How does Water Treatment result in Energy Savings for my Building?
The primary functions of a Chemical Water Treatment program for heating and cooling systems that use water as a heat transfer medium are the control of scale, corrosion and micro organisms. Left untreated, water will cause scale or mineral deposits to form on heat transfer surfaces, thereby reducing efficiency and increasing energy costs. Exposed to untreated water, metal is subject to corrosion. The corrosion of metal equipment, such as piping, boilers, cooling towers, chillers and heat exchangers, leads to costly repair and maintenance. Similarly, left unchecked, the growth of micro organisms in water systems can contribute to corrosion and efficiency loss. The growth of Bacteria such as Legionella Pneumophila can also lead to unsafe conditions for building occupants. Properly administered Chemical Water Treatment Programs have the added benefit of reducing water usage, especially in Cooling Water systems.
In heat transfer equipment, corrosion and scales deposits will interfere with the normal efficient transfer of heat energy from one side to the other. The degree of interference with this transfer of heat in a heat exchanger is called the fouling factor. The fouling factor is related to thickness of Calcium Carbonate scale as described in Table 1.
Table 1 – Fouling Factor of Calcium Carbonate Type Scale
As the fouling factor increases, heat transfer efficiency decreases and energy usage increases. Figure 1 shows the effect of scale on the condensing temperature for a typical water-cooled condenser. The condensing temperature increases in proportion to the fouling factor. An increase in condensing temperature requires a proportionate increase in energy or compressor horsepower, also shown in Figure 1.
FIGURE 1 – Effect of Scale on Condensing Temperature and Compressor Horsepower
Figure 2 illustrates the additional energy consumption required to compensate for calcium carbonate type scale on condenser tube surfaces of a refrigeration machine. A scale thickness of only .025 in (fouling factor of .002) will result in a 22 percent increase in energy consumption.
FIGURE 2 – Effect of Condenser Tube Scale on Energy Consumption
Table 2 shows the average loss of energy as a result of boiler scale. In addition to the reduced heat transfer efficiency, insulating scale deposits can also result in costly metal bulges, blisters and failure of boiler tubes.
TABLE 2 – Boiler Scale Thickness vs. Energy Loss
Calculating the Cost of Scale in a Cooling Operation
Increased Cost = T x H x P x R x E
T = System Tonnage
H = Hours of Operation
P = % increase energy consumption
R = Energy Rate ($/kWh)
E = Energy Required (kW/(h ton)
500 ton Air Conditioning Plant with scale deposit of .025 in
T = 500
H = 720 hours per month
P = 22% increase
R = $.11/kWh
E = .75 kWh/(h ton)
Increased cost = $6,534 per month