Automatic Control of Cooling Towers

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A ut omat i c Cont rol of Cool i ng Tow ers

HVAC @C India/AV

Cooling tower is a simple device. It moves air through falling water to cool the water. The air

absorbs heat by carrying away hot water vapor and thus cools the remaining water.

What makes cooling tower analysis and design seemingly difficult is the thermodynamic

characteristics of air and water.

Cooling towers are used to cool condenser water in water-cooled chiller systems, the most

efficient means of creating chilled water. The tower fans provide moving air to cool thecondenser water by evaporation.

As air moves through water, it absorbs heat depending upon the difference between the "wet

bulb" temperature of the air and the inlet water temperature. This difference dictates how

much heat is eliminated per unit of air and water. To increase heat removal at steady state

temperatures, the flow of air through the water must be increased. To decrease heat removal,

the flow of air through the water must be decreased. The amount of heat removal is directly

proportional to the flow of air through the water.

After passing through the tower, the cooled water collects at the bottom of the cooling tower

basin and is pumped back through the chiller condenser by the condenser water pump. In

many climates, cooling towers provide the most energy efficient method of removing heat from

the chillers condenser water

The function of the cooling tower is to deliver outlet water at a specific temperature

to permit optimum heat removal at the chiller or other heat source. As the "wet bulb"

temperature varies, the heat removal capacity of cooling tower varies. The colder the "wet

bulb" temperature of the air is, the less the flow of air required to remove a given amount of

heat.

A cooling tower fan is designed to supply adequate air to cool the water to a specific

temperature when the air is very hot and humid (design "wet bulb" temperature). The


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HVAC @C India/AV

atmosphere, however, is at this design point condition only 2 to 5% of the operational time.

Consequently, reductions in air flow can be achieved 95 to 98% of the time.

Existing Control Practices:

Few of the common control practices for Cooling Towers are:

1. 0n-Off control: Based on the temperature of the condenser water leaving the cooling

tower, the fans are cycled on and off. To limit the number of times the fan motors are powered

on and off, wide temperature control bands are established. When the temperature exceeds

the high limit, a fan is staged on, and then staged off upon reaching the low temperature limit.

As the wet bulb temperature or system load decreases, the duty cycle of the cooling tower fan

also decreases, within the limits of the on-off control temperature band.

2. Two-Speed control: A two-speed motor is

used to maintain the temperature of the water.

This motor works on the two fixed temperature

bands and does not control the exact watertemperature

3. Adjustable pitch fan blades: controls the

speed in case of change in air density.

4. Tower By-pass valve: provide adequate

temperature control, they are very wasteful of

energy.

Application challenge:

Extra man-power required for controlling the Sequencing of Cooling Tower(s) for day

and night operations.

Brakes or sensors are needed to ensure fan blades are not rotating in the reversedirection before starting (prevent snapped motor shaft)

Fan speed control not taken into consideration at system design stage (single speed

starter)

Increased maintenance costs due to power train shock/stresses and motor starter

replacement (systems with single and two-speed starters)

Process water temperatures are not accurately controlled (unstable operation - large

swings in fan speed create large temperature swings)


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HVAC @C India/AV

Pitch of the blades may need to be adjusted according to the season (air temperature)

to prevent motor overloading due to cold, dense air

Automatic Control of Cooling Tower Operation:

Automatic control of the Cooling can be easily achieved by using a VFD and a dedicated Logic

Controller. VFD on cooling towers eliminate many drawbacks associated with starter-controlled

fans. Reduced energy consumption (lower utility costs), reduced maintenance requirements(personnel & equipment replacement costs) and process water temperature stabilization are

among the benefits.

1. The fan may be spinning when a VFD is commanded to start. A VFD must correctly identify

motor rotation, slow the motor down to zero speed (when opposite rotation is detected),

accelerate the motor in the correct direction and not trip on an over-voltage or over-current

condition. Mechanical brakes or anti-ratcheting devices can be used to ensure that a fan

doesn't rotate in the wrong direction.A VFD eliminates both mechanical and electrical brakes

as well as anti-ratcheting devices, time delay relays, etc.

2. In extreme cold weather, the fan may have to run slower than required, raising the tower

and process water temperatures. It is also common to reverse a cooling tower fan, keeping the

heat in the tower. VFD's accomplish this function &eliminate reversing starters.

3. As the speed of the cooling tower fan is reduced below a minimum level, gearbox driven

tower fans require a minimum speed for gearbox lubrication. An adjustable frequency drive

offers a programmable minimum frequency setting to maintain this minimum speed operation.

4. In conjunction with minimum speed operation, drives offer sleep mode, a feature hat

automatically stops the fan motor when demand is at a low level for a determined period of

time. When the system demand increases, the drive restarts the motor to reach the required

output. Sleep mode has great energy savings capability and reduces wear on driven equipment

5. Likewise, on hot days, when the air is thinner, fans can be run above 50 Hz, providing

additional cooling capacity. The VFD's current and/or torque limit function will limit the current

of the motor such that the nameplate FLA rating is not exceeded. This is impossible without a

VFD.

Energy Saving on Lower Speed:

On fan loads, the HP requirement varies as the

cube of the speed, so the slower the fan speed -

the less energy required. A fan running at 80%

speed will consume only 51.2% of the power of a

fan running at full speed. At 50% fan speed,

power consumption is only 12.5%.


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HVAC @C India/AV

Features Used:

Sleep Function During Low Load Demand

Speed search and flying start

o For starting a rotating fan

Improved Power Factor (.98 across speed range)o Maximize Power Distribution System

Reduced Inrush Currents

o Minimize Peak Demand Charges

Soft Start/Soft Stop

o Eliminates Mechanical Shock and Stress on Power Train (couplings, belts, drive

shafts, gear boxes, etc.)

Reduce Utility (Operating) Costs

Reduced Energy Consumption, Process Operates at Most Efficient Point

Allows Load Shedding

Eliminates Motor Voltage Imbalance

Possible DC injection on start.

Over-speed capability for increased performance.

Automatic Control of Cooling Towers

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