Cascading Plant

Abstract

An integrated refrigeration and air conditioning plant working in a cascade cycle with two different refrigerants for the low and high temperature circuits. The low temperature segment works with an ozone friendly synthetic refrigerant with a minimal amount of refrigerant charge, while the upper stage operates with another ozone friendly refrigerant with much lesser global warming potential than the one in the low stage. The upper and/or medium temperature circuit also services the air conditioning needs of the environment where the refrigerated equipment is located, such as the shop floor of a supermarket or a laboratory housing the cabinets that keep refrigerators.

Description

FIELD OF INVENTION

The invention relates to refrigeration and air conditioning systems, including such systems when used in a supermarket setting. It specifically addresses the lacuna in integrating seemingly different, but intricately related, domains of cooling requirements at various temperature levels. The objectives are sought to be achieved in a manner that is environmentally friendly, energy efficient and operated in an intelligent way that marries the thermodynamic fluid dynamic interface at two different refrigerants.

BACKGROUND TO THE PRESENT INVENTION

In this document the numbering of refrigerants is in accordance with the protocol recommended by the learned societies relevant to the profession of refrigeration and air conditioning, such as American Society of Refrigeration, Heating and Air conditioning Engineers (ASHRAE), International Institute of Refrigeration (IIR), Australian Institute of Refrigeration, Air Conditioning and Heating (AIRAH), Japan Society of Refrigerating and Air Conditioning Engineers (JSRAE).

Whilst the following discussion is in terms of a refrigeration and air conditioning systems in supermarkets, a person skilled in the art will appreciate that the invention may be applied for non supermarket applications. Any reference to refrigerant 404a in the context of low stage cascading is also deemed to include 507a, low boiling point natural refrigerants such as carbon dioxide, ammonia and hydrocarbon refrigerants and any possible refrigerants that might be discovered in future for such applications as well.

Any reference to refrigerant HFC 134a in the context of upper stage cascading is also deemed to include other refrigerants, such as 404a, 507a, 407c, 410a, HFO 1234yf, hydrocarbon refrigerants and any possible refrigerants that might be discovered in future for such applications.

The commitment of major supermarket owners to reduction of greenhouse gas emissions makes it imminent that they use “green” cooling technologies. Refrigeration contributes to about 70% of their energy consumption which is responsible for indirect emissions and virtually 100% contribution towards direct emissions resulting from leakages of refrigerants from the cooling systems.

The current Australian refrigeration and air conditioning trade practices are unable to reduce the leakages below 10% of initial charge per year. A supermarket has anywhere between 600-1500 kg refrigerant on site leading to at least 60-150 kg of gas leaking out each year. While this may not appear much, the amount of supermarkets Australia wide is 1500 and therefore the collective damage caused by their leveraging emissions is high.

HFC 134a, the most popular air conditioning refrigerant has a global warming potential of 1300 times that of CO2 and the low temperature refrigerant R404a has 3300 for this value. CO2, although could be used in this application more effectively, will involve extensive modifications to piping and disruption to trading. This application is hence suggested for operating super markets with least disruption to trade.

There does not seem to be an alternative to synthetic but ozone friendly, albeit, high global warming potential (GWP) refrigerants 404a and 507a for low temperatures (˜−30° C.). However, the options for medium temperature (˜−10° C.) refrigeration are much wider, 404a, 507a, 134a, 407c, 410a. Further, distribution of cooling load between low and medium temperature refrigeration is 1:4. This load management is intricately related to the conditions of the store ambience because the display cases are rated for a particular operating ambient, such as 25° C. and 60% RH, and the refrigeration compressors are chosen to cater to that load. The retailers endeavour to maintain conditions even better (23° C. and 50% RH) in order to reduce the load on the refrigeration system. This is achieved through an air conditioning system that operates independently of the refrigeration system. These air conditioning systems use HFC 134a as the refrigerant for either direct expansion or chilled water based cooling and dehumidification coils.

In order to fulfil the environmental obligations, supermarkets strive to minimise the greenhouse gas emissions within the framework of current technological practices and available skill levels of manpower without compromising on requirements of the cold chain nor comfort of shoppers.

To meet such demands, this invention in one embodiment describes a low temperature refrigeration system operated with 404a/507a/CO2 cascaded with 134a/404a/507a/407c medium temperature refrigeration and air conditioning system. Further, the direct expansion cooling and dehumidification coils in air conditioning are replaced with a chilled water system. This change is endowed with double benefits of substantial reduction in the amount of refrigerant held on site and preventing possible contamination of the supply air with the refrigerant in the event of leakage in the air handling unit.

The entire heat rejection process of the plant is centralised into one system of single or plurality of heat exchangers with water being the cooling medium. Even this so called waste heat of the plant is recovered for use in heating air in the air handling unit or for preheating process water. The net effect is a close control of the cooling capacity with least energy and minimal leakage of refrigerant.

Other Draw Backs of the Present State-of-the-Art

• • The current practice is to use a separate set of compressors that provide low temperature refrigeration which operate between evaporating and condensing temperatures of −30 to 45° C., which is a very large thermal boost that is fraught with very poor volumetric and isentropic efficiencies of the compressors, very high discharge temperatures that led to rapid degradation of the lubricating oil. Further, because of large differential between the suction and discharge pressures, the leakage losses are large.
  • Because of large thermal boosts, the available latent heat at the evaporator reduces to almost 50%. This is partially compensated by sub-cooling the liquid from 45 to about 10° C. using the evaporating refrigerant of the medium temperature refrigeration system which is actually at −10° C. Thermodynamically, this large temperature differential for heat transfer in a heat exchanger generates a lot of entropy which is undesirable.
  • The medium temperature refrigeration thermal boost is from about −10 to 45° C. In both the cases, the condensing temperature is high because of the use of air cooled condensers. Generally, the same refrigerant as in low temperature is used for medium temperature as well (eg: 404a and 507a).
  • The air conditioning thermal boost is from 5 to 45° C. The refrigerant is 134a or 407c.
  • Separate plant rooms for refrigeration and air conditioning compressors duplicate the control and electrical components. Further, independent controls do not allow fine tuning of matching interfaces resulting in high energy consumption for refrigeration.

SUMMARY OF THE PRESENT INVENTION

According to a first preferred embodiment of the invention, there is provided a climate system for an enclosure, the enclosure having at least three sub-enclosures, the system comprising:

• (a) a refrigeration plant for maintaining the sub-enclosures within predetermined temperature ranges including
  • (i) a low temperature circuit for cooling at least one sub-enclosure;
  • (ii) a medium temperature circuit for cooling at least one sub-enclosure; and
  • (iii) a high temperature circuit for cooling at least one sub-enclosure;
    • wherein the medium temperature circuit and/or high temperature circuit is in heat exchange with the low temperature circuit,
  • and
• (b) an air conditioning plant for maintaining the enclosure, external of any sub enclosure, within at least one predetermined temperature range including at least one non refrigerant fluid (eg. water) circuit in heat exchange with (i) the medium temperature circuit and/or high temperature circuit and (ii) an air circulation circuit for the enclosure.

Preferably, the refrigerant in the low temperature circuit is a high rated global warming refrigerant and the refrigerant in the medium temperature circuit and/or high temperature circuit is a low rated global warming refrigerant. Alternatively, the refrigerant in the low temperature circuit is CO2 or ammonia and the refrigerant in the medium temperature circuit and/or high temperature circuit is a low rated global warming refrigerant. In a further alternative, the refrigerant in the low temperature circuit is CO2 and the refrigerant in the medium temperature circuit and/or high temperature circuit is a high rated global warming refrigerant.

In a further preferred embodiment, the system comprises a first and a second non refrigerant fluid circuit, the first circuit for circulating chilled fluid for heat exchange with the air circulation circuit to cool air and the second circuit for circulating warm fluid for heat exchange with the air circulation circuit to warm air.

In another preferred embodiment, the system further comprising a control system for optimising the operation of the system. Typically the control system includes one or more sensors to sense perturbations in the operation of the system and one or more transmitters to generate signals to the low temperature circuit and/or medium temperature circuit and/or high temperature circuit in response to sensing said perturbations.

As indicated there are differing types of sub-enclosures including high, medium temperature and low temperature sub-enclosures. In the supermarket example these may be low temperature cabinets such as those in which frozen food is stored and accessed by a closable sealed door. In such an example there may also be medium temperature cabinets of the type typically use for displaying unfrozen meat and dairy products.

From the above examples, it will be understood that when the terms “enclosure” or “sub-enclosure” are used in this specification they include partially as well as completed enclosed configurations.

The following highlights some specific technical advantages which may be achieved by the integrated approach of the invention.

Preferably the low temperature circuit for serving the freezer and other low temperature cases is operated at a condensing temperature of 0 to 5° C. such that the thermal boost between evaporating and condensing temperatures is no more than 35° C. More preferably, the low temperature circuit is operated with refrigerants 404a. A corollary aspect of this small boost is a substantial increase in volumetric and isentropic efficiencies because of smaller pressure ratios across the compressors. Another aspect is that the liquid sub-cooler used in current practice is deleted.

Even more preferably, the fraction of latent heat available at the evaporator is substantially increased compared to current practice because of a reduction in the condensing temperature and pressure. A consequent benefit is a reduction in mass flow rate of the refrigerant in the lower stage of the cascade plant which inherently necessitates a smaller compressor. The net effect is that not only the compressor size is smaller, but also it works more efficiently resulting substantial reduction in indirect greenhouse gas emissions due to electricity consumption.

According to another preferred aspect, since the condensing temperature may be lowered, the maximum operating pressure in the lower stage of cascade cycle is also reduced. This causes smaller leakages of refrigerant because the leakage is proportional to the square root of the gauge pressure.

According to one more aspect of the invention, this high temperature circuit provides cooling for the chilled water system in addition to providing cascade cooling for the low temperature circuit. More preferably the service provided for water chilling circuit operates with a pressure regulator to a set evaporator pressure marginally higher than the saturation pressure of the refrigerant at the normal freezing point of water.

As per a further aspect preferably the high temperature circuit cascade refrigeration cycle rejects heat to a stream of water circulating in a closed cycle in a compact heat exchanger. More preferably the raise in temperature of the water stream is used for heating applications in the air conditioning system or for preheating of process water used in other services in the supermarket.

According to yet another aspect of this invention, preferably the dynamic response of the interfacing equipment between low temperature circuit and other circuits, for example the heat exchangers, is made responsive to transient conditions of operation through a judicious programming of the control system. More preferably, this control system accounts for, inter alia, accounting for the slopes and curvatures of the vapour pressure curves, specific volume and heats of vaporization of the two refrigerants and impedances of the hardware.

Preferably, the refrigeration plant for servicing the chilled water system is made integral with the low and medium temperature refrigeration systems. More preferably, the upper temperature circuit cascade system operates with a nearly fixed suction temperature matching the cascade condensation temperature, but operates with a higher evaporator pressure for the water chiller through settings of a single or plurality of evaporator pressure regulators, which can be mechanical or electronic.

DETAILED DESCRIPTION OF THE EMBODIMENTS

The invention will now be further illustrated with reference to the non limiting example depicted in the accompanying FIGURE. In this respect the following possibilities exist, all of which are of the inventions disclosed here.

1. The low and high stage of cascade refrigeration units are used with the same refrigerant 404a or 507a and combine the air conditioning service with the high stage.

This embodiment eliminates the condenser for the low temperature refrigeration rack. It addressed the concerns of some users who do not want to have two synthetic working fluids on site.

2. The high stage refrigerant in case 1 above is replaced with a low global warming potential and lower vapour pressure refrigerant such as HFC 134a. This option reduces the direct global warming potential even below that of the option 1 above. The high global warming potential refrigerant is contained within the low stage circuit which caters to much smaller load than the high stage segment.

3. The low stage refrigerant is substituted with a natural working fluid such as CO2 or ammonia and the high stage is operated with a low global warming potential refrigerant such as HFC 134a. The direct global warming potential of low stage refrigerant is virtually eliminated. The upper stage contribution to direct global warming is significantly reduced because of lower vapour pressure and lower global warming potential of that refrigerant used in it.

4. The low stage refrigerant is a natural working fluid such as CO2, but the high stage is operated with a high vapour pressure refrigerant such as 404a or 507a. Despite the high global warming potential of these high stage refrigerants, their specific volumes in their pressure range of operation are large and will result in smaller sizes of the hardware which could lead to economic benefits. A prospective user can reduce the direct global warming effect by improving system practices that can reduce leaks and service requirements.

As will be appreciated, with this approach it is possible to replacing all heat exchangers where one of the fluid streams is a refrigerant with the other liquid also being either a liquid, such as water, or evaporating liquid, such as another refrigerant. This enables reduction of the total refrigerant held in the system.

Other potential benefits are:

• • Minimising the log mean temperature differentials required for thermal energy transfers in all liquid to liquid or liquid to evaporating liquid heat exchangers to a range of 2-8° C. using compact heat exchangers instead of shell and tube heat exchangers.
  • It is possible for large air cooled heat exchangers involving air and the refrigerant to be eliminated, and consequently the total amount of refrigerant contained in the condenser may be drastically reduced.
  • It is possible to eliminate large air cooled condensers used in tandem with individual compressor racks for low and medium temperatures and air conditioning and substitute them with a single dry cooler for water which is used as a secondary cooling medium.
  • Energy consumption may be minimised by operating the low and high stages across smaller thermal boosts than the present practice and thereby indirect greenhouse gas emissions of the refrigeration plant in the first embodiment where the same refrigerant is used for low and high stage cascade.

Further description in the following disclosure refers to two embodiments which cover the first option above as the embodiment 1 (same refrigerant) and the remaining three options in an embodiment 2 (namely different refrigerants). All these options include combining the refrigeration and chilled water production for air conditioning services into one plant which is the embodiment 3.

In all the embodiments, the compressor racks (10, 100 and 150) operate on a vapour compression refrigeration cycle as described in the thermodynamic concepts. The present state-of-the-art is to use positive displacement compressors (such as reciprocating, rotary, scroll or screw type) and the present invention does not preclude the use of centrifugal compressors.

The low temperature segment (designated by 10 series of numbered components) operated in a conventional vapour compressor refrigeration cycle with R404a as the refrigerant and uses a single or plurality of positive displacement type of compression system (10). The discharge gas of this compressor is directly led through an oil separator (12) and then divided into two streams (13A and 13B) which are fed to the cascade condensers (200A and 200 B). The condensed streams (14A and 14B) are fed to a liquid receiver (15). The liquid receiver allows pump down of the refrigerant for servicing and provides the fluid impedance. This is necessary during fluctuating cooling load. Firstly, the impedance allows some damping of fluctuations in flow arising out of load variations, but also to allow augmentation of flow to the display cases (600 A to N). Secondly, it allows the balance point to be attained according to a first order approach towards step changes while the compressor (10) is responding to necessary speed variations. The condensing temperature will be about 2 to 5° C. warmer than the evaporating temperature of streams 109 and 159. Notionally, cascading evaporating temperature is −7° C. implying a condensation in the range of −5 to −2° C. The notional cascade evaporating temperature is chosen to meet the temperature at which the medium temperature display case (800A to N) evaporators are maintained. This notional temperature can vary between −10 to −5° C. without prejudice to any other temperature that can be adopted and all such variations are deemed to be inclusive in the claims. The liquid receiver 15 is the source of the refrigerant for the display cases 600A to N.

In applications involving a large number of low temperature direct expansion (DX) systems (such as in biological specimen preservation) with each system operating at a different evaporating temperature (for example between −35 to −20° C.), more than one low temperature compressor rack can be used.

The condensing load of the low stage segment is divided equally between two compressor banks 100 and 150. As per the first embodiment these racks are operated with 404a as the refrigerant. According to the second embodiment either or both may be operated with HFC 134a or any other similar fluid as the refrigerant. In the supermarket context rack 100 and 150 can operate with the same saturated suction temperature, for example between −10 and −5° C., or with dissimilar saturated suction temperatures, for example, rack 100 at −10° C. and rack 150 at −5° C. Necessarily, (T109 and T159)<(T14A and T14B) and this difference is governed by the manifestation of the flow rates of the HFC134a and 404a streams. Typically a minimum difference of 2° C. will be required and those well versed in the art would appreciate that operation at differences larger than about 8° C. would lead to entropy generation although there is no bar on such an operation.

FIG. 1 shows a schematic wherein rack 100 provides cooling for cascade heat exchanger 200A, the medium temperature display cases 800A to N and the water chiller 203. Rack 150 provides cooling for cascade heat exchanger 200B, the medium temperature cool rooms 208/702 and the water chiller 204. However, the possibility of splitting the load of the cases between the racks is not precluded from the claims. For example, either of the racks may be tuned to provide cooling for medium temperature display cases requiring refrigeration at different temperatures. For example, dairy cases may be operated with an evaporation temperature of 0° C. and meat cases with an evaporation temperature of −8° C.

Those versed with water chilling would appreciate that heat exchangers 203 and 204 may not be operated at below the freezing point of water. Evaporator pressure regulators 113 and 163 are inserted in the refrigerant return lines 112 and 162 to regulate the pressure of evaporating 134a in 203 and 204 to a value which is above the saturation temperature of 134a at the freezing point of water.

In this description equal sizing of cascade heat exchangers 200A and 200B and equal water chillers are described. However, unequal sizing of 200A and 200B and/or 203 and 204 is desirable. Unequal sizing may be advantageous if the air conditioning load and/or higher evaporation medium temperature load is so large compared to the other medium temperature refrigeration load that almost a dedicated rack is required to meet those demands. This will be economical because in general the water chilling load is borne by the rack that operates at the highest saturation suction temperature to minimise the entropy generation in chilling. In this case, temperatures T109 will not be equal to T159 and hence the log mean temperature differences (LMTD) of heat exchangers 200A and 200B will be different. The intelligent control system will take into account this dissimilar sizing.

Condensation of upper stage cascade cycle refrigerant (HFC 134a) occurs in another set heat exchangers (201 and 202) which are water cooled. Water used for this purpose is again in a closed circuit (designated by 400 series of components). The warm water produced in the process is used in the air handling unit of the air conditioning system (206). It may also be used for preheating air and/or any other fluid. The air handling unit has two coils, namely, one circulating chilled water (205) produced from 203 and 204 and another for warm water in 206 produced from 201 and 202. The dry cooler 207 only needs to reject that amount of heat that is not used heating air or process fluid. However, the dry cooler is sized as though no heat recovery has actually occurred because the operational flexibility does not make it imminent that heat recovery must be done.

Inter-Relationship Between Refrigeration and Air Conditioning

The operation of the refrigeration plant (which was meant for cooling at or below the freezing point of water (such as preservation of food) was intricately related to the envelope in which they are located. For example, in a supermarket, the display cases are expected to be located in an air conditioned environment. Nearly 70% cooling load on an open fronted medium temperature display cases (800 A to N) is due to infiltration of humid ambient air into the air curtain.

One way of managing the refrigeration load of the display cases is to manage the absolute humidity of the ambient air. A typical condition for which the display cases are rated (from which the refrigeration plant load is calculated) is a temperature of 25° C. and a relative humidity of 60% yielding an absolute humidity of 12 g/kg of dry air for a normal atmospheric pressure condition. The humidity level for a −5° C. air exit temperature at the coil will be about 2.5 g/kg of dry air. The cooling coil will frost up at the rate of about 9.5 g/kg of air handled by it in the display case. This is one of the reasons for perturbations (variations) in the load on the refrigeration plant. The numerical values used herein are only for the sake of an example and this invention is deemed to include other values that are relevant a particular condition of usage.

When the external ambient conditions change such as in summer, the refrigeration plant serving the low and medium temperature cases struggles to meet the load because of higher condensing temperatures and reduced volumetric efficiencies of the compressor bank (100 and 150).

One way to reduce the load on them is to lower the absolute humidity of the store condition, to say 9 g/kg of dry air, by achieving higher dehumidification in the cooling coil (205). This can be done by lowering the dew point temperature at the coil from about 10 to 5° C. This can be done easily because the HFC compressor racks (100 and 150) that service the chilled water from 203 and 204 to the cooling coil (205) in the air handling unit is the same as the one that services the cascade condensers 200A and 200B and are already operating at a suction temperature lower than that necessary for the reduction of dew point temperature. Thus, this embodiment allows actually reducing the load on the refrigeration plant during adverse operating conditions through an intelligent linking of operating conditions of air conditioning and refrigeration circuits.

The chilled water circuit (300 series components in FIG. 1) serves the cooling needs of the air conditioning plant. The chilled water is produced in two heat exchangers (203 and 204) as described above. The pump (301) in this circuit is run through a variable speed drive such that the amount of water in circulation can be altered to meet the cooling and dehumidification load. Two types of control are envisaged, namely, variable mass flow and variable temperature of water through the coil 205.

Most refrigeration plants operate under a variable head pressure (p13, p103 and p153). The regulation of p13 is achieved by intelligent operation of heat exchangers 200A and 200B though appropriation of flows and pressures of 100 and 150 series circuits. Regulation of p104 and p154 is done by yet another intelligent operation of heat exchangers 201 and 202. The control involves manipulation of flow rates generated by the pump 401. This pump is also of variable speed type. The heat acquired by water circuit is rejected to the ambient in the dry cooler 207. The dry cooler has a set of fans (700A and 700B) which are again on variable speed drive.

The way in which the head pressures are regulated through an intelligent management of speeds of pump 401, fans 700A and 700B is a part of the intellectual property claimed here.

Modifications and improvements to the invention will be readily apparent to those skilled in the art. Such modifications and improvements are intended to be within the scope of this invention.

The word ‘comprising’ and forms of the word ‘comprising’ as used in this description and in the claims does not limit the invention claimed to exclude any variants or additions.

In this specification, including the background section, where a document, act or item of knowledge is referred to or discussed, this reference or discussion is not an admission that the document, act or item of knowledge or any combination thereof was at the priority date, publicly available, known to the public, part of common general knowledge, or known to be relevant to an attempt to solve any problem with which this specification is concerned.

Claims

1. A climate system for an enclosure, the enclosure having at least three sub-enclosures, the system comprising:

(a) a refrigeration plant for maintaining the sub-enclosures within predetermined temperature ranges including, (i) a low temperature circuit for cooling at least one sub-enclosure; (ii) a medium temperature circuit for cooling at least one sub-enclosure; and (iii) a high temperature circuit for cooling at least one sub-enclosure; wherein the medium temperature circuit and/or high temperature circuit is in heat exchange with the low temperature circuit, and

(b) an air conditioning plant for maintaining the enclosure, external of any sub enclosure, within at least one predetermined temperature range including at least one non refrigerant fluid circuit in heat exchange with (i) the medium temperature circuit and/or high temperature circuit and (ii) an air circulation circuit for the enclosure.

2. The climate system of claim 1 wherein the refrigerant in the low temperature circuit is a high rated global warming refrigerant and the refrigerant in the medium temperature circuit and/or high temperature circuit is a low rated global warming refrigerant.

3. The climate system of claim 1 wherein the refrigerant in the low temperature circuit is CO2 or ammonia and the refrigerant in the medium temperature circuit and/or high temperature circuit is a low rated global warming refrigerant.

4. The climate system of claim 1 wherein the refrigerant in the low temperature circuit is CO2 and the refrigerant in the medium temperature circuit and/or high temperature circuit is a high rated global warming refrigerant.

5. The system of claim 1, comprising a first and a second non refrigerant fluid circuit, the first circuit for circulating chilled fluid for heat exchange with the air circulation circuit to cool air and the second circuit for circulating warm fluid for heat exchange with the air circulation circuit to warm air.

6. The system of claim 1 further comprising a control system for optimising the operation of the system.

7. The system claim 6 wherein the control system includes one or more sensors to sense perturbations in the operation of the system and one or more transmitters to generate signals to the low temperature circuit and/or medium temperature circuit and/or high temperature circuit in response to sensing said perturbations.