HK1138241A - Improved heating for a transport refrigeration unit operating in cold ambients - Google Patents

Description

Improved heating of transport refrigeration units operating in low temperature environments

Technical Field

[0001] The present invention relates generally to refrigeration systems and, more particularly, to transport refrigeration systems operating in low ambient conditions.

Background

[0002] For transporting goods that need to be kept refrigerated or frozen, vehicles (such as trucks, trailers, rail cars, or refrigerated cabinets) are each provided with a refrigeration system that interfaces with the cargo space to cool the cargo to a predetermined temperature. During periods when the vehicle is located in an area having relatively low ambient temperature conditions, the temperature within the cargo space may drop to an undesirably low temperature such that the cargo may be damaged. It is therefore necessary to provide heat to the interior cargo space to avoid a temperature drop to such levels.

[0003] One method that has been used to provide heat to the cargo tanks is to use refrigerant to compress the heat. However, in extremely low temperature environments, only little heat of compression can be generated because a large amount of heat is lost to the ambient atmosphere within the condenser and interconnecting piping. If the heat of compression is insufficient to overcome the lower ambient temperature conditions, damage to the cargo may occur.

[0004] The transport refrigeration unit typically includes a diesel engine for driving the system compressor.

[0005] Diesel engines typically have a liquid coolant system that includes a radiator that cools liquid through a liquid-to-air heat exchanger or radiator. In this way, heat from the engine is transferred to the ambient environment via the radiator. The radiator is typically placed close to the condenser, and a single fan passes cooling air through the condenser and then through the radiator, and then into the ambient environment.

Disclosure of Invention

[0006] Briefly, in accordance with one aspect of the present invention, during operation of a refrigeration system in very low ambient temperature conditions, a heating system is added to a conventional heating system in which waste heat from the engine radiator is used to increase the condensing pressure and temperature to thereby increase the amount of heat of compression and the amount of heat available to maintain the cargo temperature.

[0007] In accordance with another aspect of the invention, the fan, which normally operates to move cooling air first through the condenser coil and then through the radiator coil, is operated in reverse during the heating cycle to move air first over the radiator coil and then over the condenser coil to increase the heat of compression of the system.

[0008] In accordance with yet another aspect of the invention, in addition to the heat from the radiator coil, heat from the engine is also caused to flow over the condenser coil to thereby further increase the heat of compression of the system.

[0009] In the drawings described below, preferred embodiments are described; however, various other modifications and alternative constructions can be made without departing from the true spirit and scope of the invention.

Drawings

[0010] Fig. 1 is a schematic diagram of a transport refrigeration system operating in a cooling mode in accordance with the prior art.

[0011] Fig. 2 is a schematic diagram of a transport refrigeration system operating in a heating mode in accordance with the prior art.

[0012] FIG. 3 is a side view schematic illustrating the airflow through the system during a cooling mode in accordance with the present invention.

[0013] FIG. 4 is a side view schematic illustrating the flow of air through the system during a heating cycle in accordance with the present invention.

[0014] Fig. 5 is a side view of an alternative embodiment.

[0015] FIG. 6 is a schematic side view of airflow during a cooling mode in accordance with an alternative embodiment.

[0016] Fig. 7 is a schematic side view of an alternative heating pattern according to that shown in fig. 6.

[0017] Fig. 8 is a schematic view of another alternative embodiment of the present invention.

Detailed Description

[0018] Referring now to fig. 1, a conventional transport refrigeration system is illustrated and includes the major components of a compressor 11, a condenser 12, an expansion valve 13 and an evaporator 14, all connected in series flow relationship to operate as an evaporative compression refrigeration system in a normal manner.

[0019] The compressor 14 increases the pressure and temperature of the refrigerant and forces the refrigerant through the discharge check valve 16 into the condenser tubes. The condenser fan circulates ambient air over the outside of the condenser tubes. The tubes have fins designed to enhance heat transfer from the refrigerant gas to the air. This heat removal results in liquefaction of the refrigerant. The liquid refrigerant exits the condenser 12 and flows through a solenoid valve 17 (normally on) and then to an accumulator 18.

[0020] The accumulator 18 stores additional refrigerant necessary for low ambient operation and for heating and defrost modes of operation.

[0021] The refrigerant leaves the accumulator 18 and flows through a manual liquid line service valve 19 and then to a subcooler 21. Subcooler 21 occupies a portion of the main condensing coil surface and further dissipates heat to the passing air.

[0022] The refrigerant then flows through the filter-dryer 22 where the absorbent keeps the refrigerant clean and dry, and then to the electrically controlled liquid line solenoid valve 23, which when opened allows the liquid refrigerant to flow into the "liquid/suction" heat exchanger 24 where the liquid refrigerant further reduces in temperature by dissipating some of its heat to the suction gas. The liquid refrigerant then flows to an expansion valve 13, the expansion valve 13 preferably being an external balance temperature thermostatic expansion valve, the expansion valve 13 reducing the pressure of the liquid refrigerant and metering the flow of liquid refrigerant to the evaporator 14 to maximize the use of the heat exchange surface of the evaporator 14.

[0023] The drop in refrigerant pressure caused by the expansion valve is accompanied by a drop in temperature such that the low pressure, cryogenic fluid flowing into the evaporator tubes is cooler than the air circulated through the evaporator tubes by the evaporator fan. The evaporator tubes have aluminum fins to increase heat transfer; thus, the heat of the air circulating through the evaporator tubes is removed. This cool air is circulated within the tank to maintain the cargo at a desired temperature.

[0024] The heat transfer from the air to the cryogenic liquid refrigerant causes the liquid refrigerant to evaporate. The low temperature, low pressure vapor passes through a "suction line/liquid line" heat exchanger 24 where it absorbs more heat from the high pressure/high temperature liquid and is then returned to the compressor 11 through a suction modulation valve 26. The suction modulation valve 26 controls the compressor suction pressure to match the compressor capacity to the load.

[0025] While the primary concern of a transport refrigeration system is in the operational cooling mode, it should be recognized that in certain seasons and locations, the ambient temperature may be below the desired temperature of the interior range of the cabinet. Therefore, it is necessary to provide heat to the box during these periods to avoid exposing the cargo to temperatures below the desired temperature. In addition, when operating in the cooling mode, frost often forms on the evaporator coils and needs to be removed in order for the system to continue to operate effectively. This is done by a defrost process. Both heating and defrosting are typically accomplished by using the "heat of compression" of the system. That is, when the vapor is compressed to a high pressure and a high temperature in the compressor 11, mechanical energy necessary to operate the compressor 11 is transferred to the gas being compressed. This heat is referred to as "heat of compression" and is used as a heat source during the heating cycle.

[0026] Referring to fig. 2, when the unit controller requests heating, the hot gas solenoid valve 27 is opened and the condenser pressure control solenoid valve 17 is closed. The condenser coil 12 is then filled with refrigerant and the hot gas from the compressor 11 enters the evaporator 17. The liquid line solenoid valve 23 will remain energized (valve open) until the compressor discharge pressure rises to a predetermined set point in the microprocessor. The microprocessor de-energizes the liquid line solenoid valve 23 which closes 23 to stop the flow of refrigerant to the expansion valve 13. When additional heating capacity is required, the microprocessor opens the liquid line solenoid valve 23 to allow additional refrigerant to be metered into the hot gas cycle through the expansion valve 13.

[0027] The hot gas bypass line 28 functions to increase the receiver pressure at low ambient temperatures (below-17.8 c/0F) to allow refrigerant to flow from the receiver 18 to the evaporator 14 when desired.

[0028] The applicant has realised that in a low temperature environment only a very small amount of heat of compression can be generated, which is insufficient to provide the heat necessary to maintain the desired temperature within the tank. It is therefore desirable to provide additional heat during these periods.

[0029] The compressor 14 is conventionally driven by an internal combustion engine, preferably a diesel engine. The engine requires some kind of cooling method to avoid excessive internal temperatures. This is typically accomplished by a radiator, with liquid coolant passing through the engine and the radiator, which is exposed to air flowing therethrough for cooling the coolant.

[0030] Referring now to fig. 3, the arrangement of the engine 29 and the radiator 31 in fluid connection therewith is shown relative to the condenser coil 12 and the evaporator coil 14. It will be seen that the radiator coil 13 is disposed directly behind the condenser coil 12 so that cooling air is caused to pass through the condenser coil 12 and then through the radiator 31 when the motor 33 drives the condenser fan 32. As shown, a portion of this air then passes over the engine 29, and a portion flows out of the opening 34 to the environment. A damper 36 is provided which may be used in the following manner.

[0031] To increase the amount of compression heat during low ambient conditions, the present invention contemplates using the heat rejected by the engine radiator 31 to provide an additional source of heat for this purpose. This is done in the manner shown in fig. 4.

[0032] Here, as shown, the motor direction is reversed such that the fan 32 causes air to flow in the opposite direction. That is, ambient air is caused to flow in from the opening 34 and then through the radiator 31 before flowing through the condenser coil 12, such that this warmer air that is recirculated into the condenser inlet air stream is used to increase the condensing pressure and condensing temperature. Higher pressures cause the compressor 11 to generate more heat of compression and therefore more heat to maintain cargo temperature.

[0033] In addition to waste heat from the radiator, the relative positions of the components shown in figure 4 also allow the fan 32 to draw in heat from the engine 29 and then pass it to the radiator 31 and the condenser coil 12 to further improve the thermal performance of the system.

[0034] While the damper 36 is shown in an open position in fig. 3 and 4, the damper 36 may be moved to a closed position for introducing heated air that has circulated through the hot engine into the radiator and condenser to further increase the condensing temperature and condensing pressure, as opposed to drawing cooler air from the outside environment.

[0035] Fig. 5 shows an alternative embodiment in which only a minimum depth of the unit is available due to assembly limitations. Accordingly, the evaporator section 37 includes a dedicated fan 38 and drive motor 39 to circulate air through the evaporator coil 41. The condenser fan is not positioned in the middle of the space 42 but at the lower end of the space so that the electric motor 43 is positioned in the space 42 and the fan 44 is positioned between the space 42 and the space occupied by the engine compartment comprising the engine, the generator and the compressor, which part is shown at 30 on the figure.

[0036] In operation of the heating process, the fan is operated in a direction such that hot air from the engine room flows into the space 42 and through the radiator 31 and the condenser 12 to increase the condensing pressure in the manner described above. In the cooling mode, the fan 44 is operated in the opposite direction so that air flows first through the condenser 12, radiator 31 and space 42 and then through the engine compartment.

[0037] Referring now to fig. 6 and 7, an alternative embodiment is illustrated which includes a plurality of louvers (shetters) 46 and dampers 47 as shown. During cooling mode operation, fan motor 33 drives fan 32 in a direction that draws air through condenser 12 and radiator 31, and louvers 46 open to allow air to pass through them and then through condenser 12 and radiator 31. As shown, the damper 47 is in a closed position.

[0038] During heating mode operation, as shown, shutter 46 is closed and damper 47 is open. As shown, the fan motor 33 rotates the fan 32 in a blowing direction such that air passes through the radiator 31, then through the condenser 12, and then out through the opening of the damper 47. These additional dampers serve to block air from entering the condenser and the radiator from an undesirable direction, which is opposite to the airflow path described above, when the refrigeration unit is in transit.

[0039] Fig. 8 shows another alternative in which the fan 32 is driven by a belt 48 and is unidirectional. It is therefore necessary to provide other ways of reversing the flow direction when transitioning from the cooling mode to the heating mode. To achieve this, an air recirculation passage 49 is provided at one end of the unit, as shown. A door 51 is also provided which is open during the heating mode (as shown in solid lines) and closed during the cooling mode (as shown in dashed lines). Thus, during cooling mode operation, air flows from the fan into the air circulation passage 49 and then passes through the condenser coil 12 and the radiator 31 with the louvers 46 in the closed position.

[0040] During cooling mode operation, the door 51 is in the closed position and the shutter 46 is in the open position such that air passes through the condenser coil 12, then through the radiator 31, and then out the open shutter 51. In the heating mode, the fan direction cannot be reversed due to the drive of the belt, so air is directed into the passage 49 and recirculated to the condenser.

Claims (16)

1. A transport refrigeration system of the type having an engine-driven compressor and having a fan, a condenser coil and a radiator coil in serial flow relationship such that during cooling mode operation said fan causes air to flow through said condenser coil and then through said radiator coil, comprising: means for reversing the air flow during operation in a heating mode such that the air flows through the radiator coil before flowing through the condenser coil, thereby increasing the temperature and pressure of the condenser.

2. A transport refrigeration system as set forth in claim 1 wherein said flow reversing device includes means for reversing the direction of said fan.

3. The transport refrigeration system of claim 1, wherein: the engine is disposed proximate the radiator coil, and further the flow reversing device causes the air to flow such that heat from the engine passes through the radiator coil before passing through the condenser coil.

4. A transport refrigeration system as set forth in claim 1 including at least one louver disposed proximate said condenser coil, said at least one louver being adapted to be open during cooling mode operation and closed during heating mode operation.

5. A transport refrigeration system as set forth in claim 4 and further including a damper disposed adjacent said condenser coil and adapted to be closed during cooling mode operation and open during heating mode operation.

6. The transport refrigeration system of claim 1, wherein: the flow reversing device includes an air recirculation passage and an associated door that is open during cooling mode operation, the air being caused to flow from the fan into the air recirculation passage, then through the condenser coil and then through the radiator coil; during operation in a heating mode, the door is closed and air from the fan passes through the radiator before passing through the condenser coil.

7. A method of increasing the heating capacity of a transport refrigeration system of the type having an engine-driven compressor and having a fan, a condenser coil and a radiator coil in serial flow relationship such that during cooling mode operation the fan causes air to flow through the condenser coil and then through the radiator coil, under low ambient temperature conditions, said method comprising the steps of: reversing the air flow such that the air is caused to flow through the radiator coil before flowing through the condenser coil, thereby increasing the temperature and pressure of the condenser coil.

8. The method of claim 7, wherein the reversing step is accomplished by a reversible fan.

9. The method of claim 7 including the further step of circulating air over said engine such that heat from said engine flows over said radiator coil and then over said condenser coil.

10. The method as set forth in claim 7, including the step of providing louvers adjacent said condenser coil, said louvers being adapted to open during cooling mode operation and close during heating mode operation.

11. The method as set forth in claim 10, including the step of providing a damper adjacent said condenser coil, said damper being adapted to be closed during cooling mode operation and open during heating mode operation.

12. The method of claim 7, wherein: said step of reversing said air flow is accomplished by an air recirculation passage and associated door that opens during cooling such that said air passes through said air recirculation passage and then through said condenser coil and said radiator; during operation in a heating mode with the door closed, the air passes through the radiator coil before passing through the condenser coil.

13. A heating apparatus for a transport refrigeration system of the type mounted on a cargo container and having a compressor, a condenser, an expansion device and an evaporator connected in series flow relationship, the evaporator selectively providing cooling or heating to the cargo container, the heating apparatus comprising:

a motor/generator set including an internal combustion engine for driving the generator which in turn provides electrical power to the refrigeration system, the internal combustion engine having a radiator for exchanging heat from coolant in the engine to ambient air flowing over the radiator; and

means for directing a flow of heated air from the heat sink to the inlet of the condenser to increase the temperature of air passing over the condenser to increase the condensing temperature and pressure.

14. The apparatus of claim 13, wherein the flow directing means comprises a reversible fan.

15. A method for increasing the heating capacity of a transport refrigeration system for cooling a container, the transport refrigeration system including a condenser and a compressor, the container being heated by heat of compression under low ambient conditions, the method comprising the steps of:

providing a motor/generator unit comprising a radiator and a liquid-cooled internal combustion engine, the radiator cooling the liquid by transferring heat to air passing over the radiator; and

directing a heated air stream from the heat sink to an inlet of the condenser to increase a condensing pressure and a condensing temperature, thereby increasing a refrigerant compression heat and a resulting heat provided to the container.

16. The method of claim 15 including the further step of directing a flow of heated air from the internal combustion engine over the radiator and over the condenser to increase the condensing pressure and temperature of the condenser.