Recirculating chillers are refrigerated liquid cooling systems that are used in numerous industries including medical, military, laser, and analytical instrumentation. Chillers are used to keep a component such as laser head, detector panel, or other temperature sensitive device at a constant temperature and/or to remove waste heat and prevent overheating of critical components.
When selecting a recirculating chiller, several factors should be considered that could affect cooling capacity. These factors include ambient air temperature or facility water temperature, chiller set temperature, process fluid, chiller maintenance, and more. Chiller manufacturers generally provide cooling capacity ratings based on a 20°C water delivery temperature and a 20°C ambient temperature. However, what happens if the ambient temperature is higher or lower than 20°C? What if the coolant is supplied to the process at 5°C instead of at 20°C? What if a coolant other than water is used? How do all these variations affect the cooling capacity of a chiller?
In order to understand how these factors can impact the cooling capacity of a chiller, it is necessary to understand first how a chiller operates. A compressor-based recirculating chiller works by using the latent heat properties of a refrigerant to remove heat from a process and reject it to ambient air or to facility water. (See Figure 1.) In order to transfer this process heat to the ambient air or facility water, the refrigeration system must provide a refrigerant temperature below the temperature of the process fluid to be cooled. Later in the process, the system must raise the temperature of the refrigerant to a level above the temperature of the medium that is used for rejecting the heat.
A chiller is a complex system, but the basic components are the compressor, condenser, thermostatic expansion valve (TXV), and evaporator. Starting at the compressor, the refrigerant coming from the evaporator is compressed from a saturated gas to a high-pressure, high-temperature gas. The now hot gas is passed through the condenser, where it is cooled and condensed into a saturated liquid by rejecting the heat to the cooler ambient air (air-cooled condenser) or to facility water (water-cooled condenser). The refrigerant then passes through the TXV, across which its pressure and temperature drop considerably. The refrigerant temperature is now lower than that of the process fluid and, as a result, heat is transferred from the process fluid to the refrigerant causing it to evaporate into a low-pressure gas. The cycle is once again repeated as the gas flows back to the compressor.
The condenser and evaporator are heat exchangers that transfer heat from one medium to another. In the case of an air-cooled condenser, an aluminum-finned copper tube liquid-to-air heat exchanger is typically used for rejecting heat from the hot refrigerant gas to the ambient air. A water-cooled condenser, on the other hand, uses a liquid-to-liquid heat exchanger to transfer heat from the hot refrigerant gas to the facility water. In the case of the evaporator, a liquid-to-liquid heat exchanger is typically used to transfer heat from the process fluid to the refrigerant. The performance of a heat exchanger depends on many factors, including the process fluid used, incoming fluid temperatures, flow rates, materials of construction, and design of the heat exchanger. With all other factors being equal, the driving force behind the transfer of heat from one fluid to another is the difference in incoming fluid temperatures.
Ambient air temperature or facility water temperature play an important role in the cooling capacity of a chiller. In order for the condenser to reject the total heat (process heat load plus heat of compression) to the ambient air or facility water, the temperature difference between the hot refrigerant gas and the ambient air or facility water must be sufficient. For example, Lytron chillers typically operate at condensing temperatures between 32.2°C (90°F) and 43.3°C (110°F) and reject heat to 20°C (68°F) ambient air or 24°C (75°F) facility water. (See Figure 2.) Water-cooled condensers can reject the same amount of heat to a higher facility water temperature because water is a much better heat transfer fluid than air and does not require as large of a temperature differential between the two incoming fluids.
As the ambient air or facility water temperature increases, the ability of the chiller condenser to transfer the process heat from the refrigerant to the ambient air or facility water is reduced, causing higher condensing pressures that could result in reduced system performance. Therefore, if the recirculating chiller will be exposed to ambient temperatures above 20°C, sizing calculations should be run to determine the required cooling capacity. Similarly, if the ambient temperature decreases, the performance will improve due to the larger initial temperature differential.
When sizing a chiller, it is essential to know your maximum ambient temperature or facility water temperature so that you can select a chiller with enough cooling capacity to meet your application's needs. (Consult with an applications engineer for assistance in sizing a chiller.)
Similar to the condenser, the evaporator performance decreases if the difference in incoming temperatures between the liquid refrigerant and the returning process water temperature is reduced. This occurs if the chiller is set to run at a low temperature, such as 5°C instead of 20°C. The returning process water temperature will be lower if the chiller supply temperature is lower and there will be less temperature differential to drive heat transfer. The performance of a chiller decreases as the set temperature decreases. Similarly, the performance of the chiller will improve as the set temperature increases up to the maximum temperature within the recommended temperature range. (See Figure 2.)
The process fluid used in the recirculating chiller also impacts performance. Cooling capacity for chillers is typically based on using water as the process fluid, so using a process fluid other than water could result in less cooling capacity. For example, some recirculating chillers are designed to be compatible with polyalphaolefin (PAO) as the process fluid. PAO is typically used in military applications because of its dielectric properties and/or wide operating temperature range. However, all other factors being equal, the cooling capacity of a PAO chiller will be less than the cooling capacity of a water chiller since PAO has a lower specific heat, lower density, and lower thermal conductivity than water.
Another factor that affects chiller performance is condenser and evaporator maintenance. Dust accumulation on air-cooled condensers or fouling of tubes or flow passages on water-cooled condensers or evaporators leads to decreased chiller performance. When dust or debris accumulates on the fins and fan blades of air-cooled condensers it restricts airflow, resulting in a loss of chiller cooling capacity. If the chiller will be operated in a dusty or dirty environment, routine maintenance or cleaning should be scheduled and/or the chiller should be oversized. Fouling of water-cooled condensers due to scale formation, corrosion, and/or biological growth from poor water quality could also result. Fouling forms an insulator layer on the internal walls of tubes that impedes the heat transfer between the refrigerant and the water, resulting in the deterioration of chiller efficiency. By using clean water with corrosion inhibitors the risk of fouling can be minimized. (See Lytron's application note "Recirculating Chiller Tune Ups: Operation and Maintenance of Your Coolest Equipment" for more information.)
In rare cases where an air-cooled recirculating chiller is located at a high altitude location, the lower density air will affect cooling capacity. Since mass flow rate is equal to volumetric flow rate times density, if the density decreases a higher volumetric flow rate must be supplied by the condenser fan in order to provide the same cooling capacity that you would have at sea level. One option is to oversize the recirculating chiller to ensure cooling capacity needs are met.
Humidity is another factor that affects chiller performance when the supply process coolant temperature is below the ambient dew point. In such cases, if the chiller lines, evaporator, and pump are not insulated, condensation may form on these surfaces causing a loss in cooling capacity. Untreated metals' surface can also experience damage from corrosion. Therefore, insulation is strongly recommended.
It is also necessary to keep in mind that a 230 VAC 50 Hz chiller has approximately 17% less cooling capacity than a 230 VAC 60 Hz chiller due to the slower frequency with which the pump, compressor, and fan motors rotate. (See Figure 3.)
The performance of a chiller depends on the ambient air temperature or facility water temperature, chiller set temperature, process fluid, operation and maintenance, and more. It's crucial to consider all of these factors when selecting a chiller as well as when operating one. This will help to ensure uptime of the equipment the chiller is cooling.
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