Premier Industries, Inc.
MEASURING DISCHARGE TEMPERATURES
for
PREMIER EVAPORATIVE COOLING EQUIPMENT
GENERAL:
To accurately measure the discharge temperature from an Evaporative Cooling unit, the following conditions and procedures must be followed:
1. Climatic Conditions of ambient Dry Bulb and Wet Bulb temperatures must be accurately determined at the time of the test. The wet bulb temperature is the controlling factor in direct evaporative cooling. The discharge temperature on a unit with 12" thick rigid media with 45/15 degree flute arrangement will usually be about 2 to 4 degrees above the wet bulb temperature with a face velocity of the air flow through the media between 400 and 500 feet per minute. Refer to the table below for a more specific saturation efficiency which determines the predicted discharge temperature at different media thickness and face velocities.
2. Dry bulb temperature
readings must be made within 12" of the media on the discharge side. Multiple
readings should be made on a grid pattern across the width and height of
the media. The readings should be totaled and divided by the number of
readings to obtain an accurate discharge temperature. The illustration
above describes the typical method.
Important note: Heat gain
occurs in the air stream from the point of immediate discharge from the cooling
media to the delivery point of the air. This heat gain cannot be
accurately pre-determined. The performance of the cooling unit must be
determined from a controllable, predictable point.
3. Water flow rate is important to the cooling efficiency of the media. Too little or too much water flow adversely affects the cooling efficiency. The proper flow rate is an amount equal to the evaporation rate plus an amount for washing the media. Common practice is to set the water flow rate at 3 times the evaporation rate. Since the evaporation rate changes with climatic conditions, the media manufacturers recommend a setting that equals approximately 1.5 gallons per minute for each square foot of the top surface area of the media. This is easily calculated by the formula; media width x depth (feet) x 1.5. I.E. a 10' wide unit with 12" media depth would need a flow rate of (10 x 1 x 1.5) 15 gallons per minute. Flooding the media with excess water greatly reduces the evaporation process which in turn reduces the cooling efficiency of the media. Too much water causes raw moisture carry-over in the air stream causing early deterioration of the metal parts in the air stream such as blower, motor, ductwork, equipment being cooled, etc.
4. All measurements should be made after an initial break-in period for the media of about 72 hours of wetted operation.
5. Water quality also affects cooling efficiency. Only pure water evaporates leaving minerals behind. The purer the water, the higher the evaporation rate and higher cooling efficiency attained. Build up of minerals on the face of the media can affect air flow volume cooling efficiency when it becomes excessive.
6. Even the orientation of the evaporative cooling unit can affect performance. The final test report made by professional testing of Gas Turbine inlet air evaporative coolers at Clark Station is shown below. Note the difference on North and South facing units efficiency. Time of day must also be considered. West facing units tested in the afternoon can reflect different results than when taken in the morning.
7. Evaporative coolers efficiency is controlled by the wet bulb temperature. Therefore, when the wet bulb is high, the discharge temperature will be high. When the wet bulb is low, the discharge temperature will be low. An example of this would be; 90(f) dry bulb and 70(f) wet bulb should discharge 72(f) dry bulb (based on 12" media @ 450 - 500 FPM velocity. Same example if the dry bulb/wet bulb were 110/70. The discharge temperature should be 74(f). The 20 degree rise in dry bulb only increased discharge temperature by 2(f) degrees. An extreme example would be; 120(f) dry bulb and 65(f) wet bulb should discharge 70.5(f). Understanding this basic concept helps in the testing and expectations of evaporative cooling.
8. Predicted discharge temperature is
determined by the following formula using 12" thick rigid media, 45/15 flute
arrangement and 450 - 500 FPM velocity:
Example: 100Db/70Wb with 90% saturation (cooling) efficiency
Step 1: Dry bulb - wet bulb = wet bulb depression (differential)
Step 2: Percent efficiency * Wet bulb depression = temperature drop
Step 3: Dry bulb entering - temperature drop = dry bulb temperature
leaving
100Db - 70Wb = 30Db x .9 (SE) = 27Db drop
100Db entering - 27Db drop = 73Db discharge temperature
Evaporative Cooling Media Saturation Efficiency and Pressure drop Table
|
Face Velocity |
Percent Media Efficiency at Media Depth: |
Static Pressure Drop at Media Depth: |
||||||||||
|
4" |
6" |
8" |
12" |
16" |
24" |
4" |
6" |
8" |
12" |
16" |
24" |
|
|
200 FPM |
71% |
86% |
91% |
96% |
99% |
99% |
0.02" |
0.03" |
0.04" |
0.06" |
0.08" |
0.09" |
|
300 FPM |
67% |
81% |
88% |
94% |
98% |
99% |
0.03" |
0.05" |
0.07" |
0.10" |
0.13" |
0.19" |
|
400 FPM |
62% |
77% |
84% |
92% |
96% |
99% |
0.05" |
0.09" |
0.11" |
0.18" |
0.25" |
0.31" |
|
500 FPM |
59% |
72% |
82% |
89% |
94% |
99% |
0.09" |
0.12" |
0.17" |
0.26" |
0.36" |
0.50" |
|
600 FPM |
57% |
70% |
80% |
88% |
92% |
99% |
0.12" |
0.18" |
0.22" |
0.36" |
||
Summary: As indicated above (and below) the process of accurately determining the discharge temperature from an evaporative cooling unit is more complicated that at first thought. Even the determination of ambient dry bulb and wet bulb temperatures requires accuracy. Proper instruments should be used to determine all the relevant information needed to make this determination.
Nevada Power Company
Date: May 6, 1998
To: Whom it may concern
On April 29, 1998, a test to determine the overall evaporative cooling effectiveness was performed on the following equipment:
640,000 cfm replacement evaporative cooler and filter housing. This unit was supplied to Nevada Power Company, Clark Station, Unit #7, by Premier Industries, Inc. of Phoenix, Arizona. It is situated in Las Vegas, Nev. The cooler and filter housings replaced existing cooler/filter housings manufactured by Ecodyne Corporation.
The turbine is a Westinghouse,
501B-6, dual fuel Econopac Model, rated new at 78 MW.
The evaporative cooler system is comprised of 4 separate modules,
and was guaranteed to attain 95% effectiveness overall. The following are the
results of the acceptance test.
Eastside North
attained 100.00% effectiveness
South attained 96.72%
effectiveness
Westside North
attained 100.00% effectiveness
South attained 94.63%
effectiveness
The overall cooler effectiveness was accepted as 97.92%.
Any questions, feel free to contact me,
Dale Stucki
