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Impact of Water Treatment on Chiller Energy Costs

Cooling Systems


The main energy consumers in a water-cooled air conditioning system are the chiller, recirculating pumps, tower fans, and air handler fans. Chiller efficiency is important because it reflects the amount of energy required to produce a ton of refrigeration. It is typically measured in KW/Ton and is a function of the chiller design, operating conditions, and heat transfer efficiency. Effective water treatment helps optimize chiller efficiency and minimize energy costs by keeping the evaporator and condenser heat exchange surfaces clean and corrosion free.

Compressor, Condenser, and Head Pressure
A chiller transfers the heat "picked up" from the building by the chilled water system to the cooling tower, where it is discharged to the atmosphere. The compressor provides the driving force for the refrigeration cycle and is the primary consumer of electricity in a chiller. The compressor functions to increase the refrigerant temperature and pressure. Anything that increases the workload on the compressor will increase energy consumption.
In the condenser, heat is transferred from the hot refrigerant gas to the cooling water, which causes the refrigerant gas to cool and liquefy. The temperature of the condensed refrigerant has a corresponding vapor pressure called the condensing pressure or condenser head pressure. The compressor is designed to work at a certain condensing pressure for a given load. The term "high head pressure" refers to condenser pressure that is higher than it should be for a specific load condition.
Condenser Fouling and Energy Costs
Deposition (fouling) on the condenser tubes reduces heat transfer efficiency, increases the condenser head pressure, and results in higher energy costs. Reduced heat transfer efficiency in the condenser causes the compressor to work harder, increasing the refrigerant condensing temperature and pressure to transfer the same amount of heat to the cooling water. Each additional 1oF in refrigerant temperature requires the compressor to consume 1.5% more energy. If the deposit thickness is great enough, condenser head pressure will exceed the chiller limits and the chiller will shutdown.
Some deposits are more insulating than others and thus have a greater impact on the head pressure and energy requirements. For example, calcium carbonate scale deposits transfer heat up to four times better than biofilm deposits (slime). As a result, slime increases head pressure and energy requirements and will shut down a chiller much faster than "normal" scale. Condenser deposits can be a mixture of slime, scale, corrosion by-products, and suspended solids scrubbed from the air.
Table One below shows the potential economic impact of scale deposits on a 500-ton chiller running at full load, 24 hours per day. Actual increased energy use depends on compressor type, actual operating head pressure, and percent operating load. For the same thickness, the increased cost associated with a biofilm deposit can be significantly greater than with scale, depending on the actual scale composition. It becomes clear that good microbiological control is vital for efficient chiller operation.

Deposit Thickness(inches) Fouling Factor % Efficiency Loss Increased Annual Electrical Cost (due to Scale Deposit)*
0.00 0.0000 0 $0
0.01 0.0008 9 $19,790
0.02 0.0017 18 $39,580
0.03 0.0025 27 $59,635
0.04 0.0033 36 $79,155
0.05 0.0042 45 $98,945
Table One - Condenser Deposit Thickness vs. Increased Electricity Cost

*Based on electricity cost of $0.07 per KWH, chiller efficiency of 0.65 KW/Ton, and power factor of 0.91. Scale is assumed to have thermal conductivity of 1.0 BTU/(hr)(ft2)(oF).

Other Factors Influencing Head Pressure

Besides waterside fouling, there are three other conditions which can cause high head pressure
1. Non-condensable gases (i.e. air) in the refrigerant
2. Low condenser water flow rate
3. Condenser inlet water temperature too high

Evaporator Fouling

Fouling in the evaporator tubes will also increase energy costs. Fouled evaporator tubes can cause a drop in refrigerant evaporating pressure that reduces its density. As a result, the compressor must pump the gas to a higher pressure to remove an equivalent amount of heat from the chilled water. Again, the compressor must work harder, which increases energy requirements.

% Load Refrigerant 11 (psig) Refrigerant 12 (psig) Refrigerant 22 (psig) Refrigerant 113 (psig) % Increased Electricity Used
8.0 113 198 10.0 16
11.0 125 232 11.5 32
14.5 140 270 13.0 51
50 10.0 123 223 11.0 17
14.0 137 264 12.5 34
80 10.5 126 218 11.5 6
13.5 138 260 13.0 20
100 13.0 135 236 12.5 8
15.0 142 259 13.5 16
Table Two - Condenser Head Pressure vs. Load for Different Refrigerants