Pumped single or two phase cooling is generally used to remove and dissipate heat from high-power heat sources such as electronics and lasers, or when the thermal energy must be transferred a significant distance between the heat source and the heat sink. Pumped single phase cooling is commonly used today in automotive systems and power electronics equipment, where the heat fluxes are relatively low and/or temperature uniformity is not required. In a pumped single phase loop, the liquid coolant is pumped through a cold plate which is attached to the heat source being cooled. The temperature of the liquid coolant increases as it passes through the cold plate, absorbing and storing the heat in its sensible heat capacity.
What Are Pumped Two Phase Cooling Systems?
In pumped two phase cooling systems, heat is transferred by the evaporation and condensation of a portion or all of the working fluid. Typically, a liquid near saturation is pumped into the cold plate, where it starts to boil, cooling the electronics and storing the energy in the latent heat of the fluid. The two phase (liquid and vapor) fluid then flows to the condenser, where the heat is removed, condensing the vapor, so that a single phase (liquid) exits the condenser, and the cycle repeats.
When properly designed, two phase pumped loops can do the following:
- Transfer heat over long distances
- Cool high heat flux electronics
- Accommodate & cool multiple cold plates in parallel
- Use quick disconnects to swap out electronics
- Heat can be applied and removed from any combination of cold plates, with passive flow control to each cold plate
- Operate in any orientation
- Cooling over large areas (ACT has demonstrated two phase cooling with multiple 1.8 ft2 (1700 cm2) cold plates)
Pumped two phase systems require additional design, since flow instabilities must be suppressed, and the system must accommodate both liquid and vapor flows. However, pumped two phase has the following benefits when compared with single phase cooling:
- Reduced Size, Weight and Power (SWaP), which is an important concern on aircraft and military vehicles
- Lower flow rates and pumping power
- Mini-channel heat exchangers instead of micro-channel heat exchangers, reducing pumping power and clogging issues
- Isothermal temperatures over large cold plates (±0.5°C has been demonstrated at ACT)
- Thermal management of multiple electronics cards that need to operate at the same temperature (±3°C has been demonstrated at ACT)
- When properly designed, turning some of the electronics off will not affect the temperature of the remaining cold plates
Two Phase Cooling System Layout
The basic layout of a pumped two phase cooling system is similar to a pumped single phase system, except that a two phase reservoir is used to accommodate changes in fluid volume, rather than the accumulator that is used in a single phase system. An example pumped two phase system is shown in Figure 1, where quick connects allow cold plates (and associated electronics) to be swapped out without the need to drain and recharge the system. Flexible lines allow the cold plates to be tested in any orientation, and at different elevations.
The cold plates (heat sinks) are the two evaporators in the front of Figure 1, with a transparent top plate. In the left evaporator, single phase flow enters from the top, a fraction of the liquid boils to remove the heat, and the two phase mixture exits at the bottom (note the location of the bubbles). For the right evaporator, the single phase flow enters from the bottom, with a two phase mixture exiting the top of the cold plate. ACT has demonstrated the ability of these systems to remove heat as the orientation of each evaporator is changed independently from the others.
Some electronics cooling applications have large numbers of parallel electronics boards, where it is desirable to apply electrical power and cool an arbitrary number of boards, without having to adjust the flow to each board. This is easily accommodated with a pumped two phase cooling system, where large numbers of cold plates can be cooled in parallel (series flow is not generally used when temperature uniformity is important, so that each cold plate has the same entrance conditions).
Figure 2 (A) shows the test set-up for four cold plates, each of which can be heated independently. The individual cold plates are marked with blue, orange, yellow and red stickers.
Note that the valves are used in this setup to provide a fixed pressure drop, adjusting them as flow conditions change is not necessary. Figure 2 (B) shows an individual cold plate in more detail.
Figure 3 (A) is a video demonstrating that power can be applied and removed to any of the cold plates during operation. When heater power is supplied to a given cold plate, a color coded dot appears next to the cold plate, and bubble formation can be seen. Figure 3 (B) shows the temperatures and flow rates for all four cold plates. When the electrical power to an individual cold plate is turned off, the temperature of that cold plate drops. However, as expected, the temperature of the powered cold plates is basically unaffected as the power changes.
Where to Learn More About Pumped Two Phase Cooling Systems
Further information on pumped two phase cooling is located on the following pages:
- Comparison of Pumped Single and Two Phase Loops
- Pumped Two Phase Mini-Channel Cooling for Electronics Thermal Management, which discusses pumped two phase cooling in more detail
- Mini-Channel Cold Plates for Two Phase Cooling of Electronics, which discusses some of the features of the ACT cold plate (heat sink) designs
Related technologies that ACT is working on include:
- Hybrid Two Phase Loops for high heat flux cooling, that separate the liquid and vapor streams using capillary forces and a wick.
- Vapor Compression Systems with Phase Change Material (PCM) Heat Exchangers, which use a PCM heat exchanger to reduce the size of the required vapor compression system when the cooling is intermittent.
- Liquid Jet Impingement for high heat flux cooling of small, movable hot spots.
- High Temperature Vapor Compression Systems which operate at higher than normal temperatures to dump electronics waste heat to high temperature fuel in aircraft cooling applications.
Read more about Pumped Two Phase Cooling:
- Comparison of Pumped Single and Two Phase Loops
- Pumped Two Phase Mini-Channel Cooling for Electronics Thermal Management
- Mini-Channel Cold Plates for Two Phase Cooling of Electronics
- Hybrid Two Phase Loop
Pumped Two Phase FAQs
Pumped Two Phase (P2P) Cooling, also known as Pumped Evaporative Cooling, is an active cooling technique that utilizes the latent energy associated with the boiling of a working fluid to efficiently remove high levels of waste heat. Compared with single phase cooling, P2P cooling enables:
- Lower Flow Rate
- Reduction in system size relative to many pumped single-phase solutions
- Reduction in Power Consumption
- Higher Heat Flux removal capability
- Increased isothermality across the heat source surface
- Practical use of dielectric working fluids, such as refrigerants
What is Pumped Two Phase (Pumped Evaporative) Cooling?
Pumped Two Phase (P2P) systems, also known as pumped evaporative cooling systems, use the same basic system level components as a pumped single-phase system. However, in P2P systems, the working fluid, typically a refrigerant, is allowed to boil as it passes through the cold plate and removes the heat from the hot surface of the device. More heat can be removed through the boiling process, otherwise known as latent heat, than through sensible heat with single phase cooling.
Boiling across the entire evaporator surface, offers a further advantage: the evaporator will have a very uniform surface temperature, typically within a few degrees. This near-isothermal performance is important for many applications such as laser devices, whose wavelength emissions are sensitive to temperature.
How Does Pumped Two Phase (Pumped Evaporative) Cooling Work?
A schematic of a pumped evaporative cooling apparatus is shown in Figure 1. Key components include the pump, one or more evaporators (cold plates), a back-pressure regulator to control the saturation pressure (and temperature), a condensing heat exchanger, and a reservoir (accumulator).
During operation, subcooled liquid is pumped from the accumulator to the cold plate(s). The single-phase liquid passes into the cold plate(s), where it boils to remove the heat supplied to the cold plate. The two phase flow from the evaporators travels through a back-pressure regulator, where it is condensed and then subcooled. The subcooled liquid flows into the reservoir (accumulator), and the cycle repeats. The reservoir (accumulator) accommodates the change in liquid volume during operation. The majority of the fluid in the P2P system is liquid when the evaporators are not heated. In this condition, the accumulator is mostly filled with vapor, as well as a small amount of liquid. Once boiling starts in the cold plates, the cold plates, condenser, and the line connecting them are mostly filled with vapor (on a volume basis). The accumulator fills with additional liquid, to accommodate the vapor in the remainder of the system.
Figure 2. Video showing heat transfer and boiling in a P2P cold plate with a transparent top plate. This cold plate has two parallel channels and was used to demonstrate flow control and stability features during heat load transients. Higher heat flux systems typically use multiple mini-channels, rather than dual flow channels.
When is Pumped Two Phase (Pumped Evaporative) Cooling Used?
Pumped two phase cooling is typically used when improved temperature uniformity, high heat flux cooling, or reduced pumping power versus single phase cooling is required. Pumped two phase (P2P) applications include:
- High Heat Flux Laser Diode Cooling
- Removal of high heat fluxes (>500 W/cm2) at low thermal resistances
- Cooling Parallel Boards
- Tight temperature requirements where isothermality is required across all boards
- Power Electronics Cabinet Cooling
- Dielectric refrigerants required for electrical isolation combined with need for large thermal power dissipation (10’s of kW’s)
What are the Benefits of Pumped Two Phase (Pumped Evaporative) Cooling Over Single Phase Cooling?
Three effects reduce the flow rates and pumping power for two phase systems versus single-phase systems: high latent heat vs. low specific heat, higher heat transfer coefficients, and reduced viscosity. These advantages reduce the coolant flow rates and pressure drop. As a result, smaller pumps requiring less power and weight can be used to remove higher amounts of heat, in effect, increasing the Coefficient of Performance, or C.O.P., of the cooling system. Pumped two phase (P2P) cooling has the following benefits over pumped single-phase cooling:
- Reduced Size, Weight and Power (SWaP), which is an important concern on aircraft and military vehicles
- Smaller, lower mass pump
- Lower Pumping Power
- Smaller pump and less energy consumption
- More Isothermal
- Higher heat flux capabilities
- Dielectric fluids
- No electrical shorts via system leaks
Does ACT Provide Complete Pumped Two Phase (Pumped Evaporative) Systems?
Yes. At ACT we handle all of the design and engineering and deliver complete, robust, turnkey Pumped Two Phase (P2P) solutions for military and commercial customers. The cold plates (evaporators) are always custom. For both military and commercial applications, ACT’s design philosophy is to use COTS (Commercial Off the Shelf) parts whenever feasible, to reduce costs. Our knowledgeable and experienced engineers work with our customers to identify their needs and then develop the complete system including: pumps, radiators, cold plates, fans, fluid lines, filters, and controls. Once the design is complete we will fabricate the entire system.
Many companies offer various components of complete thermal management systems, ACT does the entire system level design and analysis required to make sure all of the individual components work together to meet your specific requirements. We specify and integrate all of the components to assure flow rates, pressure drops, serviceability, structural and thermal requirements are all met in a robust long life P2P cooling system
Answering All of your Pumped Two Phase Cooling Common Questions
Now that you have the basics, we’re sure you have more complex questions. While some answers are specific to your needs and system requirements, these responses to standard questions will give you a better understanding as to how these devices operate:
What Materials are Used in a Pumped Two Phase (Pumped Evaporative) Cooling System?
A variety of materials can be used in a pumped two phase (P2P) cooling system. The main constraints are that the envelope must be compatible with the working fluid, and the envelope must be able to withstand the maximum saturation pressure. Refrigerant working fluids are compatible with aluminum, copper, steel, and stainless steel. Methanol can be used with steel, or stainless steel, but is not compatible with aluminum.
What Fluids are Used in a Pumped Two Phase (Pumped Evaporative) Cooling System?
The fluid is chosen based on the operating and storage temperatures ranges. Refrigerants, such as R134a, are the most common working fluids. Benefits include a low viscosity, low corrosion, and a low freezing temperature (-103°C R134a). Methanol (freezes at -97°C) has also been used in some pumped evaporative cooling systems. Water is typically not used, since it can damage the system when it freezes and expands. A “Figure of Merit” (boiling coefficient) for different refrigerants is shown in Figure 2. Refrigerant R134a has very favorable thermophysical properties, and is often used as the working fluid for many P2P applications.
What is the Operating Temperature Range of a Pumped Two Phase (Pumped Evaporative) Cooling System?
The operating temperature range, depends on the working fluid chosen. The minimum operating temperature is set by the sonic velocity. As the vapor temperature in the evaporator is lowered, the vapor pressure drops. To carry a given amount of heat, the vapor velocity must increase, which in turn increases the pressure drop from the evaporator to the condenser. At low vapor pressures, compressible flow effects become important. If the vapor velocity at the minimum operating temperature is too high, a different fluid should be selected.
One maximum operating temperature limit is the critical temperature. As the critical point is approached, the latent heat goes to zero, so the heat transported by phase change (evaporation and condensation) also goes to zero. More practically, the maximum operating temperature is generally set by the maximum allowable working pressure in the system.
What is the Storage Temperature Range of a Pumped Two Phase (Pumped Evaporative) Cooling System?
The storage temperature range of a pumped two phase (P2P) system depends on the working fluid selected. The minimum storage temperature is generally 10°C above the triple point, to avoid damage when the fluid freezes. There are three constraints on the maximum storage temperature range:
- Typically set at 80°C or above (U.S. Department of Transportation shipping requirement).
- System must withstand the saturation pressure (and a safety factor) at the maximum pressure.
- Accumulator must be sized so that there is a minimum of 15-percent vapor volume at the maximum temperature. As the temperature increases, the liquid density decreases, and the liquid volume increases. If the entire system is full of liquid, it is no longer saturated, and the extremely high pressure can damage (or burst) the system. To avoid this, the accumulator is sized so that there is some vapor at the highest storage temperature.
Will a Pumped Two Phase (Pumped Evaporative) Cooling Operate in any Orientation, and in Microgravity?
Yes. The evaporators (cold plates) can operate in all orientations in any properly designed pumped two phase (P2P) system. Horizontal, vertical flow up, and vertical flow down have all been demonstrated by ACT simultaneously with different cold plates in a pumped two phase system. Most ground-based P2P systems rely on gravity to separate the vapor and liquid in the accumulator. The hydrostatic head between the accumulator and pump provides Net Positive Suction Head (NPSH) at the pump inlet, preventing vapor lock. Ships can have a substantial side-to-side motion due to waves, but generally can rely on at least some hydrostatic head. However, military aircraft can have acceleration in any direction, while spacecraft operate in microgravity, with no hydrostatic head. With proper design, P2P cooling systems can operate on both aircraft and spacecraft.
How Remotely can Pumped Two Phase (Pumped Evaporative) Cooling Systems Reject Heat?
Pumped Two Phase Cooling is ideal for situations where the radiators must be located remotely. Many applications benefit from having the heat rejected remotely, so that the high power thermal loads coming off of their equipment do not burden the existing HVAC or house coolant systems. It is feasible to locate your radiator many tens of meters away from the heat source using a Pumped Evaporative system.
What is the Maximum Heat Flux That Can Be Cooled with a Pumped Two Phase (Pumped Evaporative) Cooling System?
The maximum heat flux depends on the working fluid, heated area, and any boiling enhancements in the channels. ACT has cooled 300-500 W/cm2 from the simple mini-channel evaporator shown in Figure 2. The Critical Heat Flux (CHF) is significantly increased with a simple sintered porous copper coating; see Figure 3.
How Many Cold Plates (Evaporators) Can I Use in a Pumped Two Phase (Pumped Evaporative) Cooling System?
There is no set limit to the number of cold plates that can be used. Some designs have dozens of individual cold plates. Note that the vertical distance between the top and bottom cold plate can affect the isothermality, since the bottom plate will have a higher saturation pressure (and saturation temperature) due to the hydrostatic head.
Can I Turn Evaporators (Cold Plates) On and Off During Operation of a Pumped Two Phase (Pumped Evaporative) Cooling System?
With proper design, heat can be switched on and off to some cold plates, while the temperature and cooling in the remaining cold plates are unaffected; see the video in Figure 6a. In this video four parallel evaporators are present and are somewhat randomly turned on an off. When a heat load is applied to an evaporator, a colored circle is shown and bubble generation resulting from boiling of the working fluid can be seen. Note that the outlet is at the top of the heat exchanger where the most vapor is seen, with the inlet at the bottom. When no heat is being applied, there is no colored circle and only liquid is present. The second heater from the right, with the yellow dot, is the one most frequently turned on/off. As shown in Figure 6b, the temperatures of the “on” cold plates were not affected by switching other plates on and off. This is accomplished by adding flow constrictors upstream of the individual cold plates, with ΔP’s significantly greater than the change in cold plate pressure drop for single versus two phase flow. The video at the top of the page includes video of the test.
Does All of the Liquid Need to be Vaporized in a Pumped Two Phase (Pumped Evaporative) Cooling System?
No. The best heat transfer and temperature uniformity over a cold plate are achieved with some liquid exiting the evaporator. For refrigerants, vapor qualities below 50 percent vapor are recommended. At higher vapor qualities, the heat transfer coefficient drops, harming isothermality. Excess liquid at the heat exchanger exit also simplifies the controls, since there is no possibility of a dryout, which would cause high temperatures and potential damage. The pumping power required to pump the excess liquid around the loop is minimal.
Can I use Quick Disconnects in a Pumped Two Phase (Pumped Evaporative) Cooling System?
Yes. We have implemented solutions with quick disconnects to allow for the rapid removal or connection of cold plates from the primary loop. We have also used quick disconnects successfully with central distribution units (CDU’s) to attach and detach multiple cold plates in a common system.
What is the Typical Reliability of a Pumped Two Phase (Pumped Evaporative) Cooling System?
The reliability of a pumped two phase cooling system is most often limited by the reliability of the pump that is used to circulate fluid. The pump life depends on many application specific factors and the design of the pump itself. Please contact ACT’s experts so we can learn more about your application and better inform you about the design options which can meet your reliability goals.
Can I use Pumped Two Phase (Pumped Evaporative) Cooling System to cool high voltage components?
Yes. We have developed several solutions that offer high voltage isolation through the use of dielectric and electrically isolating fluids, hoses and fittings. In fact, even in the unlikely event of a leak, the working fluids will not damage high voltage components.
Can Pumped Two Phase (Pumped Evaporative) Cooling Systems be incorporated in rack level cooling systems?
Yes, we have developed cold plate and central distribution units (CDUs) for pumping. Typically, the CDUs take up 2U to 6U worth of space while the cold plates can be manufactured in 1U to 2U form factors. The CDUs are connected to the cold plates using quick disconnects.
I Have a Question Not Answered Here
If you still have questions about pumped two phase cooling, Advanced Cooling Technologies is happy to help. While some of these concepts are complex, we have experienced professionals standing by that will be able to answer any questions or clarify any issues you may have. Customer service is our top priority, so please don’t hesitate to contact us with your questions today!
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