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Selecting a Liquid Cooling System

Glowing liquid cooled futuristic hyper fast computer CPU.

December 08, 2020

bient cooling systems, recirculating chillers, liquid-to-liquid cooling systems there are so many different cooling technologies that selecting one can be a daunting task. This simple step-by-step guide explains the different types of cooling systems available and highlights how to select the right product for your application.

Considerations to Picking a System for Your Application

The first consideration is whether you need precise temperature control or need to cool below ambient (air) temperature. If you can answer ‘no’ to both of these, you are looking for a cooling system that will simply remove bulk heat. The most cost effective solution is an ambient cooling system.

The biggest cost drivers for aluminum cold plates, after those mentioned above, are machining time and additional processing steps. Cold plate manufacturers typically have a cost associated with machining time which covers depreciation costs of the machine, power, supplies, and maintenance. Therefore, the longer the cold plate sits in the machine the more costly it is. Each additional processing step continues to drive the cost up.

Ambient Cooling Systems



An ambient cooling system is the simplest and most economical cooling system. It contains a heat exchanger, fan, pump and tank, in a compact package. The pump circulates the fluid to your system and back through the heat exchanger, and the fan blows ambient air across the heat exchanger to cool the liquid.

There is no temperature control circuit, so an ambient cooling system does not maintain a pre-set temperature. Since the ambient air provides the cooling, the ambient temperature is the lower limit for the fluid exit temperature.

Although they appear simple, ambient cooling systems are engineered for maximum capacity. The cooling system manufacturer’s thorough understanding of heat exchanger performance allows the liquid flow rate and air flow rate to be performance-matched to extract the most cooling capacity from the system. The plumbing is designed for high reliability and components are carefully selected to avoid any galvanic corrosion issues. An off-the-shelf ambient cooling system is extremely easy to use – simply connect the fluid inlet / outlet fittings to your equipment, fill the tank and turn it on!

But what if you need to control the temperature or cool below room temperature? Recirculating chillers and liquid-to-liquid cooling systems are both good alternatives.

Recirculating Chillers



Recirculating chillers offer precise temperature control (within 0.1°C) and cooling below ambient temperature. They are quiet, cover a wide range of cooling capacities, and are available with many different options and additional features. Recirculating chillers are compact, quiet and easy to install.

Recirculating chillers use refrigerant for cooling. They operate in a similar way to your refrigerator at home, except that they cool water instead of air. The process water circuit includes an evaporator, tank, and pump. The water is cooled by the refrigerant as it passes through the evaporator. On the other side of the evaporator the refrigerant evaporates to cool the water, then passes through a compressor and condenser, rejecting the heat to the ambient air.

When heat loads get high, chillers can cause the room’s air conditioning system to become overloaded as they reject the waste heat into the ambient environment. One option is to use a chiller with a liquid-cooled condenser. In this case, the refrigerant is cooled by facility-chilled water instead of air, making the chiller quieter and avoiding room-warming problems.

Another alternative for high heat loads is a liquid-to-liquid cooling system.

Liquid-to-liquid Cooling Systems (LCS)



A liquid-to-liquid cooling system cools below ambient temperature and offers similar temperature stability to a recirculating chiller. Instead of rejecting the waste heat to the room, it transfers it to chilled facility water via a liquid-to-liquid heat exchanger.

The process side circuit is completely isolated from the facility water, protecting your equipment from fluctuations in temperature, flow rate of the facility water and any contaminants that may be present. Since the facility water provides the cooling, the facility water temperature is the lower limit for fluid exit temperature.

Liquid-to-liquid cooling systems are popular for high heat load applications as they are compact – approximately 1/3 the size of a refrigerant-based chiller of similar capacity. Without a compressor, they are also very quiet and energy efficient.

Recirculating chiller or liquid-to-liquid cooling system?

For low heat loads, recirculating chillers are usually the simplest solution as installation is so easy. At high heat loads, liquid-to-liquid cooling systems are more cost effective. However, their use is restricted to situations where chilled facility water is available. The necessity to plumb them into facility water may affect the locations they can be used in and the portability of the equipment.

If you have high heat loads and need to reject the heat to facility water, the choice between an LCS and a recirculating chiller with a water-cooled condenser depends on your set-point temperature. If your set-point temperature is higher than your maximum facility water temperature, an LCS is more cost-effective. However, if you need to cool close to or below the facility water temperature, you will need a refrigerant based chiller with a water-cooled condenser.

Selecting a Modular Cooling System

Calculate Which System You Need



MCS performance is shown as Q/ITD versus flow rate. Q is heat load, and ITD is the initial temperature difference, or the difference between MCS liquid inlet temperature and ambient air temperature.

To select the correct MCS system, you first need to determine Q/ITD. Then, using the MCS performance graph, draw a horizontal line at the calculated Q/ITD value. Finally, check that the pump will provide sufficient flow rate.

Example:

A laser produces 700 W of waste heat. The water temperature exiting the laser should be less than 35°C. Ambient room temperature is 20°C. The laser equipment requires a flow rate of at least 1 gpm. Which MCS system should be selected?

First, determine Q/ITD Q/ITD = 700 W/(35°C-20°C) = 46.7 W/°C

Using the thermal performance graph, you can see that at flow rates above 0.5 gpm, MCS20 will provide adequate performance. The standard BB pump offers a flow rate of 1.3 gpm so it will work well. If you are considering an alternative pump, use the pump flow rate calculation to verify that with the given pressure drop, flow rate will be sufficient.

Selecting a Liquid-to-Liquid Cooling System

How to Calculate which Liquid-to-Liquid Cooling System is Right for Your Application

In most Liquid-to-Liquid Cooling applications, we know the temperature of facility water (TF), the desired process set-point temperature (TP), the flow rate through the process ( P) and the heat load of the process, Q.



To determine the required capacity, Q/ITD, we first need to calculate the change in temperature, ΔT, through the process. We can do this either by solving the heat capacity equation:



or by using the heat capacity graphs found in our Thermal Reference Guide.

Next, we calculate Q/ITD to find the required cooling capacity. Q is the process heat load. ITD, the initial temperature difference, is the difference in temperature between warm return water, (TP+ ΔT) and cold facility water (TF).



Finally, refer to the Liquid Cooling System (LCS) performance curves to determine the facility process flow rate required to achieve calculated Q/ITD.

Example Liquid-to-Liquid Cooling System Calculation

A solder reflow oven requires a process set point of 20 °C. The heat load is 10 kW and process water flow rate is 5 gpm. The facility water is at 10°C.

Using the heat capacity graphs, we find that the ΔT through the process is approximately 7.6°C for the condition 10 kW at 5 gpm.

We can now solve for Q/ITD as follows:



Referencing the LCS performance graph, we can see that a facility flow rate above 2 gpm will meet required performance.





Conclusions for Selecting a Liquid System

Ultimately, the required cooling capacity, temperature stability, set-point temperature, and availability of facility cooling water will dictate which system to use. For further assistance in choosing a cooling system, contact our thermal design engineers to discuss your specific application requirements. Based on inputs such as your heat load and required flow rate, it will even recommend an appropriate product.

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