Efficient Electric Chillers

Chillers are used extensively for large facility space cooling and in industrial process liquid cooling. In many commercial and industrial facilities space cooling and process refrigeration represent one of the largest energy expenditures. Improving chiller efficiency can significantly reduce your energy usage without affecting comfort or production.

A liquid chilling system cools water or some other secondary coolant for air conditioning or process refrigeration. Liquid (usually water) is supplied to the facility at a temperature of 45° (for air conditioning) and is returned at some higher temperature after it has removed heat from the facility. Under full load conditions, the water will usually undergo a 10° temperature rise. As the chiller removes heat from this water, it rejects this heat into the outdoor air, whether directly by means of a refrigerant-to-air heat exchanger, or indirectly by means of a separate water loop and a cooling tower.

A chiller is usually factory assembled and shipped to the facility where final electrical and plumbing connections are made, but may be shipped in sections for field assembly. It has four primary components: the compressor, the compressor drive, the evaporator, and the condenser. The evaporator and condenser serve as heat exchangers which transfer heat between the water and the refrigerant. Chillers can be categorized based on the type of compressor. They may be electrically driven, engine driven, or absorption-type chillers. While engine-driven and absorption chillers are utilized in commercial facilities, electric chillers have been used far more extensively.

Electrically Driven Chillers

Electrically driven chillers utilize electric motors to drive the compressor. These chillers can be further categorized according to the type of compressor which is used.

Reciprocating Compressor Chillers use cylinders with pistons acting as pumps to increase refrigerant pressure. Compressors may have anywhere from 1 to 12 piston-cylinder arrangements, which unload in pairs as load decreases. These compressors are well-suited for air-cooled condensers and low-temperature application, and dominate the market in small tonnage systems.

Centrifugal Compressor Chillers are basically fans or blowers, building refrigerant pressure by forcing gas around a scroll and through a funnel-shaped opening at high speed. They are generally quieter, require less maintenance, and have less vibration than reciprocating compressors. They are not suited for air-cooled chillers and must, therefore, be water-cooled.

Screw Compressor Chillers are more compact than either the centrifugal or reciprocating compressor. The compressor consists of two matching helically grooved rotors which turn, compressing the refrigerant gas as it passes from one end of the screws to the other. Screw type systems are better suited for low temperature applications.

Scroll Compressor Chillers use two spirals, one within the other, to compress refrigerant. They are relatively new in commercial applications, very quiet, and efficient. Scroll compressors are available up to 60 tons.

Efficiency Rating Systems

When comparing chillers for energy efficiency, auxiliary energy requirements such as condenser and chilled water pumps, cooling tower fans, as well as the cost of water treatment are not usually taken into account. Electric chillers may be compared based on the input power in kiloWatts required to deliver one ton of air conditioning, or KW/Ton. Manufacturers' literature for electric driven chillers will generally give the input KW along with the cooling capacity in tons or BTUH (Btu/Hr) at a given condenser water temperature and evaporation temperature. Although each chiller will have its own rating assigned by the manufacturer, typical efficiency ranges along with sizes are given in the following table.

Table 1. Typical Compressor Sizes and Efficiencies
Compressor Type
Compressor KW/Ton
Sizes (Tons)
Air-Cooled    
Reciprocating
1.0 - 1.3
3 - 450
Centrifugal
0.7 - 0.9
80 - 1300
Screw
0.7 - 1.2
45 - 380
Scroll
up to 1.2
up to 60
Auxiliary KW/Ton = 0.19 KW/Ton
   
Water-Cooled    
Reciprocating
0.8 - 0.9
3 - 450
Centrifugal
0.5 - 0.8
80 - 8500
Screw
0.6 - 0.7
50 - 1300
Auxiliary KW/Ton = 0.21 KW/Ton    

A water-cooled chiller refers to a chiller where a separate water loop is used at the condenser to remove heat from the refrigerant. Note that efficiencies for water cooled units are generally higher than those for air-cooled (where a air-to-refrigerant heat exchanger is utilized), but there is a slight increase in auxiliary KW/Ton. This efficiency improvement is a result of more efficient heat transfer and consequently, lower condensing temperatures.

How to improve efficiency

Water-to-Refrigerant heat transfer improvements: Major advances have been made in improving heat transfer efficiency. These include increased condenser/evaporator heat exchange area and improved heat transfer material construction. By reducing the approach temperature between water and refrigerant, compression pressure differential can also be reduced.

Reducing compression pressure differential involves reducing head pressure (condensing temperature) or increasing suction pressure (evaporation temperatures). This can be achieved by improving heat transfer conditions (as previously described) or by effectively modifying control strategies. One way to effectively increase evaporation temperatures is to install a chilled water reset. This type of control will allow chilled water temperature to climb when less load is present. Table 2 summarizes the effect of reducing head pressure or increasing suction pressure to the COP of the compressor. Recent studies show that, for constant-speed centrifugal chillers, this reset strategy saves energy at operating loads in the 40% to 80% range. Centrifugal chillers equipped with variable speed drives respond better to chilled water reset and will achieve savings down to 10% load. Water-cooled condensers achieve lower head pressures than air-cooled condensers: therefore, water-cooled chiller systems are typically more efficient than air-cooled chillers.

Table 2. Increasing Chiller Efficiency
Compressor
Type
Percent Increase in the COP
for each 1° Reset for:
 
Condenser
Temperature
Evaporation
Temperature
Reciprocating
1.30%
1.13%
Centrifugal
0.50%
1.54%
Screw
2.30%
2.08%
Absorption
0.80%
0.59%

Compressor efficiency depends on the type of compressor being utilized. As seen from Table 1, all compressors are not created equal. Some compressors operate better than others when only partly loaded. Reciprocating compressors generally operate better when partly loaded while screw compressors operate best under fully-loaded conditions.