Factories, warehouses, healthcare facilities, commercial buildings, and office spaces generate unwanted heat that must be removed from the process, equipment, or occupants inside. Chillers may be used to remove this heat by transferring it to the atmosphere either by air or water cooled chillers. At the heart of the chilling process is the evaporation of a liquid refrigerant into a gas. Two basic types of chillers include vapor compression and absorption. In this blog, we will focus on the vapor compression chiller.

Vapor Compression Chiller System

Vapor compression chillers use a mechanical compressor powered by electricity, steam, or gas turbines. They produce cooling using the “vapor compression” refrigeration cycle (similar to a home air conditioning unit). Figure 1 illustrates a typical vapor compression chiller system for building air conditioning.

Figure 1:  Vapor Compression Chiller System

The vapor compression chiller system steps, as numbered in Figure 1, are described below.

1. Chilled water (45°F) from the chiller is pumped through cooling coils located in air handling units (AHU) throughout the building. As building air is blown over the coils, heat is transferred from the air to the chilled water. This cools the air and increases the chilled water temperature.
2. The “warm” chilled water (55°F) is returned to the chiller, where it transfers the heat it has picked up from the building air to the refrigerant. This cools the chilled water back down so it can repeat the cooling cycle.
3. Heat from the building air is now contained in the refrigerant inside the chiller.
4. Cooled tower water (85°F) is pumped through the condenser side of the chiller. The heat is removed from the refrigerant and transferred to the tower water. The tower water temperature increases (95°F). Tower water temperatures are dependent on the outside air temperatures and humidity.
5. The warm water from the condenser is pumped to the top of the cooling tower, where it is sprayed or allowed to fall through the tower fill.
6. Air is blown across the water as it falls through the tower. This causes a portion of the water to evaporate and the remaining water to be cooled (85°F). The heat from the building has now been transferred to the atmosphere. Cooling occurs by both latent heat transfer (evaporation) and sensible heat transfer (heat transfer from warm water to cool air by direct contact)
7. Cooled water is again pumped through the condenser section of the chiller to cool the refrigerant.
8. Makeup water is added to maintain a constant water level in the tower basin.
9. Because the impurities that were in the evaporated water are “left behind” in the bulk water, the concentration increases in the system water. A portion of the concentrated system water must be sent to the drain to control the concentration in the system water. Makeup water requirements equal the amount of water removed via evaporation and bleed.

How a Vapor Compression Chiller Cools

Next let’s look at how a typical chiller used with a building HVAC system

operates.  Vapor compression chillers have four basic components:

1. Compressor
2. Condenser
3. Expansion valve
4. Evaporator

Figure 2:  Vapor Compression Refrigeration

The refrigeration process starts as follows, as shown in Figure 2:

1. When the low-pressure liquid refrigerant in the evaporator section is progressively “boiled” by the heat in the incoming chilled water, it becomes a low-pressure cool gas at 37°F. The evaporation of the low-pressure liquid refrigerant causes heat to be absorbed (removed) from the incoming chilled water, which changes its temperature from 55°F to 45°F. This transfers the heat from the chilled water circuit to the refrigerant.
2. At the compressor, the low-pressure cool refrigerant gas is pulled from the evaporator by the compressor and compressed into a high-pressure, hot gas at 105°F. The compression process concentrates the heat in the cool, low-pressure refrigerant vapor into a much smaller volume. This raises its temperature significantly.
3. The hot compressed refrigerant gas travels to the condenser to be cooled by cooling tower water at 85°F. The removal of heat (cooling) causes the hot, high-pressure gas refrigerant to become a warm, high-pressure liquid. This process is called condensation and occurs when enough heat has been removed from the refrigerant to lower the temperature to about 97°F. This part of the chiller is called the “condenser” and the tower water is called “condenser water.” This process transfers the heat from the refrigerant to the cooling tower water, now at 95°F, so it can be rejected to the atmosphere through evaporation.
4. The warm high-pressure liquid refrigerant from the condenser is metered through an expansion or metering valve to become a low-pressure cold liquid/gas mixture that enters the evaporators. This occurs when a small amount of refrigerant evaporates in the lower pressure evaporator lowering the liquid refrigerants temperature from 97°F to about 45°F.

Chiller Compressor

The heart of a vapor compression chiller is the mechanical compressor. Compression chillers can be classified based on the type of compressor used, with positive displacement (reciprocating, scroll, and screw) and centrifugal compressors being the main types.

Reciprocating compressors are characterized by what looks like an “engine” associated with the chiller. This engine is the reciprocating piston-type compressor. Centrifugal compressors are typically used in larger chillers because they can handle larger volumes of refrigerant. Centrifugal chillers are characterized by what looks like a large fan that creates a pressure difference from one side of the compressor to the other.

Chem-Aqua Can Help Keep Your Chiller Cool

No matter which type of chiller you use, effective water treatment is an essential part of a cooling water system’s preventative maintenance program. A customized water treatment program from Chem-Aqua does more than protect your system from the ravages of corrosion, deposits, and microbiological growth. It also helps maximize the life, efficiency, reliability, and safety of your cooling water system while keeping energy, water, and maintenance costs to a minimum. To find out how we can help, contact us at https://www.chemaqua.com/en-us/Contact-Us.

Written by: James McDonald

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