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HVAC 101: How a Chiller Works

18 September 2020 Bookmark and Share


Everyone knows what an air conditioner is. It's the machine that cools down a space and makes summer a bit more bearable indoors. However, not as many people are familiar with a chiller, which is a machine that also cools a space, but through a slightly different method. Both types of cooling units have their place in different applications, and it's important to understand why chillers are a practical choice for many buildings. To learn about the advantages of chillers, let's look at how they work.


The Refrigerant Cycle

The most common chiller is the vapor-compression type, which mechanically compresses and expands refrigerant in a system to absorb heat from an environment. Although the refrigerant runs through a continuous cycle, let's say that the cycle "begins" at the compressor. The refrigerant entering the compressor has a relatively low temperature and pressure. The compressor then takes this refrigerant and compresses it, which increases pressure and thus makes the refrigerant significantly hotter.


At this point the refrigerant is still a gas — a vapor to be exact — but this will change after the next step in the process. After the refrigerant leaves the compressor, it travels through a condenser, where heat is absorbed from the refrigerant and is eventually rejected outside of the building. The two types of condensers will be discussed below, but what is important is that the refrigerant loses some of its heat and becomes a liquid in the process. At this point, it is ready to be cooled down and to absorb heat from an environment.


Once refrigerant leaves the condenser, it makes its way to the expansion valve. The valve depressurizes the refrigerant (like covering the end of a garden hose with your thumb and only letting out a fine mist). This has the opposite effect of the compressor: the expanded refrigerant decreases pressure and becomes significantly cooler. From there, the refrigerant almost immediately goes into the evaporator, which is shaped like a big tank. Inside the tank, water runs through metal tubes that come into contact with the refrigerant (sometimes the refrigerant runs through the tubes instead). The cold refrigerant absorbs heat from the water before it eventually leaves the evaporator. From there, the relatively cool refrigerant travels back to the compressor and repeats the cycle all over again.


Chilled Water

The entire point of the refrigerant cycle is to absorb heat from a constant stream of water and to somehow reject the heat outside. So, once the chilled water is cooled inside the evaporator, where does it go?


Well, it depends on what needs to be cooled. If special equipment needs to be cooled, the water is pumped there. Otherwise, if the chiller is cooling down a space, the chilled water is pumped to an air handler unit. Inside the air handler unit, air is pushed through a coil that runs the chilled water, cooling the air much like how an evaporator cools the chilled water. Once the chilled water absorbs heat, it returns to the evaporator to be cooled again.


Water-Cooled vs. Air-Cooled Chillers

There are two types of vapor-compression chillers, and the main difference lies in how the condenser rejects heat absorbed by the refrigerant. In a water-cooled chiller, the shape and function of the condenser is similar to the evaporator. Condenser water runs through the condenser tank and absorbs heat from the recently compressed refrigerant. The warm water is then pumped to a cooling tower, which is typically installed on the roof of a building. Inside the cooling tower, the condenser water cools down by coming into direct contact with a stream of ambient air. Afterwards, the condenser water is pumped back to the condenser. Since heat rejection is performed by the cooling tower, this type of chiller is stored inside a building and away from the elements.


An air-cooled chiller, on the other hand, is installed outside of a building. This is because the condenser is not a tank, but instead a "wall" of coils that come into contact with the outside air. Fans push air through the coils, absorbing heat from the refrigerant. Both types have their advantages and disadvantages, but they both have the same function: remove heat from a continuous circuit of water.


Chillers vs. Air Conditioners

Where an AC would directly remove heat from a certain area, a chiller instead removes heat from water. This often makes a chiller the more efficient option for cooling larger buildings. Instead of placing an air conditioner on each floor of a building, a chiller allows the refrigerant cycle to occur in a basement or on a roof, far away from the main space of the building. And instead of trying to ventilate cold air across long distances, a chiller uses water as a more efficient mechanism for transferring cool media over a distance.


Now that you understand the basic function of chillers, you can make more informed decisions when determining what equipment your facility needs. If you need a chiller for one reason or another, there is still a matter of determining whether you should go for a rental unit or purchase your own. Next time, we'll discuss the reasons why you would rent a chiller.

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