WO2008111968A1

WO2008111968A1 - Prevention of refrigerant solidification

Info

Publication number: WO2008111968A1
Application number: PCT/US2007/063662
Authority: WO
Inventors: Alexander Lifson; Michael F. Taras
Original assignee: Carrier Corporation
Priority date: 2007-03-09
Filing date: 2007-03-09
Publication date: 2008-09-18
Also published as: HK1141579A1; CN101663546B; CN101663546A; EP2135017A4; US20100011787A1; EP2135017A1

Abstract

A refrigerant system may utilize CO2 as a refrigerant. Should the sensed operating conditions indicate that the refrigerant might be approaching a condition at which the refrigerant could solidify, corrective actions are taken to prevent refrigerant transformation to a solid thermodynamic state. In one embodiment, hot gas from a compressor discharge is bypassed to a location upstream of the evaporator. In another embodiment, the high-side pressure of a refrigerant system is adjusted. In yet another embodiment, an additional charge of refrigerant is delivered on demand into the refrigerant system. In still another embodiment, a defrost cycle is initiated on demand. These embodiments prevent the refrigerant from approaching the conditions at which it may solidify.

Description

PREVENTION OF REFRIGERANT SOLIDIFICATION

BACKGROUND OF THE INVENTION

This application relates to refrigerant systems, which utilize CO₂ as a refrigerant, and which take preventive steps to reduce the likelihood of the CO₂ refrigerant transforming to a solid thermodynamic state.

Generally, refrigerant systems are utilized to circulate a refrigerant throughout a refrigerant circuit to condition a secondary fluid to be delivered to an indoor environment. As one example, air conditioning systems circulate a refrigerant to condition air being delivered into a climate-controlled space or zone.

Over recent years, a heightened concern about global warming, as well as, in some cases, ozone depletion, caused by some of the most commonly used refrigerants, such as R22, R123, R134a, R410A and R404A, forced HVAC&R industry to search for alternative fluids and refrigerant system solutions. Therefore, much attention has been drawn to so-called natural refrigerants, such as R744 (CO₂), R718 (water) and R717 (ammonia). CO₂ is one of these promising natural refrigerants that has zero ozone depletion potential and extremely low (one) global warming potential. Thus, CO₂ is becoming more widely used as a replacement for the conventional HFC refrigerants. However, utilizing CO₂ as a refrigerant does raise challenges for the refrigerant system designer. One challenge raised by CO2 is that it can transform to a solid thermodynamic state at pressures which can be experienced in typical refrigerant system applications. The CO₂ refrigerant has a relatively high triple point. As an example, with a pressure of about 75.1 psia, which would correspond to a saturated temperature of -69.8 degrees Fahrenheit, the CO₂ refrigerant can solidify.

If the CO₂ refrigerant transforms to a solid thermodynamic state, the refrigerant system ceases to operate. The solid refrigerant could plug up the expansion device, the distributor and distributor tubes, the evaporator refrigerant heat exchange channels and associated refrigerant pipes. Among other undesired phenomena, it could also cause compressor damage. With the possibility that pressure in the refrigerant system could drop, on some occasions, below 75.1 psia, the potential for CO₂ solidification raises challenges for the refrigerant system designer. Such situations can occur, for example, if the refrigerant system loses substantial amount of charge, the expansion device has malfunctioned, the evaporator fan has ceased to operate properly, the evaporator heat exchanger got plugged, a low pressure sensor has malfunctioned, a low pressure switch failed, etc. or a combination of thereof.

SUMMARY OF THE INVENTION

In disclosed embodiments of this invention, various preventive steps are taken should the refrigerant system be approaching a situation wherein a CO₂ refrigerant can transition to a solid thermodynamic state. In one embodiment, a bypass line selectively bypasses hot compressed refrigerant gas upstream of an evaporator. This design concept will increase pressure and temperature in the evaporator, preventing the CO₂ refrigerant from transitioning to a solid thermodynamic state.

In another embodiment, in transcritical applications, the refrigerant system high- side pressure is reduced, should the conditions in the evaporator be approaching solidification conditions for the CO₂. By reducing the pressure on the discharge (high- pressure) refrigerant side, the refrigerant distribution throughout the system is affected, causing the evaporator pressure to change, preventing the solidification of the CO₂.

In another embodiment, a receiver may contain an additional CO₂ refrigerant charge, which can be selectively delivered into the refrigerant system to increase the evaporator pressure, when the refrigerant system operation is approaching a situation where the CO₂ refrigerant could solidify.

In yet another embodiment, should the conditions indicate that the refrigerant system is approaching a condition, which could cause solidification of the CO₂ refrigerant, a defrost operation at the evaporator is initiated, preventing the transformation of the CO₂ refrigerant to a solid thermodynamic state.

In using those techniques, the refrigerant system can still continue to operate without being shutdown, as would have been the case if the refrigerant system were stopped, for example, using a low-pressure switch, which would trip the refrigerant system if the suction pressure decreases below a certain specified pressure limit. These and other features of the present invention can be best understood from the following specification and drawings, the following of which is a brief description.

BRIEF DESCRIPTION OF THE DRAWINGS Figure 1 shows a first schematic of the present invention.

Figure 2 shows a second schematic of the present invention. Figure 3 shows a third schematic of the present invention. Figure 4 shows a fourth schematic of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Figure 1 shows a refrigerant system 20 having a compressor 22 delivering a compressed refrigerant through a heat exchanger 24. For operation above the critical point (supercritical operation), the heat exchanger 24 is normally called a gas cooler and, for operation below the critical point (subcritical operation), the heat exchanger 24 is normally called a condenser. From the heat exchanger 24, the refrigerant is delivered to an expansion device 26, and then to an evaporator 28. As shown, a pressure sensor 30 senses the evaporator pressure and transmits the sensed reading to a control 32. While the pressure sensor is shown at the evaporator, other appropriate locations, such as a suction line or a suction port of the compressor 22, can be utilized, and parameters other than pressure, such as refrigerant saturation suction temperature, may be sensed and deduced to the low-side refrigerant pressure.

In the present invention, should the control 32 sense that the refrigerant system could be approaching conditions at which the CO₂ refrigerant could solidify, the hot gas bypass line 34 will deliver hot refrigerant gas from the compressor discharge to a location upstream of the evaporator 28, by opening a refrigerant flow control device such as a valve 36. Potential locations for solidification include the evaporator 28 and the vicinity of the exit from the refrigerant system expansion device 26. In this manner, the low pressure conditions within the evaporator 28 will be avoided, and the CO₂ refrigerant will not solidify. Figure 2 shows another embodiment 40, wherein when operating conditions approaching the transformation conditions of the CO₂ refrigerant into a solid thermodynamic sate are sensed by the sensor 30, the control 32 reduces the high-side pressure for the refrigerant system 40, by controlling the opening of a variable (adjustable) orifice valve 232. When the opening of the valve 232 is strategically changed, the refrigerant is re-distributed throughout the refrigerant system such that the evaporator pressure is changed accordingly. Thus, the likelihood of the CO₂ refrigerant solidification is reduced.

Figure 3 shows another embodiment 50, wherein a receiver 52 contains additional refrigerant charge to be selectively delivered into the refrigerant system 50. Should the conditions sensed by the sensor 30 indicate that the refrigerant system is approaching a potentially problematic situation causing CO₂ refrigerant solidification, a flow control device such as a valve 54 is opened and additional refrigerant is delivered into the refrigerant system 50. In this manner, the pressure in the evaporator 28 is raised, and the solidification of the CO2 refrigerant is avoided.

Figure 4 shows yet another embodiment 60, wherein a defrost cycle is initiated by the control 32, should the conditions indicate the CO₂ refrigerant is approaching a solidification line. In the illustrated embodiment, a defrost coil 62 associated with the evaporator 28 may be actuated to raise the temperature and pressure within the evaporator 28.

It should be pointed out that many different compressor types could be used in this invention. For example, scroll, screw, rotary, or reciprocating compressors can be employed.

The refrigerant systems that utilize this invention may have various options and enhancement features, such as, for instance, tandem components, economizer branches, reheat circuits, intercooler heat exchangers, etc., and can be used in many different applications, including, but not limited to, air conditioning systems, heat pump systems, marine container units, refrigeration truck-trailer units, and supermarket refrigeration systems.

While preferred embodiments of this invention have been disclosed, a worker of ordinary skill in the art would recognize that certain modifications would come within the scope of this invention. For that reason the following claims should be studied to determine the true scope and content of this invention.

Claims

CLAIMSWhat is claimed is:

1. A refrigerant system comprising: a compressor for compressing a refrigerant and delivering it downstream to a heat rejection heat exchanger, refrigerant from said heat rejection heat exchanger passing through an expansion device and then through an evaporator, refrigerant from the evaporator returning to said compressor; and a control for said system taking a corrective action, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

2. The refrigerant system as set forth in Claim 1, wherein said control takes the corrective action to prevent system shutdown.

3. The refrigerant system as set forth in Claim 1, wherein said refrigerant system is charged with CO₂ refrigerant.

4. The refrigerant system as set forth in Claim 1, wherein said control takes the corrective action utilizing a sensor for sensing a condition at which said refrigerant could solidify.

5. The refrigerant system as set forth in Claim 1, wherein said refrigerant can solidify in said evaporator.

6. The refrigerant system as set forth in Claim 1, wherein said condition is a pressure of the refrigerant.

7. The refrigerant system as set forth in Claim 6, wherein said pressure is taken at a location associated with said evaporator.

8. The refrigerant system as set forth in Claim 1, wherein said condition is a temperature of the refrigerant.

9. The refrigerant system as set forth in Claim 8, wherein said temperature is taken at a location associated with said evaporator.

10. The refrigerant system as set forth in Claim 1, wherein a hot gas bypass line is positioned to take at least a portion of refrigerant compressed by said compressor and deliver this portion of refrigerant directly to said evaporator, and said control operating a valve on said hot gas bypass line to expand this portion of refrigerant to a lower pressure, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

11. The refrigerant system as set forth in Claim 10, wherein said portion of refrigerant is delivered to a location upstream of the evaporator.

12. The refrigerant system as set forth in Claim 1, wherein said control changes a high- side pressure of the refrigerant system, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

13. The refrigerant system as set forth in Claim 12, wherein said high-side pressure is changed by controlling a valve opening.

14. The refrigerant system as set forth in Claim 1, wherein said refrigerant system further includes a receiver for storing an additional charge of refrigerant, and a valve on a line communicating said receiver into the refrigerant system, said control opening said valve to deliver additional refrigerant into the refrigerant system, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

15. The refrigerant system as set forth in Claim 1, wherein a defrost cycle is associated with said evaporator, and said control actuating said defrost cycle, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

16. The refrigerant system as set forth in Claim 15, wherein a defrost coil is associated with said evaporator to provide said defrost cycle, and wherein said defrost coil being actuated by the control.

17. A method of operating a refrigerant system comprising the steps of: providing a compressor for compressing a refrigerant and delivering it downstream to a heat rejection heat exchanger, refrigerant from said heat rejection heat exchanger passing through an expansion device and then through an evaporator, refrigerant from the evaporator returning to said compressor; and a control for said system taking a corrective action, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

18. The method as set forth in Claim 17, wherein said control takes the corrective action to prevent system shutdown.

19. The method as set forth in Claim 17, wherein said refrigerant system is charged with CO₂ refrigerant.

20. The method as set forth in Claim 17, wherein said control takes the corrective action utilizing a sensor for sensing a condition at which said refrigerant could solidify.

21. The method as set forth in Claim 17, wherein said refrigerant can solidify in said evaporator.

22. The method as set forth in Claim 17, wherein said condition is a pressure of the refrigerant.

23. The method as set forth in Claim 22, wherein said pressure is taken at a location associated with said evaporator.

24. The method as set forth in Claim 17, wherein said condition is a temperature of the refrigerant.

25. The method as set forth in Claim 24, wherein said temperature is taken at a location associated with said evaporator.

26. The method as set forth in Claim 17, wherein a hot gas bypass line is positioned to take at least a portion of refrigerant compressed by said compressor and deliver this portion of refrigerant directly to said evaporator, and said control operating a valve on said hot gas bypass line to expand this portion of refrigerant to a lower pressure, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

27. The method as set forth in Claim 26, wherein said portion of refrigerant is delivered to a location upstream of the evaporator.

28. The method as set forth in Claim 17, wherein said control changes a high-side pressure of the refrigerant system, if said refrigerant system is approaching a condition at which said refrigerant may solidify.

29. The method as set forth in Claim 28, wherein said high-side pressure is changed by controlling a valve opening.

30. The method as set forth in Claim 17, wherein said refrigerant system further includes a receiver for storing an additional charge of refrigerant, and a valve on a line communicating said receiver into the refrigerant system, said control opening said valve to deliver additional refrigerant into the refrigerant system, if the refrigerant system is approaching a condition at which said refrigerant may solidify.

31. The method as set forth in Claim 17, wherein a defrost cycle is associated with said evaporator, and said control actuating said defrost cycle if the refrigerant is approaching a condition at which said refrigerant may solidify.

32. The method as set forth in Claim 31, wherein a defrost coil is associated with said evaporator to provide said defrost cycle, and wherein said defrost coil being actuated by the control.