Chiller Essentials

Heat Transfer

Chiller Essentials

What is a chiller?

A chiller is a heat-transfer device that uses mechanical refrigeration to remove heat from a process load and transfers the heat to the environment. Chillers are used by plastics processors when they need a lower process fluid temperature than a simpler system, such as a cooling tower, can provide.

Chillers are designed to ensure a continuous flow of coolant to the cold side of a process water system at a desired “leaving water temperature” or LWT of about 50°F (10°C). The coolant is then pumped through the process, extracting heat out of molds and machinery as it flows back to the return side of the process water system.  The coolant flow, now at an elevated “entering water temperature” or EWT (5 to 10 degrees warmer than LWT), then re-enters the chiller, which does three things:

1)         it captures the heat from process coolant,
2)         it transfers the captured heat out to the environment, and
3)         it lowers the temperature of the coolant to the desired LWT, making it ready for re-circulation.

How does a chiller work?

Portable and Central Chillers
Figure 1

 A chiller is a vapor compression mechanical refrigeration system that connects to the process water system through a device called an evaporator, as shown in Figure 1. The evaporator is a heat exchanger in which heat captured by the process coolant flow is transferred to the flow of refrigerant fluid. As the heat-transfer takes place, the refrigerant evaporates, changing from a low-pressure liquid into vapor, while the temperature of the process coolant is reduced down to the desired LWT.

Next, the refrigerant flows to a compressor, which performs two functions.  First, it removes refrigerant vapor from the evaporator and ensures that the pressure in the evaporator refrigerant line (vapor pressure) remains low enough to absorb process heat at the correct rate. Second, it raises the pressure in outgoing refrigerant vapor to ensure that its temperature is high enough to release its heat when it reaches the condenser, where the refrigerant returns to a liquid state. The latent heat given up as the refrigerant changes from vapor to liquid is carried away to the environment in a cooling medium, either water or air. Thus, there are two types of cooling condensers offered on most portable and central chillers: air-cooled and water-cooled.

To make the most intelligent and efficient choice about chilling capacity for your plant, you’ve got to weigh the benefits of two choices: