Variable input radiant heater

Abstract

A variable demand radiant heating system applies variable burner control technology to singular or mulit-burner radiant heating systems. A radiant heater consists of a burner connected to an elongated heat exchanger tube. The combustion air is supplied to the burner via blower or draft inducer. Fuel is supplied to the burner via fuel regulator. Fuel and air are mixed in burner and communicated to the inlet end of the heat exchanger tube. Spent products of combustion are expelled from the heat exchanger at the outlet end. The burner controls continuously vary gas supply pressure (volume), via a modulating gas regulator, and combustion air pressure (volume), via a variable speed blower, communicated to the burner mixing chamber, which in turn, varies the burner input on a continuous curve (not stepped or staged) within a pre-determined input range as heat demand varies.

Description

FIELD OF THE INVENTION

The present invention relates to controlling the thermal energy generated by a heating system.

BACKGROUND OF THE INVENTION

Presently, radiant heaters and radiant heating systems most commonly operate at a single preset input; and space temperature is controlled by a thermostat by turning the heater or heating system on or off. In the early to mid 1990's, radiant heaters have been developed to operate at either of two distinct preset inputs by varying the fuel pressure communicated to the burner via a two-stage fuel regulator. One radiant burner with a two-stage operation is described in U.S. Pat. No. 5,353,986 titled “Demand Radiant Heating System” by Joseph B. Wortman. In the '986 patent, Wortman describes a radiant heater with a single fuel control capable of dual regulation. The dual regulation is limited to only providing a high or low input rate to respond to a high or low heat demand.

Further advances in radiant burner input control are described in U.S. Pat. No. 5,989,011 titled “Burner Control System” by Caruso et al. Caruso et al. in the '011 patent disclose a control system. Caruso et al. describe the control system as being capable of altering the fuel pressure to the burner by varying air pressure from a blower to the fuel regulator via an air regulator. By continuously varying the air pressure communicated to the fuel regulator, the discharge gas pressure to the burner is also varied allowing a continuously variable input.

However, in neither cited patent is air pressure (volume) to the burner's fuel/air mixing apparatus varied.

Heater inputs are sized to satisfy building heat loss based on an outdoor design temperature that occurs approximately 1-5% of the time during the entire heating season. In other words, single-stage heating systems normally operate at an input that exceeds the demand. It is favorable to have the option of varying the heater input based on the heating demand to decrease the number of heater on/off cycles and to increase occupant comfort in the heated space.

Two-stage burners are alternative options to the heaters that conform to the cited references. Such two-stage burners are limited to only two distinct operating inputs, offering only coarse control of varying demands and do not match the heat demand for the majority of the time. Continuously variable modulating control allows fine control of heater input to match the heat demand closely, operating at any percentage of the heater's full rated input, within a predetermined range.

In both patents mentioned above, there are disadvantages to varying gas volume (or pressure) to the burner mixing apparatus without also varying combustion air volume (or pressure) to the burner's mixing apparatus. Without variation of the air flow to the burner simultaneously with variation of fuel flow, sacrifices are made in terms of heater performance and efficiency as well as combustion quality and efficiency. It is desirable to vary the heater's input not only by controlling the gas flow to the burner, but also the combustion air flow. By varying both the combustion air flow and gas flow (pressure or volume); combustion efficiency, combustion quality, heater efficiencies and flue emissions can be more closely regulated for optimum infrared heater performance.

In commonly assigned U.S. Pat. No. 5,211,331 entitled “Control in Combination with Thermostatically Responsive Assembly”, Timothy Seel describes a variable input system of infrared burners-in-series. The infrared system of burners possesses the ability to vary fuel and combustion air to achieve modulating system input. Seel does not disclose, teach or suggest any ability to control a single “unitary” style infrared heater with associated burner modulating controls and blower mounted internal to the burner housing.

A version of the control that can be used in the present invention is manufactured by Varidigm Corporation of Plymouth, Minn. That control has the capability of controlling a modulating gas valve and varying the speed of a single-phase shaded-pole motor. That control, however, cannot be merely inserted into the present invention without tailoring certain parameters to obtain the desired results.

Fractional horsepower DC motors are readily available in the market and can easily be controlled to vary their speed, but a DC motor is more expensive than a shaded pole motor of similar size. In addition to the DC motor costing more, a controller is needed to send a control signal to the DC motor to vary its speed; a controller would also need to send a separate control signal to vary the gas valve, adding more cost. The ability to vary the speed of a shaded pole motor allows a cost savings by eliminating the need to use a more expensive DC motor as well as incorporating motor control, gas valve control and burner ignition and sequencing.

Two-stage and modulating infrared heaters with fixed combustion air flow set the combustion air flow for the maximum input. In laboratory testing in accordance with the European Standard prEN 416-2 “Single Burner Gas-Fired Overhead Radiant Tube Heaters For Non-Domestic Use”, it has been shown that two stage heaters exhibit 9-10% lower radiant efficiency at low input due to the blower delivering an excess of combustion air, which is fixed to deliver a volume and pressure of air that is optimum only at maximum input. Besides a reduction in radiant efficiency, two-stage infrared heaters show an approximate 2% decrease in thermal efficiency at low input versus high input. By reducing the combustion air and fuel when input is reduced, a modulating infrared heater will maintain its optimum radiant efficiency at all inputs. That results in an exhibition of radiant efficiency at low fire that is 9-10% higher than a two-stage heater. That capability is not possible in current modulating or two-stage infrared heater design with single speed blowers. In addition, by maintaining heat exchanger temperature through varying fuel and combustion air flow with respect to burner pressure, the radiant efficiency of the heater can be maintained throughout the entire input modulation range. Not only is radiant efficiency improved, but also thermal efficiency increases as input decreases, thermal efficiency increases 3-4% at minimum input versus maximum input. Two stage infrared heaters typically allow for a 30-35% input turndown from high input to low input, an air and fuel modulating heater can exhibit input turndowns near 70% from maximum input to minimum input, doubling the turndown capability of a two-stage infrared heater.

SUMMARY OF THE INVENTION

The present invention relates to the use of an apparatus for continuously varying the input of radiant gas heaters that respond to heat demand. The variable input radiant heater apparatus has a burner housing having a combustion air and fuel inlet, a burner assembly for mixing the fuel and air, and conveying the mixture into a heat exchanger for combustion. Combustion takes place inside the heat exchanger and the resulting hot products of combustion are moved through the heat exchanger to the exhaust end due to air pressure from a combustion air blower providing either positive air pressure from the burner end of the heater or negative pressure from the exhaust end of the heater. At the exhaust end of the heat exchanger, the combustion gasses are vented from the heater. A signal is conveyed to a controller mounted in the burner housing from a heat demand control device. Based on the signal, the controller varies the input of the heater to satisfy the heat demand. The input of the burner is varied by changes in the combustion air (via blower speed changes) and fuel (via modulating gas valve) supplied to the burner assembly.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

Generically, the present invention is directed to a single radiant heater or multi-burner radiant heating system. In particular, the present invention is directed to a single radiant heater or multi-burner radiant heating system that modulates the burner input by varying fuel and combustion air supply to the burner's mixing apparatus. The apparatus continuously varies the input of radiant gas heaters that respond to heat demand. The variable input radiant heater apparatus have a burner housing with a combustion air and fuel inlet and a burner assembly for mixing the fuel and air, and conveying the mixture into a heat exchanger for combustion. Combustion takes place inside the heat exchanger and the resulting hot products of combustion are moved through the heat exchanger to the exhaust end due to air pressure from a combustion air blower providing either positive air pressure from the burner end of the heater or negative pressure from the exhaust end of the heater. At the exhaust end of the heat exchanger, the combustion gasses are vented from the heater. A signal is conveyed to a controller mounted in the burner housing from a heat demand control device. Based on the signal, the controller varies the input of the heater to satisfy the heat demand. The input of the burner is varied by changes in the combustion air (via blower speed changes) and fuel (via modulating gas valve) supplied to the burner assembly.

1. Some Objectives of the Present Invention

It is an object of the present invention to combine patented burner control technology and detailed laboratory analysis of infrared heaters specifically, to customize the operation and settings of the control for the purpose of optimization of performance, efficiencies and safety unique to an infrared heater.

It is an object of the present invention that a modulating gas valve controls fuel supply. The gas valve may have either pneumatic or electronic modulation. The fuel volume and pressure issued from the outlet of the gas valve to the burner can either be controlled by an electronic signal from the controller or a pneumatic (air pressure) signal from the blower. An advantage of the present invention using an electronic modulating gas valve is that the control of the gas valve is independent of the air pressure generated by the blower allowing for customization of the fuel to combustion air ratio. An advantage of the ability to customize this ratio is that heater performance, efficiencies and safety can be maximized for various burner fuel types and inputs.

It is an object of the present invention that the combustion air pressure and volume supplied to the burner is variable and is controlled by varying the speed of the blower motor. An advantage of the present invention is that the motor may be DC, permanent split capacitor (AC, single phase) or shaded pole (AC, single phase). The option allows for the most economical choice as the motor market dictates. The controller varies the speed of the motor by electronic signal. Currently there is no other control readily available that can vary the speed of a fractional horsepower shaded-pole motor.

It is an object of the present invention to be able to control motor speed of a standard single-phase shaded pole motor that is commonly used in single and two-stage infrared heaters. Achieving motor speed control by purchasing a more expensive DC motor is not required.

It is an object of the present invention to incorporate the burner control into infrared burner design such that the compact, lightweight control allows mounting of the control inside the burner housing and also allows optional mounting of the blower inside the burner housing without the need to increase the housing size.

It is an object of the present invention to control the input to any point within a predetermined range of inputs. The burner may operate at any input between and including full rated input to 30% of full rated input. The input range may be narrowed by reprogramming of the control's logic chip(s) if desired.

It is an object of the present invention to vary the burner input based on any one of various demand control devices.

It is an object of the present invention to detect heated area conditions with a traditional mechanical thermostat. By recording input and duration of past heating cycles, a programmed algorithm can pre-determine the initial heater input of a new heating cycle. During a new heating cycle the controller can adjust this pre-determined heater input based on timing of the new heating cycle and/or additional limit sensors or thermostats.

It is an object of the present invention to detect heated area conditions with a temperature sensor in the space. By calculating the difference between a set point temperature and an actual air temperature the controller can vary the heater input to respond to sensed heat demand.

It is an object of the present invention to detect user-controlled settings from a manually operated potentiometer to select heater input based on user demands.

It is an object of the present invention to control the combustion characteristics at the continuously varying input by continuously varying the fuel flow and combustion air flow to the burner's mixing means. Continuously changing condition inputs communicated to the burner control dictate the desired heat input. Combustion air flow and fuel flow are continuously varied to achieve changing input requirements to satisfy the desired heat demand.

It is an object of the present invention to monitor burner pressure and correct fuel flow and/or combustion air flow to maintain proper combustion under varying burner pressure conditions and to control blower speed and gas valve position independent of each other. The controller is pre-programmed with the required gas valve positions for every burner pressure. In response to changing demands, blower speed adjusts first to achieve a desired burner pressure, as correct burner pressure is sensed the fuel is immediately adjusted for desired combustion based on the pre-programmed settings. If adequate burner pressure cannot be achieved by changing blower speed, the fuel supplied will adjust according to the burner pressure that is achieved. If burner pressure decreases during a heating cycle, the controller senses the pressure drop and the controller will adjust the gas valve to supply the fuel necessary for correct combustion at the lower burner pressure.

2. Heater

The heater or multi-burner heating system 10 in this invention includes a burner housing 12 to which a heat exchanger 14 is connected, as shown in FIG. 1. The heat exchanger's 14 length and shape may be various. Examples of shapes include and is not limited to straight, U-shaped, J-shaped, L-shaped, and polygonal shaped. The heat exchanger 14 is of conventional construction and will typically be mounted below a reflector 16 covering at least a significant portion of the length of the heat exchanger 14. The entire heater 10 including burner housing 12, heat exchanger 14 and reflector 16 is typically suspended, as shown in FIG. 2, with conventional suspension instruments like cables, rods, cords and the like 102 from and/or attached (clamps, brackets, screws, bolts, nails and the like) to a ceiling 100 of a structure.

In accordance with this invention, the housing 12 is provided with a single fuel delivery system, as shown in FIG. 3, including (1) a modulating gas valve 121, (2) a gas manifold 122 whose inlet side 122a is connected to the outlet side 121a of the gas valve 121 and (3) a burner assembly 123 whose inlet side 123a is connected to the outlet side 122b of the manifold 122.

The burner assembly 123 includes suitable apertures 123b, and an apertured stem 123c connected to the manifold 122 outlet 122b fitted with a suitable gas orifice 124. Mounted either downstream of the burner 123 or inside the burner 123 is a flame igniter 123d and flame sensor 123e.

The burner assembly 123 is positioned at the inlet end 141 of the heat exchanger 14.

3. Blower

A blower 18 is provided for causing a draft through (1) the combustion air inlet 125 of the burner housing 12, (2) the burner assembly 123, and (3) then the heat exchanger 14. The blower 18 may be positioned between the combustion air inlet 125 of the burner housing 12 and the burner assembly 123, forcing air through the burner housing 12 and heat exchanger 14. Alternately, the blower (draft inducer) may be positioned at the outlet end 142 of the heat exchanger 14, providing vacuum to pull air through the combustion air inlet 125 of the burner housing 12, through the burner assembly 123 then through the heat exchanger 14. An air restriction plate 20 is placed before or after the blower 18 to meter the combustion air delivered from the blower 18 to the burner assembly 123. Obviously, the blower 18 can be any conventional blower capable of providing the above-described attributes for conventional heating systems.

4. Controller

In accordance with this invention, a single controller 22 (control board) controls the operation and sequencing of the modulating gas valve 121, the blower 18 and the igniter 123d. The circuit board 22, manufactured by Varidigm Corporation of Plymouth, Minn., is powered both from a line voltage source 220 and from a 24V transformer 221 mounted in the burner housing 12 connected to line voltage 220. A pressure (or vacuum) switch 222 being sensitive to burner pressure via pressure lines 223 is electrically connected to the control board 22. The control board 22 monitors the opening and closing of the pressure switch circuit 222 to verify proper operation and calibration of a pressure transducer 224 on the control board 22. The pressure transducer 224 is also sensitive to burner pressure communicated via pressure hoses 223, which allows the controller 22 to alter blower 18 and gas valve operation 123 according to the current burner pressure. A pressure hose 223 is also connected to a tap on the modulating gas valve 121 to communicate a reference burner pressure to the valve for proper valve operation.

A conventional thermostat 225, as shown in FIG. 4, can be used to communicate heat demand to the controller 22. The control board 22 can collect data relating to a thermostat 225 circuit closing and opening cycle timing. Based on this timing the controller 22 can command ignition, modulation or shutting off of the burner 123. Alternately, an air temperature sensor or group of such sensors 226 can be used to communicate heat demand. The controller 22 can process the sensed temperature to command ignition, modulation or shutting off of the burner 123. Alternately, the user can initiate ignition and shut down the heater 10 with an on/off switch 227 as well as set the input rate during operation with a manual potentiometer 228.

By using the modulating burner control 22 and making modifications for application on an infrared heater 10, fuel and combustion air can be varied in correct proportions for optimum safety, performance and efficiency. The compact size allows for mounting in the burner housing 12 of the heater 123. By tailoring the controller's 22 fuel and combustion air settings specifically for infrared heaters through performing detailed laboratory analysis of burner performance characteristics individual to infrared heaters, burner efficiencies and safety can be maximized as never before.

At minimum input, the present invention achieves a thermal efficiency 5-6% higher than a two-stage infrared heater at low input. In addition, the controller 22 allows for a greater range of modulation between high and low input than a two-stage heater.

The burner controller 22 has pressure-sensing capability that greatly improves the safety and reliability of an infrared heater. Since the controller 22 has independent control of the combustion air and fuel supplies, it can adjust the blower speed to compensate for additional flue lengths or for partial flue or inlet blockage in an effort to optimize combustion quality. If proper combustion is not achievable by increasing blower 18 speed, the controller 22 will command the gas valve to reduce gas flow maintaining proper burner combustion. This ability maintains the quality of emissions for the modulating infrared heater and corrects situations that would otherwise result in elevated heat exchanger temperatures of infrared heaters. Not only does this increase the overall safety of the heater, but also potentially increases the service life of the heat exchanger 14.

5. Operation

In operation, the heater 10 is operated in a similar fashion to other thermostatically controlled heating appliances. A thermostat 225 or temperature sensor 226 or on/off switch 227 initiates the operation. Upon activation the blower 18 is energized and will operate at full speed. Once the pressure switch 222 proves flow of air through the burner and the pressure transducer 224 senses adequate pressure, the controller 22 allows an air purge period prior to ignition. After the purge period, the controller 22 energizes the igniter 123d then opens the gas valve 121. Gas flows through the gas valve 121, manifold 122 and orifice 124 then into the burner 123 where it mixes with the combustion air and the mixture is ignited by the igniter 123d. Ignition is detected by the flame sensor 123e, which signals the controller 22 to maintain the gas valve 121 in an open position. If the flame is extinguished at any time during operation, the flame sensor 123e will signal the controller 22 to close the gas valve 121 and stop the flow of gas to the manifold 122. Upon ignition, initial input is 100% of full rated input or maximum input allowed as dictated by achieved burner pressure. The heater 10 will continue to operate at maximum input for a predetermined duration for heat exchanger warm-up. Following the warm-up period, the heater will modulate based on achieved burner pressure and/or signals from demand control devices 225, 226, 227, 228. At all times during the operation of the heater 123, the burner pressure is monitored. Burner pressure, as realized by the blower operating speed, will dictate the appropriate gas pressure and volume as pre-determined by detailed laboratory testing for maximum safety, performance, efficiency and combustion and emissions quality. The heat and fire and associated flue gasses are pushed or drawn downstream through the heat exchanger 14, away from the burner 123 towards the exhaust end 142 of the heat exchanger 14. The fire and hot flue gasses heat up the heat exchanger 14. The heat exchanger 14 releases this energy through convective and radiant heat transfer from the tubes outer surface in all directions. The reflector 16 over the heat exchanger helps contain the convective heat to maintain desired tube temperature, it also reflects and directs the radiant energy down toward the heated space below the heater 10.

The heater described could also be grouped into a multi-burner heating system. In such a configuration, the exhaust ends 142 of multiple heat exchangers 14 are coupled together through a common draft inducer that is located at the exhaust end of the coupled heat exchanger. In this configuration, the draft inducer creates negative pressure through heating system drawing the flame and heated gasses toward the end of the coupled heat exchanger. All burners would modulate simultaneously as a result of connection to the same draft inducer.

While a preferred form of this invention has been described above and shown in the accompanying drawings. It should be understood that the applicant does not intend to be limited to the particular details described above and illustrated, but intends to be limited only by the scope of the invention as defined by the following claims.

Claims

1. A gas-fired radiant tube heater comprising:

at least one unitary burner housing connected to an elongated heat exchanger;

a burner controller mounted inside the burner housing;

a modulating gas valve, and a burner controller in the burner housing;

the burner controller capable of simultaneously varying gas and combustion air inputs to continuously deliver proper fuel/air ratio at every input rate.

2. The gas heater of claim 1 wherein the elongated heat exchanger is configurable in a shape that is straight.

3. The gas heater of claim 1 wherein the elongated heat exchanger is configurable in a shape that is U-shaped.

4. The gas heater of claim 1 wherein the elongated heat exchanger is configurable in a shape that is selected from the group consisting of straight, U-shaped, J-shaped, L-shaped, and polygonal shaped.

5. The gas heater of claim 1 wherein the heat exchanger is coupled to heat exchangers of additional heaters and vented through a common exit.

6. The gas heater of claim 1 wherein the burner modulates an input within a preset range burner inputs.

7. The gas heater of claim 1 wherein the burner input responds to at least one demand control device.

8. The gas heater of claim 1 wherein the burner senses burner pressure and automatically adjusts gas and combustion air inputs based on pre-programmed settings for optimum safety, efficiency and performance.

9. The gas heater of claim 1 wherein the burner adjusts the speed of a blower to allow combustion air adjustment for flue or vent blockage and/or various inlet flue or vent lengths.

10. The gas heater of claim 1 wherein the burner is capable of a 5:1 turndown.

11. The gas heater of claim 1 further comprising a blower.

12. The gas heater of claim 11 wherein the blower draws air into the heater.

13. The gas heater of claim 11 wherein the blower pushes air into the heater.

14. An apparatus for continuously varying the input of at least one radiant gas heater that responds to heat demand comprising:

a burner housing having (1) a combustion air and fuel inlet and (2) a burner assembly for mixing the fuel and air and conveying the mixture into a heat exchanger for combustion;

at least one conduit in conjunction with a blower that directs at least the energy formed from the combustion of the fuel and the air from the burner housing to a heat exchanger;

the energy formed from the combustion of the fuel and the air are vented from the heat exchanger;

a heat demand control device is positioned in the burner housing transmits a signal to a controller and based on the signal, the controller varies the input of the fuel and the air into the burner housing to satisfy the heat demand;

the controller varies the input of the fuel and the air into the burner housing by being able to simultaneously (1) alter the speed of the blower which controls the air input into the burner assembly and (2) modulate a fuel valve that controls the fuel input into the burner assembly.

15. The apparatus of claim 14 wherein the blower provides positive air pressure from the burner end of the heater.

16. The apparatus of claim 14 wherein the blower provides negative pressure from the exhaust end of the heater.

17. The apparatus of claim 14 further comprising (1) a pressure switch completes or breaks an electrical circuit due to differential pressure of air in the burner housing and (2) a pressure transducer that measures pressure within the burner housing.

18. The apparatus of claim 17 wherein if the values of the pressure switch and the pressure transducer meet a predetermined parameter, the controller allows an air purge period prior to ignition.

19. The apparatus of claim 17 wherein if the values of the pressure switch and/or the pressure transducer exceed or are below predetermined parameters, the controller will adjust or terminate the flow of air and/or fuel into the burner housing.

20. The apparatus of claim 17 wherein the energy generated by the fuel and the air is radiant and convective heat.