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July 28, 2016

Chiller Basics: Understanding the Advantages and Drawbacks of Five Types of Chillers

Chillers are the most common type of machines used for cooling large commercial spaces. There are several types, and each type offers specific pros and cons based on the application, facility budget and available energy sources.

Below, we have listed some of the most common types of chillers available to you in the market today:

Centrifugal Water Chillers

These chillers are generally available from 100 to 3,000 tons if prefabricated and up to 8,500 tons as built-up machines. The Coefficient of Performance (COP) requirements for centrifugal water chillers are:

  • <150 tons = 5 COP (.703 kW/ton)

  • 150 to 300 tons = 5.55 COP (.634 kW/ton)

  • ≥300 tons = 6.1 COP (.577 kW/ton)

  • See peak COP > 7 (.502 kW/ton)

Centrifugal water chillers use dynamic compression to convert kinetic energy to static energy. To increase the pressure and temperature of the refrigerant, the impeller blades draw the refrigerant in. Once it has passed through the impeller, the refrigerant is at a higher pressure and in a condition that allows its heat to be rejected from the chiller.

An economizer can be used in conjunction with multiple expansion devices to improve the efficiency of a multi-stage chiller. This reduces the amount of compressor power required for the chiller and can provide energy savings of 3-4 percent.

Maintenance requirements for centrifugal water chillers are fairly simple, with no regular maintenance or calibration required for the compressor, motor and controls. Recommended maintenance is visual inspection looking for any leaks and examining connections and components as well as regular replacement of filters, strainers and seals.

Absorption Water Chillers

Absorption chillers make use of the absorption refrigeration cycle and do not use a mechanical compressor. They range in size from 20 to 1,500 tons. The absorption refrigeration cycle differs from the vapor-compression refrigeration cycles in that the compressor is replaced by an absorber, pump and generator, while the condenser, expansion device, and evaporator are the same. Absorption chillers typically use distilled water as the refrigerant plus a secondary fluid, called the absorbent, which is a Lithium Bromide (LiBr) solution].

In an absorption chiller, the generator uses a high-temperature energy source, usually steam or hot water, which flows through the tubes and boils off the refrigerant into vapor. The vapor moves to the condenser and the concentrated solution returns to the absorber. In the absorber, refrigerant vapor is absorbed by the solution and condenses into a vapor, releasing the heat it acquired in the evaporator.

There are three types of absorption chillers, each with slight difference from the others:

  • Single-Effect Chiller: regular absorption refrigeration cycle; low-pressure steam (15 psig) or medium-temperature liquids (270°F); COP 0.6 to 0.8 (5.86 to 4.40 kW/ton)

  • Double-Effect Chiller: same basic components as single-effect, but it includes an additional generator, heat exchanger and pump; medium-pressure steam (115 psig) or high-temperature liquid (370°F); COP of 0.9 to 1.2 (3.91 to 2.93 kW/ton)

  • Direct-Fired Chiller: combustion of a fossil fuel in the high-temperature generator; uses natural gas, number 2 fuel oil, or liquid petroleum; COP of 0.9 to 1.1 (3.91 to 3.20 kW/ton); can also be used for heating applications

There are two applications where absorption chillers can be advantageous. One is to use the absorption chiller during peak electric rates to improve total system cost. They are also effective in combination chiller plants because partially chilling at higher temperature increases the absorption chiller’s efficiency and capacity while reducing the cooling load on an electric chiller. Operators can also preferentially load the absorption chiller during peak hours.

The most common maintenance issue with an absorption chiller is crystallization. Because lithium bromide is chemically a salt, if the solution ever falls below the saturation temperature the salt becomes a solid. The causes for crystallization include air and non-condensables leaking into the chiller, cooling water that is too cold or fluctuates too much, or an electric power failure. Outside of normal inspections, additional maintenance includes pump teardown and inspection every 5 to 10 years and inspecting the firing rate and safety of direct-fired burners.

Helical-Rotary Chiller

Helical-rotary chillers operate similarly to other chiller types and are available from 70 to 450 tons with a COP 2.9 – 3.1 (1.21 to 1.13 kW/ton). In these chillers the oil separator, which is more than 99 percent efficient, can be used to enhance the effectiveness of the refrigeration cycle.

The helical compressor features two screw rotors driven by a male rotor. The rotation of the rotors drives refrigerant towards the discharge port. Volume is reduced throughout the process, compressing the refrigerant. If there is a ported compressor then no valves are used. Otherwise, the capacity of the compressor is controlled by a slide valve, or other method that acts in a similar function. In this case, the position of the valve controls the refrigerant vapor being delivered by the compressor, allowing it to operate efficiently over a wide range of conditions. The slide valve positions include:

  • Full Load: Valve is closed, the compressor pumps the maximum volume of refrigerant, discharging through radial and axial ports

  • Part Load: Slide valve moves toward open position, depending on demand. Allows refrigerant vapor to bypass from rotors back to suction side of compressor, limiting the vapor available for the compression process

  • Unloading: Valve fully open, all of refrigerant vapor is discharged through the axial port. Allows compressor to always start fully unloaded after shutdown.

Recommended maintenance for helical-rotary chillers includes visual inspection to spot leaks, check connections, and components as well as regular replacement of filters, strainers, and seals. Oil changes and oil filter and strainer replacements are also required periodically. No regular maintenance is required for the compressor, motor and controls.

Reciprocating Compressor

Reciprocating Compressor chillers work similarly to an automobile engine and are available from 20 to 200 tons. With an inexpensive first cost these machines were common in the past. Issues with noise, efficiency, reliability and maintenance, however, have made them much less popular than other chiller types today.

Scroll Compressor

With a scroll compressor chiller a driven scroll rotates around a stationary scroll, drawing in refrigerant and compressing it. These machines are sized from 20 to 200 tons and have a COP 4.9 to 5.9 (.718 to .596 kW/ton).

Price Considerations

Prices for various chiller types vary widely:

  • 240 ton screw chiller typically costs $70,000 to $75,000

  • 240 ton centrifugal chiller costs $107,000 to $130,000

  • Two 120 ton scroll chillers cost $65,000 to $75,000

To determine the best chiller to use in a specific application, factor in efficiency, equipment and installation cost, maintenance cost and recurring costs.