CHAPTER 1 | Thermal Characteristics

The latent heat of vaporization has long been used to transfer heat to the atmosphere. It is safe to say that the process has existed since the beginning of man with our own bodies providing perhaps the best example. Our sweat glands spread water over our skin and our bodies benefit from the cooling effect that occurs when the liquid evaporates into vapor.

Suppose someone was issued a bucket of water and told to make the water change to vapor as quickly as possible. Several strategies would likely develop.

One strategy might be to find a second empty bucket and pour the water back and forth to expose more of the water to air. The falling water could be made to splash into droplets to increase the surface area exposed to the air. Better yet, the bucket could be dumped out onto a large surface for maximum exposure to air.

Next, a fan could be added to flow air over the water to encourage moisture laden air to exit and be replaced with new air that is better able to accept more vapor.

Lastly, if the quality of air could be manipulated, 'dry' air- air that contains very little moisture- would be used because of its ability to accept greater amounts of water vapor.

If the scenario were modified such that:
-New water was added to replace that lost to evaporation,
- The water was continuously recirculated over the surface, and,
- The runoff was continuously heated...

We would have all the elements of an operational cooling system.

For each pound of water that a cooling tower evaporates, it removes somewhere near 1000BTU's from the water that remains. The more evaporation that takes place, the more heat that is removed. Additional heat is taken away by the air by virtue of its temperature increase but this sensible heat exchange is minor compared to the latent component provided by the water's phase change.

To select a cooling tower, the water flow rate, water inlet temperature, water outlet temperature and ambient wet bulb temperatures must be known.

Wet bulb temperature is a site condition measured by constructing what amounts to a tiny cooling tower that can be held in one hand. This small 'cooling tower' has no heat input and is used to determine the lowest leaving water temperature a cooling tower could possibly attain accurately predicting the performance of a larger, operational counterpart. This tiny 'cooling tower' is called a sling psychrometer. It places a thin film of water on the bulb of a thermometer. The thermometer is twirled in the air. After a few seconds, the thermometer begins to show a reduced temperature reading. Twirling it more will yield successively lower temperature readings until a final 'low' temperature reading can be made after about one minute. Additional twirling is no help; This low reading is called the 'wet bulb' temperature.

It is necessary to insure that the thin water film be maintained. A cotton sock connected to a small water reservoir is typically employed.

Some psychrometers use a small battery operated fan so that the operator doesn't have to twirl the device in the air. Both types also have a non wetted thermometer that reads what is called the 'dry bulb' temperature.

A comparison of wet and dry bulb readings allows the relative humidity to be determined from a psychrometric chart. The wet bulb temperature is always lower than the dry bulb value except when the air is already saturated with water - 100% relative humidity. This is when the wet and dry bulb temperatures are the same. The air will no longer accept water and the lack of evaporation does not allow the wetted bulb to reject heat into the air by evaporation. This situation would be similar to operating a cooling tower at 100% relative humidity. The only rejected heat is that which is responsible for increasing the air temperature.

A single wet bulb reading will allow a prediction of cooling tower performance at that unique condition but the wet bulb changes throughout the day and year.

The design wet bulb is typically determined by reviewing a chart that has been prepared by taking numerous readings in a particular area over several years and determining the maximum wet bulb readings.

The wet bulb can be thought of as the heat sink temperature to a cooling tower. The lower the wet bulb, the drier the air, the more moisture it will accept and the more heat a given cooling tower is capable of rejecting.

When sizing a cooling tower, then, the highest anticipated wet bulb should be used. During the rest of the time, the cooling tower is oversized for the duty. The leaving water temperature will simply be less than design which is typically desirable.

A wet bulb chart is arranged to show the frequency of occurrence. At the Los Angeles airport, for example:

67 degree wet bulb is exceeded 2% of summertime hours
69 degree wet bulb is exceeded .5% of summertime hours
71 degree wet bulb is exceeded .1% of summertime hours

Generally, the designer would select 71 degrees as the design wet bulb for a situation like this but some installations aren't critical allowing the use of a reduced design values and smaller cooling towers. Other installations may work only in the winter or at night when the wet bulb temperature is low. The designer must select the design wet bulb for his/her project. When in doubt, select the highest anticipated wet bulb temperature to insure satisfactory year around operation.

The designer should only consider towers with independently certified capacities. The Cooling Tower Institute (CTI) lists towers that subscribe to their test standard STD-201. Alternately, the designer should specify a field test by an accredited independent test agency in accordance with CTI Acceptance Test Code ATC-105 or ASME PCT-23. (See Ch. 13.)

Manufacturer's catalogs have cooling tower selection charts with easy to follow instructions that begin with the calculation of two values:

Range = Inlet temp. - Outlet temp.

Approach = Outlet temp - Wet Bulb Temp.

These values coupled with the flow rate and wet bulb temperature allow the selection of a cooling tower.

Those new to cooling towers should make several selections at different wet bulb temperatures to test how wet bulb relates to cooling tower size. It becomes clear that the tower size increases as the wet bulb rises and that the size increase becomes dramatic as the approach is in the 'less than ten degrees' area. This exercise demonstrates how to oversize a cooling tower... just use an inflated design wet bulb temperature. This is better than artificially inflating the flow rate and possibly oversizing the nozzles.

Increasingly, manufacturers offer software to make selections easier. Calculating the heat transfer and water evaporation rates are best shown by example.

ex.: A cooling tower cools 900gpm from 95 to 85 degrees.
-What is the heat rejection?
-What is the evaporation rate?

Heat rejection = 900gal/min x 10 DegF x 8.33#/gal x 60min/Hr x 1BTU/lb. DegF = 900 x 10 x 500 = 4,500,000 BTU/Hr

Or,Heat RejectionBTU/hr = FlowGPM x RangeDegF x 500

Evaporation Rate = Heat Rejection / 1000BTU/# = FlowGPM x RangeDegf x 500 / 1000BTU/# x gal/8.33# x hr/60min

Or, Evaporation RateBTU/hr = FlowGPM x RangeDegF / 1000

People will sometimes have the mistaken notion that the cooling tower dictates the rate of heat transfer- it doesn't. A cooling tower simply gives up the heat it is given. If the cooling tower is 'big', it may accomplish the job by cooling water from 90 to 80DegF. If it is 'small', it might cool the water in the same process from 100 to 90 DegF. In either case, the heat transfer and evaporation rates are the same (as expected when reviewing the previous equations). The size of the cooling tower, the flow rate and the wet bulb temperature determine the inlet and outlet water temperatures- but not the difference between them.

Increased cooling tower performance can be achieved by adding surface area or by boosting the cfm. The former is considerably more expensive than the latter inasmuch as a cfm increase can be as simple as employing a bigger fan motor allowing increased fan speed. Cooling towers must be evaluated on a life cycle cost basis. Spending a little more for a tower that uses less horsepower or lasts longer is almost always the wisest decision.

The most common use for cooling towers is in air conditioning as the heat rejecter in a mechanical refrigeration system.

The word 'ton' comes from this application and deserves some discussion here. Its origin goes back to earlier days when theaters, concert halls and the like were cooled with ice. Typically, this ice was harvested from lakes and stored for summer use. At its eventual destination, it would be placed in bunkers where circulated air would melt the ice and cool the air. We can imagine a theater owner ordering a ton of ice one day and maybe 1.5 tons on another.

With the introduction of mechanical refrigeration, the term 'ton' was retained. The owner could now buy a system capable of providing the equivalent capacity of however many 'tons' he wanted.

Since one pound of ice absorbs 144BTU when melting, one ton of ice melting over a period of 24 hours has a heat transfer rate of:
2,000# x 144BTU/# x 1/24hrs = 12,000BTU/Hr.
So, when the occupants of the room experience heat removal at the rate of 12,000BTU/Hr, they are enjoying one ton of cooling.

The mechanical refrigeration system utilizes a compressor that adds its heat energy- basically the motor horsepower- to the refrigerant. The cooling tower must reject not only the 12,000BTU/Hr/Ton from the space but also the heat of compression as well. This added load typically amounts to about 3,000BTU/Hr/Ton for air conditioning systems. So, while the occupants are enjoying 12,000 BTU/Hr/Ton heat removal, the cooling tower is rejecting heat at a rate close to 15,000BTU/Hr/Ton.

This gives rise to the term 'Cooling Tower Ton' which is defined as 15,000BTU/Hr. This definition is only valid for typical air conditioning conditions.

Another common term is 'nominal capacity'. It also has its roots in air conditioning and involves the following assumptions:
- The cooling tower will circulate water at the rate of 3gpm/ton
- Water enters the tower at 95 degreesF and exits at 85 degreesF
- Design wet bulb temperature is 78 degreesF.

Evaluating cooling tower capacity at the nominal conditions is an easy way to determine which tower is 'bigger' when comparisons are made.

Someone employing mechanical refrigeration at 95/85 degreeF condenser water conditions in a city that actually has a 78 degreeF wet bulb temperature can use the nominal ratings as an accurate prediction for his/her cooling tower performance.

ex: A space to be air conditioned has a heat load of 300 tons. A cooling tower with a 300 ton nominal rating cools 900gpm from 95 to 85 degreesF with an ambient wet bulb temperature of 78 degreesF. The space has heat removed at the rate of 300 tons x 12,000BTU/Hr/Ton =3,600,000 BTU/Hr and the cooling tower rejects 4,500,000 BTU/Hr
( = 900Gpm x 10DegF x 500) or, 15,000BTU/Hr/Ton.

Someone in another city, say Los Angeles, where the design wet bulb may be 72 DegF, may employ a cooling tower with a substantially lower nominal rating when serving the same size installation due the fact that the cooling tower works 'easier' when rejecting heat to a lower wet bulb temperature.

ex: A space to be air conditioned has a heat load of 300 tons. A cooling tower with a nominal rating of 235 tons cools 900gpm of water from 95 to 85 DegF with an ambient wet bulb of 72 DegF (Six degrees lower than the previous example.) Heat from the space is removed at the rate of 300 Tons x 12,000 BTU/Hr/Ton = 3,600,000 BTU/Hr and the cooling tower rejects 4,500,000 BTU/Hr

The examples are the same except for wet bulb temperature and cooling tower size. A reduced wet bulb temperature allows the use of a smaller cooling tower. The student will find the converse true at wet bulb temperatures above the nominal 78 degree value... The nominal rating of the cooling tower on a 300 ton project at 80 degree wet bulb will be more than 300 tons (actually, about 360 tons).

Absorption refrigeration is another method of making chilled water for air conditioning. Instead of a compressor, the design utilizes heat energy to increase the pressure of a refrigerant. An absorption system by its nature requires a cooling tower that will remove about 50% more heat than a mechanical refrigeration system of the same capacity. The cooling tower flow rate, range, or a combination of the two must be increased so that their product is about 50% more than that for a mechanical refrigeration system.

Altitude has an effect on cooling tower performance but in a unique way. Air handlers, air cooled condensers and the like are typically made to operate at higher speeds (or, with a steeper fan pitch) as altitude increases in order to maintain the same mass flow. Air at 5,000ft, for example, is approximately 17% less dense than at sea level and the fan speed is typically increased 17% to compensate.

Surprisingly, though, a cooling tower designed for operation at sea level will work just fine at 5,000 ft elevation without modification. This is because air at reduced atmospheric pressure will accept increased amounts or water. [Anyone with an air compressor knows that compressed air doesn't hold much moisture; Water collects in the tank. It seems reasonable that the opposite occurs at reduced air pressures.]

The increased ability for the air to accept more water offsets the reduced air mass resulting in a small net gain in capacity at altitude. This is why manufacturers do not make altitude corrections with their small, package towers.

The astute designer will recognize the potential for squeezing more out of such a tower. The fan motor is not fully taxed when moving less air so why not load the motor more fully by speeding up the fan (or repitching its blades) thereby achieving even more capacity? This in fact can be done but the manufacturer must be consulted to avoid any problems with critical speeds, bearing loads, eliminator performance, etc.


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