Process Cooling
Overview
Process cooling can be an expensive. In general, we use the following guidelines when
trying to reduce cooling costs.
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4.
Eliminate "once-through" cooling.
Use cooling towers rather than chillers when feasible.
Apply for sewer exemption on cooling tower make-up water.
Use gas-powered chillers rather than electric chillers when cost-effective.
Cooling Towers
Tower Performance
A cooling tower is a counter-flow or cross-flow heat exchanger that removes heat from
water and transfers it to air. Cooling towers come in many configurations. Induced-draft
cooling towers, such as the one shown below, generally use less fan power and have short
circuit less air than forced-draft cooling towers.
Figure 1. Induced-draft cross-flow cooling tower (Source: ASHRAE Handbook: HVAC
Systems and Equipment, 2000)
The temperature difference of water through a tower, dT = Tw1-Tw2, is determined by
the load, Ql, and the mass flow rate of water, mw. Neither the size of the tower nor the
state of the outside air influences the temperature difference; however, larger towers or
lower outdoor air wet-bulb temperatures will decrease the exit water temperature, Tw2.
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Typically, most towers are sized for a 10 F temperature difference and about 2.4 gpm/ton
of cooling. Fan motor hp is about 0.1 hp/ton and air flow rates are about 2,000 cfm/hp.
The temperature of water from a cooling tower, Tw2, can be calculated based on tower
performance data such as that shown below, water flow rate, cooling load, and the
ambient wet-bulb temperature. This process can be automated in software to predict
cooling tower performance with varying ambient conditions. For example, CoolSim
(Kissock, 1997) calculates exit water temperatures, and the fraction of time that a cooling
tower can deliver water at a target temperature, based on entering water temperature,
Tw1, and TMY2 weather data. This information is useful in determining how often a
cooling tower can replace a chiller in cooling applications.
Figure 2. Typical cooling tower performance curve (Source: ASHRAE Handbook:
HVAC Systems and Equipment, 2000).
Sensible and Latent Cooling
Depending on the entering air and water temperatures, the water may be cooled by
sensible and latent cooling of the air, or simply by latent cooling of the air. In either case,
latent, i.e. evaporative, cooling is dominant. For example, consider the case in which the
air enters at a lower temperature than the water (Figure 3a). The air will leave
completely saturated and the cooling is part sensible and part latent. The sensible portion
occurs as the air temperature increases by absorbing heat from the water. The latent
portion occurs as some of the water evaporates, which draws energy out of the water.
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If the air enters at the same wet bulb temperature as before, but at a higher dry-bulb
temperature than the water, then the air will cool as it saturates (Figure 3b). Thus, the
sensible cooling component is negative, and the all the cooling is due to evaporation. In
general, cooling is dominated by latent cooling.
Figure 3. Psychrometric process lines for air through a cooling tower, if the entering air
temperature is a) less than the entering water temperature, and b) greater than the
entering water temperature.
The total cooling, ma (ha2 – ha1) is the same for both cases since enthalpy is a function
of wet-bulb temperature alone. However, the dry-bulb temperature significantly
influences the evaporation rate, mwe = ma (wa2-wa1). The rate of evaporation increases
as the dry-bulb temperature increases for a given wet-bulb temperature.
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Evaporation Rate
As discussed in the previous section, cooling in cooling towers is dominated by
evaporation. The evaporation rate can be calculated from the pyschrometric relations in
the previous section, if the inlet and exit conditions of the air are known. For example,
consider the case in which the cooling load, Ql, mass flow rate of air, ma, (which can be
calculated based on the fan cfm and specific volume of the inlet air), and inlet conditions
of air are known. The enthalpy of the exit air, ha2, can be calculated from an energy
balance.
Ql = ma (ha2 – ha1)
ha2 = ha1+ Ql / ma
The state of the exit air can be fixed by assuming that it is 100% saturated with an
enthalpy ha2. The evaporation rate, mwe, can be determined by a water mass balance on
the air.
mwe = ma (wa2- wa1)
The fraction of water evaporated is:
mwe / mw
Using this method for entering air temperatures from 50 F to 90 F, we determined that the
fraction of water evaporated typically ranges from about 0.5% to 1%, with an average
value of about 0.75%.
Another way to estimate the fraction of water evaporated is to assume that all cooling, Ql,
is from evaporation, Qevap. The cooling load Ql, is the product of the water flow rate,
mw, specific heat, cp, and temperature difference, dT. The evaporative cooling rate is the
product of the water evaporated, mwe, and the latent heat of cooling, hfg.
Ql = Qevap
mw cp dT = mwe hfg
Assuming the latent heat of evaporation of water, hfg, is 1,000 Btu/lb, and the
temperature difference of water through the tower, dT, is 10 F, the fraction of water
evaporated is:
mwe / mw = cp dT / hfg = 1 (Btu/lb-F) x 10 (F) / 1000 (Btu/lb) = 1%
If on average, 75% of the cooling were from evaporation and 25% from sensible cooling,
then the evaporation rate would be:
75% x 1% = 0.75%
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Thus, both methods suggest that 0.75% is a good estimate of for the rate of evaporation;
however, we have seen manufacturer data indicating average evaporation rates as low as
0.30%. Water lost to evaporation should not be subjected to sewer charges. Typical
sewer charges are about $2.20 per hundred cubic feet.
Some water may be lost as water droplets are blown from the tower by oversized fans or
wind. This type of water loss is called “drift”. Drift rates are typically about 0.2% of
flow (ASHRAE Handbook, HVAC Systems and Equipment, 2000); however, we
generally assume that drift losses are included in the 0.75% evaporation rate.
Water Treatment and Blow Down Rate
Cooling tower water must be treated to prevent bacterial growth and maintain the
concentration of dissolved solids at acceptable levels to prevent scale and corrosion.
Bacterial Growth
The typical method of controlling bacterial growth is to add biocides at prescribed
intervals and to keep the cooling tower water circulating. If the tower will not be
operated for a sustained period of time, then the cooling water should be drained.
Dissolved Solids
Water evaporated from a cooling tower does not contain dissolved solids. Thus, the
concentration of dissolved solids will increase over time if only enough water is added to
the tower to compensate for evaporation. To maintain the dissolved solids at acceptable
levels, most towers periodically discharge some water and replace it with fresh water.
This process is called blow down. It the level of dissolve solids increases too high, scale
will be begin to form, and/or the water may become corrosive and damage piping,
pumps, cooling tower surfaces and heat exchangers. Usually, the primary dissolved solid
to control is calcium carbonate CaCO3.
Blow down can be accomplished by continuously adding and removing a small quantity
of water, periodically draining and refilling the cooling tower reservoir, or by metering
the conductivity of water and adding fresh water only when needed. By far the most
efficient method is to meter the conductivity of water, which increases in proportion to
the level of dissolved solids, and add fresh water only when needed.
The required quantity of blow down water depends on the acceptable quantity of
dissolved solids in the tower water, PPMtarget, the quantity of dissolved solids in the
makeup water, PPMmu, and the evaporation rate, mwe. The target level of dissolved
solids is typically quantified in cycles, where:
Cycles = PPMtarget / PPMmu
For example, if the quantity of dissolved CaCO3 in the makeup water, PPMmu, is 77 ppm
and the maximum level to prevent scaling, PPMtarget, is 231, then the cooling tower
water must be maintained at three cycles:
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Cycles = PPMtarget / PPMmu = 231 ppm / 77 ppm = 3
By applying mass balances, it can be shown that the blow down water required to
maintain a certain number of cycles is
mwbd = mwe / (Cycles –1)
The total makeup water required mwmu, is the sum of the water added for evaporation
and blow down:
mwmu = mwe + mwbd
For example for a 1,000 gpm tower with a 0.75% evaporation rate and CaCO3
concentration at 3 Cycles, the quantity of makeup water required would be about:
mwe = (mwe/mw) x mw = 0.75% x 1,000 gpm = 7.5 gpm
mwbd = mwe / (Cycles –1) = 7.5 gpm / (3 – 1) = 3.75 gpm
mwmu = mwe + mwbd = 7.5 gpm + 3.75 gpm = 11.25 gpm
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