The Evaporative Condenser – New Developments
by Art Sutherland T
he condenser plays a vital role in the refrigeration process, as it is the component that rejects all of the heat absorbed by the chiller, plus the heat of compression
added by the compressor. If not handled properly, the refrigeration capacity, energy efficiency and longevity of the entire refrigeration system suffers. The evaporative condenser is matched to the system
compressor capacity and takes into account the maximum local ambient wet bulb temperature as provided by local weather station historical data. A condenser with higher capacity will provide lower condensing temperatures. Te following chart shows the same refrigeration compressor operating at a variety of condensing temperatures.
CONDENSING TEMP
75 F 85 F 95 F
105 F
DISCHARGE TEMP
193 F 214 F 235 F 254 F
CAPACITY
HORSE- POWER
59.3 tons 53.6 56.5 tons 60.0 53.5 tons 66.1 50.5 tons 71.8
COEFFICIENT OF PERFORMANCE
5.22 4.44 3.82 3.32
Te left-hand column shows the discharge temperature
starting at 75 F and rising in 10 F increments to 105 F. Te second column illustrates the increase in the compressor discharge temperature as the condensing temperature rises. It is a well- accepted rule of thumb that you cut oil life expectancy by half for every 20-degree rise above normal operating temperature. Compressors operating at higher discharge temperatures experience a significantly higher rate of component failures. Te third column illustrates the significant decrease in refrigeration capacity with rising condensing temperatures and the fourth column shows the negative effect rising condensing temperature has on compressor horsepower. Te coefficient of performance
8
The Green Box fluid cooler with adiabatic cooling design keeps the heat exchanger fins dry while still benefitting from the efficiency of evaporation.
is a summary of the refrigeration capacity and horsepower requirement in a single figure and shows that for every degree the condensing temperature increases it results in a loss of system performance of approximately 1.5 percent. In selecting a larger, more efficient condenser, it is also
important to consider existing heat recovery heat exchangers such as those used for snow melt pits. If your condensing temperature is too low there is a chance that the existing heat recovery initiatives will be reduced. With proper integrated system design you can operate at lower condensing temperatures for improved system performance and your heat recovery systems can work properly as well. An evaporative condenser takes advantage of evaporating
water to assist with the cooling process. Te benefit is driven by the fact that the wet bulb temperature is almost always lower than the corresponding dry bulb temperature. In many geographic areas, the depressed wet bulb temperature can be significantly lower than the dry bulb temperature. As an example, consider that the design dry bulb temperature of Las Vegas in summer is 108.4 F, whereas the corresponding wet bulb temperature is 67.8 F. Tis 40.6 F difference would easily improve system performance by over 50 percent if using an evaporative condenser rather than an air-cooled condenser! When water evaporates, the distillation process leaves minerals
behind. Te potential for the quantity of minerals left behind is dependent on the hardness of the water in your geographic area and the amount of run time that your system operates daily. In some areas, as much as 20 pounds of mineral per day can be produced, which can result in reduced refrigeration efficiency if left on the condenser coils. One manufacturer has claimed that a scale deposit as thin as 1/32” can reduce heat transfer by 27 percent. Te condenser evaporates approximately 0.24 pounds of
water per minute per ton of refrigeration while in operation. So if your facility has a 100-ton plant that operates 14 hours per
ISI EDGE SUMMER 2016
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