Terminology You Should Know about Chillers

In a typical commercial or industrial building, chillers consume more electricity than any other device, except for an extremely large fan. Thus, inefficient chillers can waste significant electricity, and even modest improvements in your chiller’s efficiency can be worth the investment.

However, increasing the efficiency of your system isn’t as simple as purchasing the most efficient unit possible. Choosing a chiller that’s most efficient when operating at full or part load might be counterproductive because the ratings don’t measure the efficiency of the overall cooling system. To maximize cost-effectiveness, we recommend analyzing the entire chilled-water system before selecting and specifying equipment—it may be wiser to buy a less efficient chiller and spend money on efficient auxiliary equipment and improved operating strategies.

Chiller terminology

AHRI

This manufacturer trade organization certifies chillers using several efficiency metrics. AHRI tests equipment in standard conditions to determine its coefficient of performance (COP), energy-efficiency ratio (EER), and its full-load and part-load efficiency (given in kilowatts [kW] per ton). Use these conversion factors to relate COP, EER, and kW per ton:

COP = EER ÷ 3.412; EER = 3.412 COP

kW per ton = 12 ÷ EER; EER = 12 ÷ (kW per ton)

kW per ton = 3.517 ÷ COP; COP = [12 ÷ (kW per ton)] ÷ 3.412

Water Cooled Screw Chiller

AHRI standard conditions

AHRI defines standard operating conditions in order to measure efficiency metrics consistently and accurately. For water-cooled chillers, AHRI defines standard conditions as constant flow rates of 2.4 gallons per minute (gpm) for 44° Fahrenheit (F) water leaving the evaporator and 3.0 gpm for 85°F water entering the condenser.

COP

This metric is the ratio of the cooling output power to the total power input. Because cooling output and power input are both in watts, COP is a unitless number. The higher the COP, the more efficient the equipment is.

EER

This metric is frequently used for smaller chillers—it’s the ratio of the cooling capacity to the total power input at any given set of rating conditions, expressed as Btu per watt-hour.

Full-load efficiency

This metric, given in kW per ton, is the efficiency of the chiller when it’s operating at peak load and AHRI standard conditions. The lower the kW per ton rating is, the more efficient the equipment is.

Air Cooled Scroll Chiller

Part-load efficiency

This is the efficiency of the chiller when it’s operating at part load. The industry measures this metric in kW per ton using the equation for integrated-part-load value (IPLV) and nonstandard part load value (NPLV).

Tons

One ton of cooling is the amount of heat absorbed by 1 ton of ice melting in one day, which is equivalent to 12,000 Btu per hour, or 3.516 kW in thermal energy.

What are the options?

Air or water cooled

Most large commercial and industrial buildings use water-cooled chillers. This equipment cools water with a vapor-compression refrigeration cycle and circulates it to provide air-conditioning. Central chiller plants consist of one or more chillers and their auxiliary systems, which include chilled-water pumps, condenser water pumps, and cooling towers. The chillers produce cold water, which the system pumps to one or more air handlers throughout the building. As the water passes through the building, it absorbs heat from warm indoor air. The system then distributes the cool air around the building through a network of ducts.

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