Practical considerations

Five major problems must be faced with cooling tower installations:

(i) Corrosion

(ii) Scale formation

(iii) Protection against freeze up

(iv) Cavitation at the pump

(v) Health risks

The answer to the corrosion problem is two-fold: first, a tower constructed from materials that are likely to be adequately resistant to corrosion for the expected life of the tower should be selected; secondly, due thought should be given to the use of water treatment.

Continuous evaporation means that the concentration of dissolved solids in the tank of the cooling tower will increase. Unless steps are taken to nullify the effects of this, in due course some scale or sludge formation is inevitable. This will reduce the performance of the cooling water system by being deposited on the heat transfer surfaces in the condenser and on the fill of the tower. Remedial action takes the form of allowing an adequate continuous bleed from the tower (usually arranged in the discharge pipe, as shown in Figure 11.1, so that water is not wasted when the pump is off) coupled with some form of water treatment. If the bleed rate is high enough, it may, under certain circumstances, be sufficient to minimise scale formation without the use of water treatment.

Bleed rates are often quite high. The rate desirable may be related to the hardness of the water, expressed in parts per million CaC03. For example, water which has a hardness of 100 ppm might need a continuous bleed equal to 100 per cent of the evaporation rate. At 200 ppm, it might require to be 200 per cent of the evaporation rate, and at 300 ppm, to be ten times the evaporation rate. This can sometimes mean a formidable consumption of water.

Water companies in the UK may sometimes demand that a break tank be interposed between the cooling tower (or other device, using mains water for cooling) and their mains. The break tank must be large enough to provide 4 hours’ or half a day’s supply of cooling water, in the event of a failure of the mains supply. They have three reasons for insisting on this:

(i) A degree of stand-by is offered to the user.

(ii) A break is necessary between the mains in order to prevent back-siphonage and contamination of the Company’s water supply.

(iii) Fluctuations in demand are averaged; thus, the Company’s mains can be sized for average, rather than peak demands.

There is a fourth possible reason why such a tank might be required. In a large office block (for instance), if the cooling tower supply is taken from the cold water storage tank used for supplying water to lavatories, there is reduced stand-by water available for lavatory flushing purposes. If the mains supply failed, the cooling tower would use this water, to the detriment of the hygiene in the building.

EXAMPLE 11.3

Estimate the capacity of the cooling-tower break tank necessary for a 720 kW air conditioning installation. The hardness of the mains water is 100 ppm and the plant runs for an 8-hour working day.

Answer

Assuming a coefficient of performance of 4, the heat rejected in the cooling tower is therefore

720 x 1.25 = 900 kW

Taking an approximate value of 2450 kJ kg-1 as the latent heat of water, then the evaporation rate will be 0.367 kg s-1.

For the hardness quoted, a continuous bleed of 100 per cent is probably required. Thus, the break tank must have a capacity of 11 000 litres.

To prevent freeze-up, two precautions are necessary: first, the provision of an immersion heater with an in-built thermostat in the tank, and secondly the protection of all exposed pipework which contains water with weather-proofed lagging. Below the static water line (when the pump is off), such pipework should be traced with electric cable, thermostatically controlled to keep the water above 0°C. As an alternative, the tower may be drained in winter, if the refrigeration plant is off. But such drainage is a tedious maintenance chore and may not be acceptable to a user. Operation of a cooling tower in winter requires considerable forethought.

As much of the piping as possible must be kept beneath the static water level, otherwise when the pump stops, water above the level will drain from its piping into the tank and flood through the overflow on to the surrounding roof. In any case, the tank should have enough freeboard to accommodate the flow-back from the small amount of piping that must be above the water level, when the pump stops. Non-return valves are seldom an answer to this sort of problem because the available head is usually insufficient to give a tight shut off.

Vortices of air bubbles entrained in the water may form at the outflow from the cooling tower tank. UP to 10 per cent of the volume of the outflow may be air, in bad cases. The formation of vortices is thus very undesirable. It can be prevented, according to Denny and Young (1957), by having a large enough outflow diameter, or a greater depth of water above the outflow, or by the suitable positioning of baffles (see Figure 11.7).

A screen should be provided around the outflow pipe to filter out the larger pieces of debris which collect in the tank of any cooling tower. The size of the holes should be just

Baffle

Plate

Practical considerations

‘ ► Outflow

]

Cooling tower tank

Fig. 11.7 A vortex is possible at the outlet from the tank of a cooling tower with a

Shallow water depth.

Smaller than the internal diameter of the water tubes in the condenser. Holes that are too small may be undesirable because the screen may clog up and frequently stop water flow. Birds seem to favour cooling towers in some districts and their droppings and feathers may prove a hazard. Frequent maintenance inspections should resolve this problem.

It is extremely bad practice to put a strainer at the suction side of the pump. The purpose of the strainer is to collect dirt and debris. Consequently, if the strainer is doing the job for which it was installed, the pressure drop across it will increase as time passes. After a while the pressure at pump suction will be so low that, for the particular water temperature, the water within the pump will boil. This is cavitation (see Jones (1997) and Pearsall (1972)): the performance of the pump diminishes significantly and, eventually, the impeller of the pump is destroyed. The purpose of the strainer is to protect the tubes of the condenser and the strainer must therefore always be put on the discharge side of the pump. The screen in the pond of the tower protects the pump.

To ensure that cavitation does not occur the pump should be located so that it is sufficiently below the static water level in the tank and always blows through the condenser. The pump manufacturer will quote the value of net positive suction head (abbreviated NPSH) required but the NPSH provided should always be in excess of this by a factor of at least three according to Grist (1974).

There is a health risk with the use of cooling towers and evaporative condensers, if contaminated spray water droplets of the right size are inhaled. (Five or 6 micron droplets may be inhaled into the deepest part of the lungs and 10 micron droplets can reduce to 3 microns in less than a second by evaporation.)

The risk that has received most attention is Legionnaires’ Disease, described by the CIBSE (1991). This is an uncommon infection in the UK, there being about 100 to 300 cases reported annually, of which most seem to be associated with hot water service systems (particularly showers), rather than with cooling towers. It is a form of pneumonia and people most at risk are: smokers, alcoholics and those suffering from a respiratory disease, diabetes or cancer. The fatality rate is about the same (12 per cent) as with other forms of pneumonia and the disease is effectively treatable with the appropriate antibiotics.

The bacterium (Legionnella pneumophila) exists in water naturally, throughout the UK and abroad, and has done so for many thousands of years. At temperatures below 20°C the bacterium is dormant and the multiplication rate is insignificant. Above this temperature multiplication increases and is greatest at 37°C, in laboratory specimens. At higher temperatures it reduces and ceases at 46°C. When the water temperature reaches 70°C the bacterium is killed instantaneously. Since the typical range of cooling water temperatures provided by cooling towers in the UK is from 32°C to 27°C, it is evident that steps must be taken to prevent contaminated spray from becoming a risk.

Of importance is the provision of a drift eliminator, as Wigley (1986) explains, and which, if properly specified and selected, can reduce the emission of liquid droplets from the outlet of a tower to 0.001 per cent of the cooling water circulation rate. Using a splash water distribution system for the packing, rather than nozzles, helps to reduce the drift. Correct commissioning is essential and first class operation and maintenance are vital. The Report of the Expert Advisory Committee on Biocides (1989) provides an exhaustive list of recommendations for good maintenance and generally offers excellent advice on water treatment, except for the matter of the location of strainers. Strainers must always be positioned on the discharge side of pumps.

It must not be forgotten that the location of cooling towers, evaporative condensers and air-cooled condensers should be right. The position chosen must not compromise in any way the free airflow into and out of the equipment, otherwise heat will not be rejected properly from the refrigeration plant and failure of its high pressure cut-out will be continual. The location chosen for cooling towers and evaporative condensers is also particularly critical in relation to the prevailing wind direction and the configuration of adjoining buildings; air that might convey small droplets of contaminated water (in spite of the use of drift eliminators) must not be discharged from towers or condensers in such a way that it could flow into fresh air intakes or open windows.

Exercises

1. Compare the merits of a forced convection evaporative condenser with those of a shell- and-tube water cooled type.

2. Compare briefly the relative merits of a shell-and-tube condenser with a cooling tower, and an evaporative condenser. Make a sketch of an evaporative condenser, indicating the main components.

3. (a) With the aid of neat sketches, distinguish between cooling towers and evaporative condensers. Name two other methods of rejecting the heat from the condenser of a refrigeration plant used for air conditioning.

(b) Discuss the practical limitations of the four methods mentioned in part (a) of this question. Give emphasis to application, running cost, maintenance and noise.

Notation

Symbol

Description

Unit

A

Cross-sectional area of the tower

M2

Cw

Specific heat capacity of water

KJ kg"

/ia

Enthalpy of humid air

KJ kg“

Hw

Enthalpy of a thin film of saturated air at a

Temperature? w

KJ kg-

Kg s“1

K

Coefficient of vapour diffusion for a unit value of Ahm

Ks

Volume transfer coefficient

Rate of mass flow of air

Kg s”1

Mw

Rate of mass flow of water

Kg s"1

S

Wetted surface area per unit volume of packing

Nr1

T

Air temperature

°C

Cooling water temperature

°C

Z

Height of a cooling tower

M

A hm

Mean driving force

KJ kg

Posted in Air Conditioning Engineering


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