Absorption and Engine Driven 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, either 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 that transfer heat between the water and the refrigerant. Chillers can be categorized based on the type of compressor used and the source that drives the compressor. They may be electrically driven, engine driven, or absorption-type chillers. While electric chillers have been extensively utilized throughout the years, engine- driven and absorption chillers are occasionally used.

Engine Driven Chillers

These chillers utilize a gas-fired engine (much like you would see in an automobile) to drive the compressor. Like electrically driven compressors, the engine may drive a reciprocating, centrifugal, scroll, or screw-compressor. They cannot, however, operate below 25% load.

Absorption Chillers

Absorption chillers utilize steam, water, or very hot gas to concentrate a solution which is used to absorb a secondary solution and are more appropriate for facilities with excess high pressure steam. A separate steam boiler may be utilized, or the unit may have its own direct-fired combustion chamber installed. These chillers have two components: the generator and the absorber, performing the same function as a compressor. The chilling effect is obtained through the interaction of two connected, closed tanks containing lithium bromide (a salt solution) in one and water in the other (water and ammonia may also be utilized). It is necessary to maintain a vacuum in the system, so the smallest hole in one of these tanks will eliminate the refrigeration effect. In addition to the energy requirement for cooling, absorption systems require large condenser water pumps and cooling tower fans since they reject more heat from the condenser than electrically or engine-driven systems. With this large heat rejection, there will be more water evaporated and, therefore, higher water and water treatment costs than with other systems.

Table 1. Typical Sizes and Efficiencies
Steam Absorption    
0.6 - 0.7
1.0 - 1.25
Auxiliary KW/Ton = 0.24 - 0.27 KW/Ton
Direct Fired Absorption
0.95 - 1.0
20 - 1100
Auxiliary KW/Ton = 0.24 KW/Ton
1.0 - 1.7
55 - 450
Auxiliary KW/Ton = 0.24 KW/Ton

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 must be taken into account. Gas-fired, engine-driven and absorption chillers will be rated by the coefficient-of-performance (COP) of the unit. The COP can be defined as the BTUs per Hour of useful cooling the unit delivers, divided by the BTUs per Hour of fuel input. Although each chiller will have its own rating assigned by the manufacturer, typical efficiency ranges along with sizes are given in the Table 2.

All chillers evaluated in Table 1 are water-cooled. A water-cooled chiller refers to a chiller where a separate water loop is used at the condenser to remove heat from the refrigerant. There are some air-cooled absorption and engine-driven chillers available, but efficiency is sacrificed. Efficiencies for water cooled units are generally higher than those for air-cooled (where an 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

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 for engine-driven chillers.

Reducing compression pressure differential for engine-driven chillers 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 improving or 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. 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
Percent Increase in the COP
for each 1° Reset for:

Utilize heat recovery for absorber concentration. A double effect absorption unit recovers heat from water that has been "boiled off" in the 1st stage generator in order to further concentrate the lithium bromide solution. As seen from Table 1, double-effect units have a much higher efficiency.

Compressor efficiency depends on the type of compressor being utilized and will effect the efficiency of engine- driven systems. Some compressors operate better than others when only partly loaded. Reciprocating compressors generally operate better when partly loaded while screw compressors operate better under fully-loaded conditions.