All of us have been guilty at one time or another of defining what a cooling tower is doing in terms of range hot water
temperature (HWT) minus the cold water temperature (CWT). Quite often this being accepted as a level of performance of the
cooling tower. Nothing could be further from the truth. Heat load in BTU’s per hour is equal to 500 times the circulating water
flow in gallons per minute (GPM) times range (Q=500 x GPM x Range).
Now heat load, of course, is supplied to the unit being served by the cooling tower. The cooling tower itself is neither a heat
source nor a heat sink. In the usual circulating system the heat load is independent of the cooling tower. The number 500 is a
constant to convert (gallons per minute) to [(pound per hour) (specific heat)]. Therefore, it is also independent of the cooling
tower. The circulating water flow in gallons per minute is determined by the number of pumps running and the pressure drop
in the overall circulating water system. Therefore, it likewise is independent of the cooling tower.
All these foregoing items can quite easily be understood. However, the next step that now becomes obvious is probably the
most misunderstood of all cooling tower performance information. This is the fact that if heat load, the constant, and the
circulating water flow are all independent of the cooling tower, than by mathematical deduction the range is also completely
independent of the cooling tower.
Now this may be somewhat hard to accept, but a quick recheck of the basic equation will prove the validity of the statement.
Therefore, the range is the same whether there is a two cell cooling tower or a four cell cooling tower. The range would be the
same if the fans were on full speed, half speed, or turned off. Such a statement as “My cooling tower is not performing
because I bought it to cool 20°F and it is only cooling the water 10°F” has no validity. Likewise, the converse is true.
Someone who has a cooling tower that is cooling the water 30°F whereas it was designed to cool 20°F, may not be in such a
fortunate position as he or she might imagine.
Both of these cases show no indication whatsoever of actual thermal capacity of the cooling tower. What then is a measure of
the thermal capacity of the cooling tower? It is not the amount of heat being rejected, rather it is the level at which this heat is
rejected. The measure of the performance of the cooling tower is the resultant cold water temperature, or even more
specifically, the approach (CWT - WBT) under the given conditions.
Cold water temperature is the primary independent variable, and vividly indicates the cooling tower capability. For example, a
cooling tower might be designed for cooling water from 110°F to 85°F at a 78°F wet bulb. This gives us a design range of 25°
F at an approach of 7°F. If the cooling tower in actual operates at these temperatures, then it is performing. However, it could
be operating at the conditions of 120°F hot water to 95°F cold water at a 78°F wet bulb. In this instance, the cooling range
would be the same 25°F. So it could be said that the cooling tower was cooling the water 25°F in strict accordance with
design. However the approach would have opened up from 7°F to 17°F and this cooling tower would be operating at about
50% of the original performance design.
Now on the surface this all appears rather straightforward and we have the tendency to think, “No, we would not have fallen
into this misconception”. However, if you will honestly think about it, how often have you mentally evaluated your cooling tower
by considering how many degrees it cooled the water being pumped over it?
We often blame the cooling tower or praise the cooling tower, when in truth, all we are seeing is a reflection of the percent of
design heat load at which the unit is being operated. A unit being operated at 50% over its design capacity will automatically
load the cooling tower to 50% over design range as long as the water flow is maintained constant. However, this by no means
indicates the cooling tower is operating at a 50% increased capacity over its design. Conversely, and quite importantly, a
cooling tower can make and dissipate this load in accordance with our previous statements. When someone says, “Will you
please consider upgrading my cooling tower so that it may handle an additional 20% of heat load or BTU’s per hour?”, this is
quite easily done. In fact, the waving of a ‘magic wand’ and ‘Blessing’ the cooling tower can quite easily accomplish this feat.
The cooling tower will naturally and automatically cool the water through whatever range is being demanded by the unit.
Therefore, we must always augment our request for additional heat load capacity with one more stipulation. Namely, the cold
water temperature, or more specifically, the approach. Therefore, at a given heat load, GPM, and WBT, the temperature level
at which the heat is rejected is the measure of the cooling tower performance. Please contact us if you should have any
further questions.
temperature (HWT) minus the cold water temperature (CWT). Quite often this being accepted as a level of performance of the
cooling tower. Nothing could be further from the truth. Heat load in BTU’s per hour is equal to 500 times the circulating water
flow in gallons per minute (GPM) times range (Q=500 x GPM x Range).
Now heat load, of course, is supplied to the unit being served by the cooling tower. The cooling tower itself is neither a heat
source nor a heat sink. In the usual circulating system the heat load is independent of the cooling tower. The number 500 is a
constant to convert (gallons per minute) to [(pound per hour) (specific heat)]. Therefore, it is also independent of the cooling
tower. The circulating water flow in gallons per minute is determined by the number of pumps running and the pressure drop
in the overall circulating water system. Therefore, it likewise is independent of the cooling tower.
All these foregoing items can quite easily be understood. However, the next step that now becomes obvious is probably the
most misunderstood of all cooling tower performance information. This is the fact that if heat load, the constant, and the
circulating water flow are all independent of the cooling tower, than by mathematical deduction the range is also completely
independent of the cooling tower.
Now this may be somewhat hard to accept, but a quick recheck of the basic equation will prove the validity of the statement.
Therefore, the range is the same whether there is a two cell cooling tower or a four cell cooling tower. The range would be the
same if the fans were on full speed, half speed, or turned off. Such a statement as “My cooling tower is not performing
because I bought it to cool 20°F and it is only cooling the water 10°F” has no validity. Likewise, the converse is true.
Someone who has a cooling tower that is cooling the water 30°F whereas it was designed to cool 20°F, may not be in such a
fortunate position as he or she might imagine.
Both of these cases show no indication whatsoever of actual thermal capacity of the cooling tower. What then is a measure of
the thermal capacity of the cooling tower? It is not the amount of heat being rejected, rather it is the level at which this heat is
rejected. The measure of the performance of the cooling tower is the resultant cold water temperature, or even more
specifically, the approach (CWT - WBT) under the given conditions.
Cold water temperature is the primary independent variable, and vividly indicates the cooling tower capability. For example, a
cooling tower might be designed for cooling water from 110°F to 85°F at a 78°F wet bulb. This gives us a design range of 25°
F at an approach of 7°F. If the cooling tower in actual operates at these temperatures, then it is performing. However, it could
be operating at the conditions of 120°F hot water to 95°F cold water at a 78°F wet bulb. In this instance, the cooling range
would be the same 25°F. So it could be said that the cooling tower was cooling the water 25°F in strict accordance with
design. However the approach would have opened up from 7°F to 17°F and this cooling tower would be operating at about
50% of the original performance design.
Now on the surface this all appears rather straightforward and we have the tendency to think, “No, we would not have fallen
into this misconception”. However, if you will honestly think about it, how often have you mentally evaluated your cooling tower
by considering how many degrees it cooled the water being pumped over it?
We often blame the cooling tower or praise the cooling tower, when in truth, all we are seeing is a reflection of the percent of
design heat load at which the unit is being operated. A unit being operated at 50% over its design capacity will automatically
load the cooling tower to 50% over design range as long as the water flow is maintained constant. However, this by no means
indicates the cooling tower is operating at a 50% increased capacity over its design. Conversely, and quite importantly, a
cooling tower can make and dissipate this load in accordance with our previous statements. When someone says, “Will you
please consider upgrading my cooling tower so that it may handle an additional 20% of heat load or BTU’s per hour?”, this is
quite easily done. In fact, the waving of a ‘magic wand’ and ‘Blessing’ the cooling tower can quite easily accomplish this feat.
The cooling tower will naturally and automatically cool the water through whatever range is being demanded by the unit.
Therefore, we must always augment our request for additional heat load capacity with one more stipulation. Namely, the cold
water temperature, or more specifically, the approach. Therefore, at a given heat load, GPM, and WBT, the temperature level
at which the heat is rejected is the measure of the cooling tower performance. Please contact us if you should have any
further questions.
|
| Headquarters: 1310 West Main Street La Porte, TX 77571 (office) 281-484-2665 (fax) 281-484-2371 |
| (C) 2010 Cooling Towers of Texas, All rights reserved. |
| ARTICLES |