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 to 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?
Figure 1. Pumped Two Phase Cooling Apparatus
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 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 Cooling System?
The fluid is chosen based on the operating and storage temperature ranges. Refrigerants, such as R134a are the most common working fluids. Benefits of these fluids 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.
Figure 3. Figure of Merit (Boiling Heat Transfer) of Working Fluids for Two-Phase Heat Transfer
What is the Operating Temperature Range of a Pumped Two-Phase 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 (P2P) 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 Can Remotely Placed Pumped Two-Phase 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 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.
Figure 6. (A) Heat fluxes of up to 250 W/cm2 were removed with this simple mini-channel evaporator with a boiling enhancement coating.
Figure 7. Adding a porous sintered powder coating increases the maximum heat flux. (A) Sintered copper coating. (B) Critical heat flux is higher for the porous-copper-coated mini-channel evaporator.
How Many Cold Plates (Evaporators) Can I Use in a Pumped Two-Phase 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 the Operation of a Pumped Two-Phase 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.
Figure 6. (A) Video showing two phase cooling of 4 cold plates, with intermittent power. A dot shows when the heat is applied, and vanishes when the heat is turned off. (B) Turning off heater power to some cold plates (Blue and Yellow) does not affect the temperature of the other cold plates.
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 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 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 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 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 (P2P) cooling, we’re 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!
