Heat-activated pump with integrated evaporator for electronic chip heat removal

Description

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application is a divisional of prior U.S. application Ser. No. 17/471,618, filed Sep. 10, 2021, which claims priority under 35 U.S.C. 119(e) from U.S. Provisional Application 63/076,546, filed Sep. 10, 2020.

TECHNICAL FIELD

The present invention relates to fluid-operated heat activated pumping devices for the transport of heat, and especially those adapted for the removal of the waste heat generated by electronic devices, at the chip, board, rack, or datacenter level.

BACKGROUND ART

Electronics, microelectronics, and opto-electronic devices convert most of the power they consume into waste heat. The devices need to be maintained within a narrow operating temperature to ensure optimal performance and long-term reliability. If they run too hot, they will degrade faster. If they are cooled excessively, the amount of cooling infrastructure required can be prohibitively expensive.

Electronics cooling can be classified by the amount of heat being dissipated. For instance, data center level cooling is performed on the order of tens to thousands of kilowatts. Printed Circuit Board (PCBA) level cooling is on the order of tens to thousands of watts. Chip level cooling can be order of milliwatts to tens or hundreds of watts. Different cooling solutions are deployed at each level depending on the amount of heat dissipated and the geometric space available.

Going deeper from the PCBA to the silicon chips, the closer the cooling solutions get to the heat source itself, the more efficient they can be. At the chip level, electronics cooling is facing several challenges:

• • 1. With exponentially growing computation needs, the number of transistors in a chip has been doubling every 18 months. This increases the power density of the chips that need to be cooled.
  • 2. With each new silicon technology, as transistors are packed closer together (for faster performance), there is increasing “leakage” of current across transistors: parasitic losses. To compensate for these parasitic losses, the devices require even more power to run, which results in even higher heat dissipation.
  • 3. In addition, with each new silicon technology, since the transistor feature size is reduced, the tolerance to prolonged high temperature operation is reduced. This means that advanced logic chips cannot operate at high temperatures (>105° C.-250° C.) for long periods of time without failing prematurely.
  • 4. Logic chips need to be placed as close as possible to each other and to memory and optical devices, to reduce delay and operate at faster speed. However, memory and optical devices are particularly sensitive to temperature and cannot operate at very high temperature. Some memory devices are limited to less than 95° C. operating temperature whereas some optical devices are limited to 70° C. operating temperature. Since they are placed so close to each other, cross-heating from a logic chip to a memory chip needs to be minimized to avoid overheating the memory and/or optical devices.

Conventional chip level cooling solutions vary depending on the amount of heat to be dissipated. Pumped direct liquid cooling through chip microchannels, two phase jet impingement cooling or immersion-cooling are used in applications where the heat dissipation is on the order of hundreds to thousands of watts. These solutions rely on connectivity to the larger system to enable the circulation of fluid in and out of the chip and off-chip fluid cooling.

Low wattage devices (on the order of 0.1 milliwatts to tens or a few hundreds of watts) require specific cooling solutions especially if they are in a constrained space. Smartphones are becoming more sophisticated and power hungry. Smartphone capability is limited by power consumption and heat loss. More computation capability means more power consumption, which means more heat generation and larger battery size.

Heat from smartphones is dissipated by conductive heat transfer from the transistors through the phone body to the surrounding atmosphere, human skin etc. Due to space limitations, there are no active cooling devices like fans or pumps to remove the heat faster. In some advanced smartphones, miniature heat pipes/vapor chambers are used to speed up the heat transfer from the transistors to the smartphone chassis. Heat pipe fluid return capability is limited by the amount of pumping power the capillary wicks can provide. This capillary return capability is both space and material limited. A miniature wick structure can only pump so much fluid even in best case conditions. While heat pipes are somewhat more effective than thermal interface materials, there is still a need for better heat dissipation without consuming too much space. There is a need to use active cooling techniques (like pumped liquid cooling) to speed up the heat transfer without significantly changing the size, thickness, cost, or reliability of the smartphone device.

Wearable devices (fitness monitors, watches etc.) have the same, if not even more critical, challenges of power consumption, size, cost, and battery power as smartphones. With such limited space and close contact with human skin, there is even greater need for better cooling of electronics in a small space. A typical smartwatch is expected to last at least 15 hours with a 2200 mAh battery. Operating at 1.1V, that translates to 2.42 Wh. This energy is mostly converted into heat and dissipated from the battery chassis to ambient air and human skin. Human skin is very sensitive to high temperature and a typical chip in the smartwatch can run as high at 40° C. This limits the capability of the wearable device. Current efforts are focused on using advanced thermal interface materials (graphite etc.), heat pipes, vapor chambers etc. to enhance heat transfer. These techniques are passive solutions which have limited effectiveness for temperature control and heat transfer.

Due to limited power availability, there is also a need to recapture the heat lost so it can be used to power the device rather than being dissipated/wasted into the environment. Even a small amount of energy recapture can help prolong the operating time of the device or reduce the size or cost of the battery. Efforts have been made in the past to use thermoelectric generators that utilize the difference in temperature between the device and human skin to generate electricity. However, this approach has been shown to be insufficient to generate enough useful energy to justify the space consumed by the thermoelectric generator and the cost involved.

There is a need for a scalable solution that can be deployed at the system level (e.g., datacenter), PCBA and chip level to address several of these issues and be cost effective and reliable.

SUMMARY DISCLOSURE

A heat activated pump is provided for the removal of waste heat from an electronic chip. An evaporator integrated into the electronic chip package contains a working fluid and is directly coupled against the chip to receive external heat therefrom to convert the working fluid into vapor. A pressure-control valve is coupled to an exit port of the evaporator, maintains the working fluid in the evaporator at a set target pressure and allows vaporized working fluid to escape through the exit port whenever the target pressure is exceeded. A liquid-piston chamber coupled to the pressure-control valve receives the vaporized working fluid from the evaporator at the target pressure. The vaporized working fluid expands adiabatically and displaces liquid within the liquid-piston chamber, expelling it therefrom in a pumping stage of a thermodynamic cycle. A unidirectional pump-exit check valve is coupled to an exit port of the liquid-piston chamber to allow the displaced liquid to exit the liquid-piston chamber. A unidirectional liquid suction-entry check valve is coupled to a return port of the liquid-piston chamber. A heat exchanger with a heatsink is coupled to the pump-exit and suction-entry check valves to receive displaced liquid and allow it to return to the liquid-piston chamber in a suction stage of the thermodynamic cycle. The heatsink radiates heat away to a flow of cooler air. Finally, a unidirectional vapor-injector return check valve is coupled to both an exit port of the liquid-piston chamber and to an input port of the evaporator. Periodic pressure pulses from the liquid-piston chamber that temporarily exceed the pressure in the evaporator facilitate jets of condensed working fluid to return to the evaporator in successive brief spurts.

The solution presented with this heat-activated multiphase fluid-operated pump technique for electronic chip cooling provides all the benefits of liquid cooling without some of the challenges of conventional pumps, piping and connectors involved in liquid cooling:

• • 1. High power dissipation capability—the same as with active liquid cooling and even higher if a refrigerant based working fluid is used instead of water/glycol.
  • 2. Versatile—it can be used to cool any device and ensure controlled operating temperature.
  • 3. Flexibility in component placement on a board—because it can be incorporated in any geometry upfront in the design process.
  • 4. No electrical pumps are required. Thus, no additional electricity consumption to drive the pumps.
  • 5. Much less board space is lost to accommodate electrical pumps.
  • 6. High reliability—due to the absence of electrical pumps.
  • 7. Minimal risk of leaks—no more than that with heat pipes, vapor chambers or thermo-siphons, which are commonly used in all applications.
  • 8. Easy to replace in field operation—can be replaced just as heatsinks can be replaced.
  • 9. Much lower cost due to simpler design and less key working elements.
  • 10. Less heat lost to the atmosphere as some of the heat is used to drive the pumping. This is in strict contrast to electrical pumps which actually consume more electricity to cool the chips. For instance, if a typical 1000 W chip could require a 10-watt pump to be kept cool. If redundant pumps are used, 20 watts. With heat activated pump of the present invention, the 20 watts of electrical pumping power would not be required.

In terms of operating conditions, the present heat activated pump solution sits between the two extremes of active and passive cooling solutions. It uses vapor, not liquid, pressure to drive the pumping.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic plan view of a heat-driven fluid-operated pump in accord with the present invention for providing a heat activated pump for electronics cooling.

FIG. 2 is a schematic view of a datacenter cooling arrangement using fluid-operated heat activated pumps in accord with the present invention.

FIG. 3 is a perspective view of a heat activated pump as in FIG. 1 for cooling of individual electronic chip components.

FIG. 4 is a close-up of the evaporator portion of the heat activated pump of FIG. 3.

FIGS. 5 and 6 are respective exploded and assembled side sectional views of the heat activated pump body and pressure control valve assembly of the heat activated pump of FIG. 3.

FIGS. 7A-7C are respective perspective, top exterior, and side sectional views of the heat activated pump body of the heat activated pump of FIG. 3 with special focus upon the various pump valves of the piston-suction chamber.

FIGS. 8 and 9 are perspective views of two embodiments of heat activated pump cooling constructions for use with multi-chip circuit boards.

FIG. 10 is an exploded perspective view of a heat activated pump as in FIG. 1 for cooling of space-constrained electronics (as in a smartphone or wearable) FIG. 11 is a top view of the pressure control valve, piston chamber, and heat exchanger portion of the heat activated pump of FIG. 10.

FIG. 12 is a close-up view of piston chamber valves of FIG. 11.

DETAILED DESCRIPTION

Nomenclature

Working Fluid: The fluid whose change in phase is utilized to performing the pumping operation. The working fluid could be selected from a variety of fluid options: water, as well as commonly used or new refrigerants (e.g., R-130, R-245fa, R-407c, R-410a, R-454b, R-1234yf, etc.). In addition to performance and desired temperature range, safety (flammability and exposure limits) will be factors in the choice of working fluid.

Pumped Fluid: The fluid which needs to be pumped from one location to another. This could be the same as the working fluid or it could be a different fluid material altogether. If the pumped fluid is different, it would need to be immiscible with the working fluid. In that case, a separator 113 may be provided (as seen in FIG. 1) at the return port 110 of the liquid-piston chamber 104 to direct only the working fluid material, and not the pumped fluid material, back to the evaporator(s) 101.

Ambient Temperature: The temperature in the general environment around the region or device in focus.

The Heat-Activated Multiphase Fluid-Operated Pump

The HAMFOP pump utilizes heat to pump a fluid. The heat is used to convert a working fluid from liquid to vapor. The vapor is then used to displace the fluid that needs to be pumped.

The principle of operation is as follows, as referenced by the numbered elements in FIG. 1, assuming that both the working fluid and the pumped fluid are the same:

• • 1. Heat is applied to the hot chamber or evaporator 101, which contains a small quantity of the working fluid in liquid form. As it is heated, the working fluid is transformed into vapor 102 at a high pressure based on the amount of heat applied and temperature and sizing of the hot chamber 101.
  • 2. The pressurized vapor pushes against the base of the pressure control valve 103, which is counteracted by a deadweight or equivalent load applied on top of it. Once the pressure of the vapor 102 exceeds the deadweight load, the valve 103 opens and the vapor 102 enters a piston/suction chamber 104, which has a larger area. The larger area ensures that the force applied by the vapor 102 remains high enough to keep the valve 103 open until a desired amount of the vapor 102 exits the hot chamber 101. This operation is similar to the principle of operation of a typical safety valve.
  • 3. When the vapor 102 enters the piston/suction chamber 104, which already contains the fluid 114 that needs to be pumped, the high-pressure vapor 102 rapidly expands and displaces the pump fluid 114 from the entry side of the piston chamber 104 towards an exit side (on the right in FIG. 1).
  • 4. The rapid adiabatic expansion of the vapor 102 induces a periodic pressure pulse that flows through the pumped fluid 114 in the piston chamber 104. Since the pumped fluid 114 is incompressible, this temporarily raises the pressure in the piston chamber 104, while the pressure in the hot chamber 101 temporarily decreases due to the loss of the vapor 102. This creates a positive pressure differential between the piston chamber 104 and the hot chamber 101, which results in the opening of a return valve 111. Due to the pressure pulse generated by the rapid entry of the vapor 102 and the incompressibility of the pumped fluid 114, the pressure in the piston chamber 104 transiently rises to a value above the vapor entry pressure. The increased pressure in the piston chamber 104 causes the return valve 111 to open (acting like an injector). Some of the pumped working fluid 110 flows through the return valve 111 and enters via a replenishment passageway 112 back into the hot chamber 101 for re-heating. Once enough working fluid 110 has flowed into the hot chamber 101 to equilibrate the pressure, the return valve 111 closes. This operation is similar to the function that an injector performs in steam engines: high-pressure vapor is used to push a fluid into a high pressure and high temperature evaporator. In the optional case where the pumped fluid 114 displaced by the working fluid (originally vapor 102, but now again in liquid form after the adiabatic expansion) are different materials that are immiscible, a separator 113 will be provided at the return port 110 of the piston chamber 104 to separate the working fluid from the pump fluid (now both liquids). The construction of separators of immiscible liquids is known. The key is to only allow the working fluid material to return to the hot chamber or evaporator 101, keeping out the different pumped fluid material. If the working and pumped fluids are identical materials, then the separator 113 is not needed.
  • 5. Since the piston chamber 104 now has slightly less fluid, the working vapor 102 continues to expand adiabatically and push against the pumped fluid 114. As the pressure in the piston chamber 104 continues to remain high, the pump exit valve 109 opens and lets out the fluid 114 via a passageway 108 into the heat exchanger 107. This continues until the fluid 114 has been evacuated from the piston chamber 104 and the pressure in the piston chamber 104 reduces to a pressure close to that in the heat exchanger 107.
  • 6. Upon displacing the pumped fluid 114, the vapor 102 remaining in the piston chamber 104 expands into a larger volume, so it condenses, resulting in a reduction in pressure in the piston chamber 104, to a value below the pressure in the heat exchanger 107. As a result, the pump valve 109 closes, completing the pump portion of the cycle and beginning a suction phase. The suction valve 105 now opens due to the negative differential pressure between the heat exchanger 107 and the piston chamber 104, letting fresh working fluid 106 in. Due to the negative differential pressure, fluid 106 is drawn into the piston chamber 104 from the heat exchanger 107.
  • 7. While the pressure in the piston chamber 104 is below that of the heat exchanger 107, the pressure control valve 103 remains closed to ensure no new vapor 102 enters the piston chamber 104 until the piston chamber pressure returns to above the heat exchanger pressure, concluding the suction phase of the cycle.
  • 8. Once the fluid in the piston chamber 104 has been replenished and the pressure is raised to the pressure of the heat exchanger 107, the pressure control valve 103 re-opens, letting in vapor 102 to repeat the cycle.
  • 9. In some embodiments, an electricity generator module may generate an electrical current induced in winding coils around the piston chamber during the continuous cyclic operation of the pump.

In the case where the pumped fluid 114 (and 106) is composed of a different material from the working fluid 102 (and 110), and not merely a different liquid-vapor phase of the same material, a difference in density between the working and pumped fluids can be leveraged to separate them. The elements of such an embodiment are largely identical to structure and operation to those in FIG. 1, except for the addition of a separator, in which the pumped fluid and the working fluid are separated. The pumped fluid 114 and 106 being of lower density would float to the top while the working fluid 102 and 110 would sink to the bottom. Accurate tuning (balancing the precise quantity of working and pumped fluid) would need to be performed to ensure that the separator chamber never ends up without any working fluid at all. If that were to happen, then some of the pumped fluid could enter the hot chamber 101, thus interrupting the cycle.

Heat Activated Pump for Cooling of Large Systems such as racks of PCBAs, Datacenters and other such large electronic systems A heat activated pump system can be used to move fluid around a datacenter as illustrated in FIG. 2, in which multiple heat activated pump units are interconnected together.

Heat activated pumps on devices attached to PCBAs in a rack pump the cooling fluid around in piping running throughout the data center. The datacenter-level heat activated pump system uses this hot pumped fluid from each individual heat activated pump to pump the fluid to the heat exchanger and return the fluid back to the PCBA level devices at a cool temperature to close the circuit as shown in FIG. 2. Individual heat activated pumps 152 are mounted on circuit boards 151 positioned in racks 150. Details of the individual heat activated pumps 152 are as referenced in FIG. 3. Evaporator lines 154 from groups of chips, boards, and racks connect through one or more pressure control valves 155 to one or more associated piston-suction chambers 156. Groupings may encompass an entire datacenter or various subsets of a data center, each grouping having a heat exchanger 159. The piston-suction chamber(s) 156 have a corresponding working fluid return valve 157 coupled to working fluid return lines 153 leading back to the grouping of chips, boards, and racks. Each piston-suction chamber 156 also has a pump valve 161 coupled to a heat exchanger 159, with a suction return line 160 back to the chamber 156 via a suction valve 158. Each grouping operates as described above with reference to FIG. 1.

Heat Activated Pump for Cooling of Electronic Devices Mounted on PCBAs

As seen especially in FIG. 3, key heat activated pump elements for device cooling on PCBAs are:

This specific embodiment is for a chip and board horizontally oriented with the heat activated pump mounted on top of the chip. This design can easily be modified to operate with a chip and board vertically oriented as well. In situations where multiple chips are mounted close together (as in chiplets on a single substrate) and some of the chips have lower operating temperature requirements as compared to others (as in memories and logic devices); then their heat paths can be completely separated into two distinct pumps operating independently. The vapor pressure of each pump can be tuned to match the target operating temperature of each chip to be cooled. This way, cross heating is mitigated, and each chip operates close to its own target temperature.

Evaporator

The details of the evaporator are outlined in FIG. 4. It is comprised of three key elements:

• • 1. An enclosure 203 that holds the evaporator core 204 and facilitates the transfer of heat from an external heat source to the evaporator core 204. The region of the enclosure 203 exposed to the heat source needs to be designed to be very high thermal conductivity and low thermal resistance. The rest of the evaporator enclosure 204 is insulated to prevent heat loss.
  • 2. An evaporator core 204, in which the working fluid is converted from liquid to vapor or maintained in vapor form. The key design features of the core 204 are very high surface area to enable effective heat transfer to the liquid. Some embodiments of the core include but are not limited to:
    • a. Metal Foam: porous metallic structures designed to have very high surface area and high heat transfer.
    • b. Metal Mesh/Wool/Screen: high thermal conductivity metal mesh can be used as an effective core. Copper, Aluminum, or other metal mesh structures can be brazed or soldered to the evaporator enclosure to achieve the desired high heat transfer area.
    • c. Serpentine Trench: a metal trench can be machined or stamped into the enclosure to allow the working fluid to move from entry to exit. Using a single directional serpentine trench requires less of the fluid and ensures that most of the heat is used for phase change rather than simply heating the liquid. The trenches can be filled with metal mesh/wool to further enhance heat transfer.
    • d. Cold plates: microchannel or tubular cold plates can be used.
  • 3. Top cover plate 205: this is designed to keep the enclosure 204 sealed and allow exit of the vapor and entry of the liquid. The plate 205 could be welded, bolted, brazed, soldered etc. to the evaporator or be an integrated part of the evaporator body itself.
    Pressure Control Valve

The details of the pressure control valve are outlined in FIGS. 5 and 6. It is comprised of these key elements:

• • 1. A gasket 208 that seals the vapor in the evaporator 203, 204 and 205. In very precisely machined structures, the gasket 208 can be dispensed with, and pure solid-solid pressure could be relied upon.
  • 2. A valve shaft 216 that transmits the load from the counterweight 213 to the gasket 208 to keep the pressure below a set value as determined by the weight of the counterweight load 213.
  • 3. A larger second cavity in which the valve shaft 216 sits, which ensures that a force applied by the vapor is still high enough to keep the seal open as the vapor exits the evaporator into the piston.
  • 4. A diaphragm 210 that deflects when the vapor pressure exceeds the counterweight load 213. The diaphragm 210 also prevents the vapor from leaking out of the valve.
  • 5. A load seat 209 that applies a mechanical advantage: one end has a small area and the other has a much larger area to accommodate the larger load 213. The seat 209 also has a cavity in it to hold the load 213 in place.
  • 6. A load 213 that could be a dead-weight or a liquid that keep the valve closed at a set pressure. The load 213 could also be a spring-loaded structure to achieve the same end objective: an applied force.
  • 7. A load cover 214 that encases the dead weight load 213 and holds it all in place. A small hole can be made in the cover to ensure that the air around the dead-weight load 213 always remains constant (atmospheric).
    Piston Chamber

The details of the piston are outlined in FIGS. 7A-7C. It is comprised of these key elements:

• • 1. The encasing body 206 (seen in FIG. 3) that holds the chamber in place and houses the different valves 207a-207c. The body 206 needs to be insulated to prevent losses and pre-heat the incoming fluid to improve pumping output.
  • 2. A piston chamber 207d in which the vapor expands and pushes out the pumped fluid. A key benefit of a liquid piston is that infinite potential shapes can be used to make the design compact, space efficient and versatile.
  • 3. The chamber walls of the piston-suction chamber 207d need to be smooth and can be coated with fluid repelling coating to reduce friction losses. The curves in the structure need to be smooth to prevent localized sudden area increases which may cause the vapor to condense prematurely.
    Valves

The details of the valves 207a-207c are outlined in FIG. 7C. For compactness, the valves are embedded in the body 206 of the piston pump chamber. This is just one example of the implementation of the valves, they could be implemented in infinite possible geometries and commercially available check valves could also be used. While the image shows the valves only actuated with balls 207e, the balls could be augmented with O-rings, discs, or gaskets depending on the quality of the metrological finish. The valves are actuated by the differential pressure across them.

Heat Exchanger

The details of the heat exchanger and heatsink 211 are outlined in FIG. 3. In this embodiment, the heat exchanger and heatsink 211 are shown as bonded to each other horizontally. Alternatively, the heat exchanger and heatsink could also be in the form of a radiator/heat exchanger, placed remotely from the pump depending on the space available and the access to cooling air. The fluid channels in the heat exchanger could also be structured in infinite possible geometries similar to the piston-suction chamber 207d in FIGS. 7A-7C.

Multiple Chips on PCBA Implementation

The aforementioned implementations are for cooling only one individual electronics device on a PCBA 200. When multiple devices 217 are mounted on a PCBA 200, an example arrangement is shown in FIG. 8.

The fluid-operated heat activated pumps 218 of multiple devices can be joined together to a common heat exchanger 219, as seen in FIG. 9. This ensures that if any one device is running hotter than the others, the cooler device's thermal budget can be consumed by the device requiring more cooling. Thus, the overall thermal performance of the PCBA is optimized. If all the devices are running hotter than target, the heat activated pumps on all the devices will run faster (higher flowrate) while maintaining the devices at their target operating temperature.

The heat exchanger 219 can also be placed in front of the target device so the heat exchanger receives the coolest incoming air and the hot air it releases downstream can be directed at the device to ensure it runs hot.

The heat activated pumps on each device interconnected, convert part of the collective heat dissipated by the devices into useful work to pump the cooling fluid around the devices, to maintain them at a constant target temperature.

Heat Activated Pump for Cooling of Low Wattage Electronics (Smartphones, Wearables, Flat Panels, Etc.)

A fluid-operated heat activated pump for space-constrained devices (smartphones, wearables, etc.) is shown in FIGS. 10-12. Details of each constituent are:

In vertical space constrained applications, a deadweight load cannot be used, especially if the operating temperature is high. In such situations, the diaphragm itself could be stiffened to act as the load by selecting the appropriate thickness, material, and diameter. In situations where a diaphragm cannot be used, a spring or a liquid dead weight (like Gallium) can be used. The operating principle is Pascal's law: the pressure in a fluid is always the same and the pressure is a function of the height of the fluid.

Taller columns produce higher counterweight. In some situations, the counterweight load may need to be dynamically changed (increased or decreased) while the unit is operating. For instance: the ambient temperature temporarily increases beyond nominal values. A higher ambient temperature means a higher flowrate is required through the heat exchanger to transfer the same amount of heat. If the duty load on the chip is low, the temperature of the chip can be raised slightly, so the pump does more work and increases the flowrate through the heat exchanger. This increase in chip temperature can be accomplished by raising the deadweight value: by increasing the height of the liquid column. This can be achieved by using a bimetallic strip which contracts in diameter, thus raising the height of the liquid column.

In applications where the ambient temperature is too high (e.g., a smartphone left on a car dashboard or an electronic device in a higher than specified ambient environment), the pump will act as a “thermal switch” where heat from outside the device will not be conducted back into the device. It accomplishes this because the pump is designed to transfer heat from the evaporator to the heat exchanger. If the heat exchanger temperature exceeds the evaporator temperature, the pump will cease to function until the heat exchanger temperature drops below the evaporator temperature.