Figure 1. The refrigerant absorbs heat from the process water as it moves through the chiller. The boiling refrigerant turns into a hot vapor, and the compressor pressurizes the vapor so it can be moved to the condenser, where it is cooled with either air or water. The cooled refrigerant returns to liquid form and is cycled back into the evaporator.

Do You Need A Mechanical Chiller?

Alternative methods of evaporative cooling such as wells, ponds and city water can be considered, but each has drawbacks. City water tends to be prohibitively expensive. Wells, particularly deep ones, can be effective cooling water sources, but they are limited in availability, and their maintenance requirements usually are more expensive. Ponds also can be used as long they are big enough to provide enough water and to sustain a uniform temperature; however; water from ponds may not be sufficiently clean for use in a process cooling system. Heat exchangers normally are added to evaporative cooling water sources to create a closed-loop system.

Mechanical refrigeration equipment is required to produce chilled water in the lower temperature ranges. Chillers can furnish water in the leaving water temperature (LWT) range from 70 to 20^oF (21 to -6^oC). Temperatures as low as -30^oF (-34^oC) can be obtained with specialized equipment. If the LWT will be less than 45^oF (7^oC), an antifreeze solution such as ethylene or propylene glycol is needed to prevent freezeup.

The refrigeration circuit of a mechanical chiller consists of three basic components:

• Evaporator.
  
• Compressor.
  
• Condenser.
  

In the evaporator, the refrigerant absorbs heat from the process water as it moves through the chiller. The boiling refrigerant turns into a hot vapor, and the compressor pressurizes the vapor so it can be moved to the condenser, where it is cooled with either air or water. The cooled refrigerant returns to liquid form and is cycled back into the evaporator (figure 1). This change of state from gas to liquid and back to gas is what gives the refrigeration system its work energy.

Figure 2. In this compressor design, the mobile scroll makes three orbits around the fixed scroll. During the first orbit, gas is trapped and enclosed in two pockets at the opening of each scroll. The second orbit moves the gas closer to the center of the scroll, where its volume is reduced. The third orbit compresses the gas further, and the gas is then discharged through a port in the center of the fixed scroll.

Sizing the Chiller

Portable or Central? Mechanical chillers primarily are classified as portable or central. Portable chillers are self-contained and can be moved from one point in a plant to another. Generally, portable chillers are used only where capacity requirements are 20 tons or less, but many models are available up to 40 tons. Because they can easily be moved and interchanged, portable chillers provide flexibility and can minimize the effects of downtime.

Portable chillers most commonly are used where additions are made to an existing plant or where an existing central chiller installation needs supplementation. The fact that they are mounted on wheels makes it simple to roll them to the point of use. When purchasing a new central chiller with higher capacity is too costly, existing equipment can be supplemented with a portable chiller to increase production. Portable chillers also are used when different temperatures are required at different locations. They are especially advantageous for low temperature cooling applications (below 50^oF) where operating a central chiller below 100 percent efficiency may be too expensive. Sometimes a set of independent portable chillers can be tied together with a central tank system, giving the system redundancy and improved efficiency.

Central water chillers are used as a common chilled water source for a number of machines or an entire plant when only one temperature is required. Sizes for central chillers range from 30 tons to 300 tons or larger. This type of chiller generally has multiple compressors, more efficient unloading and more sophisticated controls. One central chiller running at full capacity is more efficient and less expensive to run than several portable chillers with smaller, partially loaded compressors.

Figure 3. The hot air produced by a heat reclaim condenser can be used to supplement plant space heating needs during cold weather. When space heating is not required, the hot air can be vented outside.

Compressors

Reciprocating compressors use a piston design to increase the pressure of the refrigerant gas. Various combinations of piston sizes (bore), length of piston travel (stroke), number of cylinders and shaft speed determine the compressor's power. The piston creates pulsating bursts of compressed air, making reciprocating compressors relatively noisy. The friction created by the piston can reduce efficiency and cause high vibration levels. Reciprocating compressors have become obsolete because of energy efficiency concerns and the availability of rotary compressors, including scroll and screw designs. The smooth compression used by rotary compressors reduces vibration and noise.

Scroll compressor technology is based on two scrolls: the first is fixed while the second scroll orbits around the first. The interlocking scrolls trap the gas between them, and as the mobile scroll moves around the fixed, the gas is compressed. The mobile scroll makes three orbits around the fixed. During the first orbit, gas is trapped and enclosed in two pockets at the opening of each scroll. The second orbit moves the gas closer to the center of the scroll, where its volume is reduced. The third orbit compresses the gas further, and the gas is then discharged through a port in the center of the fixed scroll (figure 2). Scroll compressors require less electricity but are only available up to 25 hp. Multiple scroll compressors often are used together to increase the capacity of the chiller or to create a central chiller.

Screw compressors work by trapping gas between the threads of one (single-rotor) or two (twin-rotor) screws. In a twin-rotor screw compressor, the most common type, the two screws rotate against each other, trapping and condensing the gas between them. In a single-screw design, the screw rotates against two stars, trapping gas in the threads of the single screw. Screw compressors generally are used from 30 hp.

Figure 4. A fluid cooler works in conjunction with a central chiller, using cold ambient air instead of refrigerant to chill the process water before it enters the chiller.

Condensers

Water-cooled equipment is more efficient than air-cooled equipment. The lower head pressures made possible by water-cooled condensers increase equipment capacity and reduce the required compressor horsepower per ton of cooling. Water-cooled condensers tend to be more efficient, smaller and quieter than air-cooled types and should be used whenever cooling water is available. Water-cooled chillers are usually available with or without an integral pump tank.

If a source of cooling water is not available, air-cooled condensers are required. Because air-cooled condensers give off a considerable amount of heat, much thought must be put into where to locate the condenser. Outdoor air-cooled chillers are located outside to keep hot air away from temperature-controlled areas inside the plant. Split system chillers are available with remote condensers located outside, connected to the evaporator and compressor inside the plant. Split systems are sometimes thought to be advantageous because of the cooling properties of outside air, but fluctuations in ambient temperature may interfere with condensing efficiency. These systems require more components and specialized personnel for installation. Heat-reclaim chillers are located inside the plant. Hot air from this type of condenser can supplement plant space heating needs during cold weather. When space heating is not required, the hot air can be vented outside (figure 3).

Refrigerant

Because of these restrictions, R134A and R410A refrigerants are increasingly available, especially for large, screw-compressor chillers. R410A is a blend of HFCs, which do not deplete the ozone but may contribute to global warming. R134A refrigerant is environmentally safe and operates under lower pressures, reducing the amount of energy consumed.

Supplemental Chilling

Whatever your cooling needs, familiarizing oneself with chiller technologies can help to choose the right system. While appropriate sizing is critical to choosing a chiller, other factors, including compressor efficiency, condenser cooling and refrigerant choices, also are important. In addition to evaluating current needs, the future must be considered. A chiller that will be obsolete in a few years may -- or may not -- suit you now. Before making a decision about a process cooling system, always be sure of the options, and consult with an expert.

Links

• AEC Inc.