Energy Efficient Buildings

Cooling Towers

Introduction

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. An induced-draft cooling tower, which is common in HVAC and industrial applications, is shown in Figure 1. As warm water from the process falls through the tower, some of it evaporates, which cools the remaining water. The cooled water collects at the bottom of the cooling tower and is returned to the plant where it is used for cooling. Figure 2 shows an evaporative condenser, which is common in industrial refrigeration applications. Water, which is cooled by evaporation, falls over a closed heat exchanger (usually carrying refrigerant) in the top part of the tower. It then falls over more fill to enhance evaporation in the lower part of the tower. A small pump circulates water from the bottom to the top of the tower.

Figure 1) open circuit cooling tower

Figure 2) closed circuit evaporative cooling tower

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.

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.

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.

Cooling Towers as Heat Exchangers

Based on the previous discussion, it is clear that cooling tower performance is a function of the wet-bulb temperature of the entering air. In an infinite cooling tower, the leaving air wet-bulb temperature would approach the entering water temperature, and the leaving water temperature would approach the web-bulb temperature of the entering air. The difference between the leaving water temperature and the entering air wet-bulb temperature is called the approach. The relationship between air wet-bulb and water temperature is shown in the figure below. In an infinite cooling tower, the approach would be zero.

Source: ASHRAE Handbook, HVAC Systems and Equipment, 2004.

Neglecting fan power and assuming steady state operation, an energy balance on a cooling tower gives:

mw1 cpw Tw1 – mw2 cpw Tw2 + ma (ha1 – ha2) = 0

Assuming steady state operation, a mass balance on water flow gives:

mw1 – mw2 + ma (wa1 – wa2) = 0

mw2 = mw1 + ma (wa1 – wa2)

Substituting mw2 into the energy balance gives:

mw1 cpw Tw1 – [mw1 + ma (wa1 – wa2)] cpw Tw2 + ma (ha1 – ha2) = 0

mw1 cpw Tw1 – mw1 cpw Tw2 - ma (wa1 – wa2) cpw Tw2 + ma (ha1 – ha2) = 0

The fraction of incoming water that is evaporated, ma (wa2-wa1) / mw1, is typically less than 1%. Thus, ma (wa1 – wa2) is much less than mw1, and the term ma (wa1 – wa2) cpw Tw2 can be neglected with negligible error to give:

mw1 cpw (Tw1 – Tw2) = ma (ha2- ha1)

Both sides of this equation represent the total cooling capacity of the tower.

The effectiveness, E, of a heat exchanger is the ratio of the actual to maximum heat transfer.

E = Qactual / Qmax

For a heat exchanger, Qmax occurs if the air leaves the cooling tower completely saturated at the temperature of the incoming water. Thus, cooling tower effectiveness is

E = Qactual / Qmax = [mw1 cpw (Tw1 – Tw2)] / [ ma (ha,sat,tw1- ha1)]

With negligible error (due to water evaporation), the cooling tower effectiveness can also be expressed as

E = Qactual / Qmax = [mw1 cpw (Tw1 – Tw2)] / [mw1 cpw (Tw1 – Twb1)]

E = Qactual / Qmax = (Tw1 – Tw2) / (Tw1 – Twb1)

Example: Calculate the approach and effectiveness for a cooling tower with inlet water at 95 F, outlet water at 85 F, and air wet-bulb temperature = 78 F.

Approach = leaving water temperature - entering air wet-bulb temperature

Approach = 85 F – 78 F = 7 F

E = (Tw1 – Tw2) / (Tw1 – Twb1)

E = (95 F – 85 F) / (95 F – 78 F) = 58.8%

Note that the leaving water temperature can be above or below the entering air dry-bulb temperature. For example, for the conditions specified here, if the entering air were (Twb = 78 F, RH = 90%), the entering air dry-bulb temperature would be about 80 F. Thus in humid conditions like this, the leaving water temperature (85 F) would be greater than the air dry-bulb temperature (80 F). However, if the entering air were (Twb = 78 F, RH = 40%), the entering air dry-bulb temperature would be about 99 F. In dry conditions like this, the leaving water temperature (85 F) would be less than the air dry-bulb temperature (99 F).

Example: Calculate the approach and effectiveness for the same cooling tower now operating with inlet water at 69 F, outlet water at 59 F, and air wet-bulb temperature = 40 F.

Approach = leaving water temperature - entering air wet-bulb temperature

Approach = 69 F – 40 F = 29 F

E = (Tw1 – Tw2) / (Tw1 – Twb1)

E = (69 F – 59 F) / (69 F – 40 F) = 34.5%

Thus, cooling tower approach increases and effectiveness decreases at lower wet-bulb temperatures.

Example: Calculate the approach and effectiveness for the same cooling tower now operating with inlet water at 91 F, outlet water at 71 F, and air wet-bulb temperature = 40 F.

Approach = leaving water temperature - entering air wet-bulb temperature

Approach = 91 F – 40 F = 51 F

E = (Tw1 – Tw2) / (Tw1 – Twb1)

E = (91 F – 71 F) / (91 F – 40 F) = 39.2%

Thus, cooling tower effectiveness increases at higher inlet-outlet water temperature ranges.

Energy Efficiency of Counterflow and Crossflow Towers

The two most common tower designs for HVAC applications are forced-air counterflow and induced air cross-flow. Cooling tower energy use is a function of fan and pump power. To generate the same quantity of cooling, forced-air counterflow towers require more fan and more pump energy then induced-air crossflow towers. Thus, induced-air crossflow towers are almost always more energy efficient.

Forced-air counterflow towers require more fan energy because centrifugal fans are made to generate low flow against high pressure, but cooling towers generally need high flow at low pressure. In comparison, induced air crossflow towers use propeller fans, which generate high flow against low pressure, which is more suited to cooling towers.

Forced-air counterflow towers require more pump energy because these towers are taller in order to facilitate the counterflow heat transfer as the water falls through the tower. This height increases elevation head in the piping system. In addition, forced-air counterflow towers spray water through nozzles, which increases pressure drop. In comparision, induced-air crossflow towers are shorter and wider since the path of the air through the water is horizontal. In addition, the supply water simply drains from feeding pans into fill, which eliminates the need for nozzles.

A comparison of cooling tower energy use for the same loads is shown below.

Comparison of F.D. Blower Tower vs I.D. Propeller Tower for 400 Tons

Source: Marley Technical Report H-001A, “Cooling Tower Energy and Its Management”, October, 1982.

Cooling Tower Control

In HVAC applications, chiller evaporator loads vary depending on weather and building occupancy, and the quantity of heat rejected by the condenser varies accordingly. The cooling tower will always reject the all the heat from the condenser. However, the temperature of the cold water return to the condenser will decline at lower loads.

Various methods are used to control cooling tower capacity to generate the desired cold water return temperature. The two control points for cooling towers are water flow and air flow. However, cooling tower manufacturers strongly recommend that water flow remain constant at all times. Thus, primary control methods generally rely on varying air flow. The common control methods are listed below.

Run Fans Continuously

This type of control results in the coldest possible return water temperature, which reduces chiller energy use. However, it also results in the highest cooling tower fan energy use. Because the improvement of chiller efficiency with lower condenser water temperature is asymptotical at some minimum temperature, this method of control rarely results in the best overall energy efficiency.

Cycle Fans On and Off

This type of control reduces excess fan energy use at cold outsider air temperatures, and is widely used. At relatively cold temperatures, however, the fan may cycle on and off too frequently. The maximum number of fan cycles is about 8 per hour. Thus, many cooling towers are equipped with water bypass loops. In most applications, water bypass control is only used at low temperatures when fan cycling could be a problem.

Use Two-Speed Fan

This method of control adds an intermediate level of cooling between full-on and full-off. This results in considerable fan energy savings, since fan energy varies with the cube of flow. Thus, fan energy at 50% air flow is only 12% of the fan energy at full air flow. This type of stepped control can be further extended with two cell towers with one fan in each cell. This leads to four possible steps of control. A typical relationship between cold water temperature and fan flow is shown below.

Continuously Control Fan Speed with VSD

This method results in the lowest fan energy use by continuously achieving savings, due to the fan law that fan energy varies with the cube of flow.

Vary Air Flow Using Inlet Air Dampers

Before VSDs, cooling towers were sometimes controlled by running the fan at full speed while varying the inlet air dampers to modulate air flow. This method of control results in intermediate energy savings between fan cycling and continuous VSD control. However, is rarely used now that the VSD control is now commonplace.

Comparison of Energy Use with Various Methods of Cooling Tower Control

Total chiller and cooling tower energy use for these control methods for a typical HVAC application are shown below.

Comparative Energy Usage with Various Methods of Control

Source: Marley Technical Report H-001A, “Cooling Tower Energy and Its Management”, October, 1982.

Variable Cold Water Set-Point Temperature

The energy efficiency of all the control discussed above can be improved by varying the cold water set-point temperature with the outdoor air wet bulb temperature. This type of control takes into account the fact that towers can only produce water at a few degrees above the wet-bulb temperature (this temperature difference is called the “approach”); hence fan energy can be reduced when that temperature is achieved, since continued fan operation results in minimal further reductions in cold water temperature.

Fan Motor Power with Fan Speed and Air Volume Flow Rate

The figure below shows fan motor power draw as a function of input frequency for a cooling tower fan equipped with a VFD. The fan affinity laws would predict a relationship between fraction power (FP) and fraction speed (FS) of:

FP = FS3

Regression of the data show a slightly better fit using the exponent 2.8:

FP = FS2.8

Since fan speed is proportional to volume flow rate, this relation also hold for fraction volume flow rate, FV.

FP = FV2.8

The slightly reduced exponent is caused by declining VFD, motor and fan efficiencies at reduced speed.

Source data: “An Application of Adjustable Speed Drives for Cooling Tower Capacity Control”, Welch, W. and Beckman, J.

Variable-speed cooling tower fans generate the least savings compared to constant-speed fans during warm weather and when the cooling tower set point temperature is low because the fan runs more frequently at these times. Alternately, variable-speed fans generate the greatest savings during cool weather and when the cooling tower set point temperature is high because the fan runs less frequently at these times. The CoolSim output screens shown below demonstrate these concepts. Thus, variable frequency drives on cooling tower fans will generate the greatest savings on year-round cooling applications with relatively high set-point temperatures characteristic of industrial process applications.

Cooling Tower Bypass Plumbing

Bypass control is typically used only at low outdoor air wetbulb temperatures in order to reduce fan cycling. Bypass should not be used in sub-freezing temperatures since this can lead to tower freeze up. The preferred tower bypass plumbing is shown below. The preferred valve is a single two-way butterfly valve placed in the bypass line.