacing between the coils were not accounted for in the initial calculations of the Nusselt's numbers. The complexity of the design prevents the heat transfer behavior from being fully understood in theory alone, experimentation is needed. From the tests it was determined that the theoretical Nusselt's number should be increased to properly model the actual performance of the heat exchanger. When this change is applied to the full scale design, the outlet temperature of the cold water increases from 79.4° F. to 93.8° F. this updated analysis can be seen in able 5. The outlet temperature corresponds to an annual savings of $1400 for a typical gym with 10 showers and 200 uses per day.

The difference in results between the two configurations can be attributed to the column of water that builds up in the PVC pipe when the flow is restricted. The column of water immerses the bottom half oldie copper coil completely which increases contact area, but it comes at a loss of hot water temperature. The differences between the inlet and outlet temperatures of the grey water for each test are shown in Table 4. The average change in the grey water temperature for unrestricted flow was 3.44° F. compared to 3.72° F. for restricted flow. This difference is translated to the cold water flow, which is very sensitive to the changing hot water temperature.

Table 6 presents results of analysis for a full scale unit.

Filter

A heat exchanger is a device used to transfer heat from one medium to another, in our case, the energy from the grey water to the incoming cold water. The design's geometry was conceptualized to efficiently transfer this heat energy, and at the same time, not drastically affect the discharge of waste water.

The design was to put a helical pipe inside of a larger drainage pipe. The design calls for the grey water to flow over the inner helical pipe which contains the incoming cold water that is being sent to the showers. This allows for gravity to drive the hot wastewater through the system and out of the heat exchanger. This can be seen in FIG. 20.

The helical pipe design maximizes the area of contact as it is placed directly in the flow of the grey water, capturing the largest amount or energy from the water. Due to the nature of the helical pipe, debris build up can be prevented by using an appropriate spacing between the coils and by including a filtering system described later. The outer pipe size was determined by equating the unmodified drainage pipe size to a larger pipe with the helical pipe inside. This allows for the same flow rates that a typical unmodified drain pipe can accommodate as well as preventing additional clogging.

The next feature of the grey water heat recovery system provides protection to the systems helical coil by filtering the grey water. The filter prevents debris buildup which not only leads to clogging but also inhibits the transfer of energy. The filter is modular and is inserted into the drainage pipe above the helical coil using a unique system of clasps and seals to ensure water tight fit.

Another development to prevent debris build up in the heat exchanger consists of a filtering device. This was done by creating a slot for the filter module in the large drainage pipe. The filter module is a simple yet effective design of a cup with a filter on the bottom. The filter prevents debris buildup which ensures the efficiency of the machine is not affected. When time to replace, the customer would realize a slower draining rate, signaling the need to replace the filter, all while keeping the coil protected. The filter will be placed in a circular recess that has the same inner radius and thickness as the large drainage pipe. This creates a tight fit allowing for complete debris collection. The makeup of the outer portion oldie filter will consist of a half-pipe with the same radius as the large pipe. The filter will be secured with two latches at either end of the half-pipe that will hook into the heat exchangers drainage pipe, locking the filter in place for a water tight fit. The filter module will include an indicator mechanism to confirm to the user, after slower draining is experienced, that the filter needs to be cleaned. The filter module without the actual filter can be seen in FIG. 21.

There are several advantages of incorporating a filter into the system design. The filter will reduce the speed of the incoming grey water in addition to creating a more turbulent flow and filtering the water in a centralized location. Slowing the velocity of the water allows for longer exposure to the coils, thereby increasing the heat transfer. The only downside to the modular filter design is the added maintenance to the system. The frequency of maintenance will depend on the system use and the amount of debris from the environment the system is used in.

The design with the use of the filter has the potential to back up with debris once the filter is filled. This is solved by using a pipe connection called a tee, which has two outlets, similar to the shape of the letter “T”. By placing the tee with the single end going into the heat exchanger system, the second branch of the tee connector allows for the start of the bypass, which runs down the side of the pipe. Plumbing the system this way allows for a place where the backed up water can go if the filter is clogged, creating a bypass of the heat exchanger. From the tee, the bypass pipe is plumbed to down the reverse side of the system. This pipe provides rigidity where the filter insert gets inserted. The bypass pipe runs parallel to the heat exchanger until it reunites with the sewage line. This can be seen in FIG. 20.

FIG. 22 is an exploded diagram of one embodiment of a helical coil counterflow heat exchanger that illustrates the parts of the invention. A tee pipe fitting 1 admits waste water at the top and connects to both the heat exchanger and the bypass pipe. The path leading to the heat exchanger then has a pipe increaser 2 which then leads to the filter module 3. The grey water proceeds down pipe 7 and flows over the exterior of a helical coil 8. The helical coil 8 has a connector 5 at each end that is provided for ease of manufacturing and to position the helical coil 8 in the center of the drainage pipe 7. Clean cold water flows upwardly in the helical coil 8 when recovery of heat from the grey water is taking place. Once the grey water has passed over the helical coil 8 it proceeds to an elbow 9, followed by a short length of pipe 10 of the same diameter. Finally the water passes through a pipe reducer 12 which leads to the rest of the water discharge plumbing. The bypass route starts at the teel, to an elbow4, followed by a straight pipe 6. The bypass is completed with an elbow 4 and then can be connected to the rest of the water discharge plumbing.

Flow Control and Thermal Management

The flow system includes a switch and a controller to control a valve to direct the flow of grey water through the heat exchanger. In some embodiments, the controller is a thermal switch. In some embodiments, the thermal switch is a bimetallic element that operates to allow grey water to flow through the discharge pipe when the grey water has a temperature above a predetermined value (e.g., is warm enough to provide useful thermal energy), and to cause the grey water to flow thorough a bypass pipe when the grey water has a temperature below a predetermined value (e.g., is not warm enough to provide useful thermal energy). In some embodiments, the controller is an electronic microprocessor-based controller configured to operate under instructions provided on a machine-readable medium. In some embodiments, the controller further comprises a thermal sensing element, such as a thermocouple, that can provide a non-volatile electrical signal representative of a temperature (e.g., the temperature of the grey water). In some embodiments, the controller further comprises a flow sensing element that can sense whether grey water is flowing in the discharge pipe or in the bypass pipe. When no grey water is flowing at all, there is no point in running clean water through the heat exchanger. For example, the controller controls the bypass valve to cause grey water to flow through the bypass pipe when the filter becomes clogged. The controller can indicate such a condition by announcing that the filter is clogged and should be cleaned or replaced.

DEFINITIONS

Recording the results from an operation or data acquisition, such as for example, recording results at a particular frequency or wavelength, is understood to mean and is defined herein as writing output data in a non-transitory manner to a storage element, to a machine-readable storage medium, or to a storage device. Non-transitory machine-readable storage media that can be used in the invention include electronic, magnetic and/or optical storage media, such as magnetic floppy disks and hard disks; a DVD drive, a CD drive that in some embodiments can employ DVD disks, any of CD-ROM disks (i.e., read-only optical storage disks), CD-R disks (i.e., write-once, read-many optical storage disks), and CD-RW disks (i.e., rewriteable optical storage disks); and electronic storage media, such as RAM, ROM, EPROM, Compact Flash cards, PCMCIA cards, or alternatively SD or SDIO memory; and the electronic components (e.g., floppy disk drive, DVD drive, CD/CD-R/CD-RW drive, or Compact Flash/PCMCIA/SD adapter) that accommodate and read from and/or write to the storage media. Unless otherwise explicitly recited, any reference herein to “record” or “recording” is understood to refer to a non-transitory record or a non-transitory recording.

As is known to those of skill in the machine-readable storage media arts, new media and formats for data storage are continually being devised, and any convenient, commercially available storage medium and corresponding read/write device that may become available in the future is likely to be appropriate for use, especially if it provides any of a greater storage capacity, a higher access speed, a smaller size, and a lower cost per bit of stored information. Well known older machine-readable media are also available for use under certain conditions, such as punched paper tape or cards, magnetic recording on tape or wire, optical or magnetic reading of printed characters (e.g., OCR and magnetically encoded symbols) and machine-readable symbols such as one and two dimensional bar codes. Recording image data for later use (e.g., writing an image to memory or to digital memory) can be performed to enable the use of the recorded information as output, as data for display to a user, or as data to be made available for later use. Such digital memory elements or chips can be standalone memory devices, or can be incorporated within a device of interest. “Writing output data” or “writing an image to memory” is defined herein as including writing transformed data to registers within a microcomputer.

“Microcomputer” is defined herein as synonymous with microprocessor, microcontroller, and digital signal processor (“DSP”). It is understood that memory used by the microcomputer, including for example instructions for data processing coded as “firmware” can reside in memory physically inside of a microcomputer chip or in memory external to the microcomputer or in a combination of internal and external memory. Similarly, analog signals can be digitized by a standalone analog to digital converter (“ADC”) or one or more ADCs or multiplexed ADC channels can reside within a microcomputer package. It is also understood that field programmable array (“FPGA”) chips or application specific integrated circuits (“ASIC”) chips can perform microcomputer functions, either in hardware logic, software emulation of a microcomputer, or by a combination of the two. Apparatus having any of the inventive features described herein can operate entirely on one microcomputer or can include more than one microcomputer.

General purpose programmable computers useful for controlling instrumentation, recording signals and analyzing signals or data according to the present description can be any of a personal computer (PC), a microprocessor based computer, a portable computer, or other type of processing device. The general purpose programmable computer typically comprises a central processing unit, a storage or memory unit that can record and read information and programs using machine-readable storage media, a communication terminal such as a wired communication device or a wireless communication device, an output device such as a display terminal, and an input device such as a keyboard. The display terminal can be a touch screen display, in which case it can function as both a display device and an input device. Different and/or additional input devices can be present such as a pointing device, such as a mouse or a joystick, and different or additional output devices can be present such as an enunciator, for example a speaker, a second display, or a printer. The computer can run any one of a variety of operating systems, such as for example, any one of several versions of Windows, or of MacOS, or of UNIX, or of Linux. Computational results obtained in the operation of the general purpose computer can be stored for later use, and/or can be displayed to a user. At the very least, each microprocessor-based general purpose computer has registers that store the results of each computational step within the microprocessor, which results are then commonly stored in cache memory for later use.

Many functions of electrical and electronic apparatus can be implemented in hardware (for example, hard-wired logic), in software (for example, logic encoded in a program operating on a general purpose processor), and in firmware (for example, logic encoded in a non-volatile memory that is invoked for operation on a processor as required). The present invention contemplates the substitution of one implementation of hardware, firmware and software for another implementation of the equivalent functionality using a different one of hardware, firmware and software. To the extent that an implementation can be represented mathematically by a transfer function, that is, a specified response is generated at an output terminal for a specific excitation applied to an input terminal of a “black box” exhibiting the transfer function, any implementation of the transfer function, including any combination of hardware, firmware and software implementations of portions or segments of the transfer function, is contemplated herein, so long as at least some of the implementation is performed in hardware.

Theoretical Discussion

Although the theoretical description given herein is thought to be correct, the operation of the devices described and claimed herein does not depend upon the accuracy or validity of the theoretical description. That is, later theoretical developments that may explain the observed results on a basis different from the theory presented herein will not detract from the inventions described herein.

Any patent, patent application, or publication identified in the specification is hereby incorporated by reference herein in its entirety. Any material, or portion thereof, that is said to be incorporated by reference herein, but which conflicts with existing definitions, statements, or other disclosure material explicitly set forth herein is only incorporated to the extent that no conflict arises between that incorporated material and the present disclosure material. In the event of a conflict, the conflict is to be resolved in favor of the present disclosure as the preferred disclosure.

While the present invention has been particularly shown and described with reference to the preferred mode as illustrated in the drawing, it will be understood by one skilled in the art that various changes in detail may be affected therein without departing from the spirit and scope of the invention as defined by the claims.

Claims

1. A grey water to clean water heat exchanger, comprising:

a discharge pipe having an exterior surface, an interior diameter, a grey water inlet and a grey water outlet, said discharge pipe configured to convey grey water from a location of use to a discharge location;

a helical coil having a clean water inlet and a clean water outlet, said helical coil situated within said interior diameter of said discharge pipe with said clean water inlet and said clean water outlet configured to be accessible from locations outside said exterior surface of said discharge pipe, said helical coil configured to carry clean water from a water source to a location of use of the clean water, said discharge pipe and said helical coil configured to allow heat energy to flow from said grey water to said clean water;

a filter configured to be removably positioned within said grey water flow, said filter configured to filter material from said grey water;

a bypass valve and a bypass pipe configured to allow grey water to controllably bypass said discharge pipe and said helical coil and to reach said discharge location; and

a controller configured to activate and deactivate said bypass valve in response to a non-volatile control signal.

2. The grey water to clean water heat exchanger of claim 1, wherein said discharge pipe and said helical coil are configured to have said grey water and said clean water flowing in counterflow to each other.

3. The grey water to clean water heat exchanger of claim 1, wherein said controller is a thermal switch.

4. The grey water to clean water heat exchanger of claim 3, wherein said thermal switch is a bimetallic element.

5. The grey water to clean water heat exchanger of claim 1, wherein said controller is an electronic microprocessor-based controller configured to operate under instructions provided on a machine-readable medium.

6. The grey water to clean water heat exchanger of claim 5, wherein said controller further comprises a thermal sensing element.

7. The grey water to clean water heat exchanger of claim 6, wherein said thermal sensing element is a thermocouple.

8. The grey water to clean water heat exchanger of claim 1, wherein said controller further comprises a flow sensing element.

9. The grey water to clean water heat exchanger of claim 1, wherein said helical coil comprises copper.

10. The grey water to clean water heat exchanger of claim 1, wherein said filter comprises a nylon filter element.

11. A thermal energy recovery method, comprising the steps of:

providing a grey water to clean water heat exchanger, comprising: a grey water discharge pipe configured to carry grey water and a helical coil situated within said discharge pipe, said helical coil configured to carry clean water, a removable filter configured to filter material from said grey water, a bypass valve and a bypass pipe configured to allow grey water to controllably bypass said discharge pipe, and a controller configured to activate and deactivate said bypass valve in response to a non-volatile control signal;

providing a source of grey water and a source of clean water;

setting said bypass valve to convey grey water through said bypass pipe;

sensing whether a flow of grey water having a temperature above a predetermined value is present in said bypass pipe;

in response to sensing that a flow of grey water having a temperature above a predetermined value is present in said bypass pipe, causing said controller to operate said bypass valve to flow said grey water through said discharge pipe; and

recovering thermal energy from said grey water and heating clean water in said helical coil with said recovered energy.

12. The thermal energy recovery method of claim 11, wherein said controller controls said bypass valve to cause grey water to flow through said bypass pipe when said filter becomes clogged.

13. The thermal energy recovery method of claim 11, wherein said controller indicates that said filter requires cleaning or replacement when said filter becomes clogged.