WATER TREATMENT

All water supplies contain a proportion of dissolved salts. These will tend to be deposited at the hottest part of the system, e. g. the furring of a kettle or hot water pipes. Also, these impurities do not evaporate into an airstream, so where water is being evaporated as part of the cooling process, the salts will remain in the circuit and increase in concentration, thus hastening the furring process.

It is possible to remove all solids from the make-up water, but it is much cheaper to check the concentration by other means. Two general methods are employed. The first relies on physical or chemical effects to delay deposition of scale on the hot surfaces; the second restricts the concentration to a level at which precipitation will not occur. In both cases, the accumulation of solids is removed by bleeding off water from the circuit to drain, in addition to that which is evaporated (see Figure 6.9).

Water evaporated

Figure 6.9 Limitation of solids concentration by bleed-off

The concentration of solids in the circulating water will increase until the amount carried away by the bleed water compensates for that not carried away in the water vapour. So, if

Cm = concentration of solids in make-up water (kg/kg) cb = concentration of solids in bleed-off (kg/kg) we = mass flow of water evaporated (kg/s) wm = mass flow of make-up water (kg/s)

Mass of solids entering = mass of solids leaving

Cm X Wm = cb X (wm — We)

C

B

If water hardness is 560 ppm (parts per million), the water treatment can permit a concentration of solids to 1200 ppm, the cooling capacity is 700 kW and the compressor power 170 kW, how much water should be bled to waste and what is the total make-up required? Cooling tower capacity = 700 + 170 = 870kW Latent heat of water vapour = 2420 kJ/kg 870

Rate of evaporation

0.36 kg/s 0.0012

2420 0.36

Rate of make-up

0.68 kg/s

0.0012 — 0.00056 Rate of bleed-off = 0.68 — 0.36 = 0.32 kg/s

W„	W„
The concentration of mains make-up water, cm, is obtained from the water sup­ply authority. The permissible concentration, cb, will be decided by the method of water treatment or the assumed concentration of untreated water which will prevent precipitation.
Example 6.4
In all cases where water is used for cooling, but more especially where it is being evaporated, the hardness figure should be obtained from the local water supply authority. Enquiries should also be made as to possible variations in the supply, since many cities draw their water from two or more catchment areas, and the type and quantity of hardness may change. Many suppliers now offer water treatment for use in refrigeration condenser circuits, and the merits of different methods need to be assessed before making a choice. The reader is referred to specialist works on the subject. There are several methods of providing a percentage ‘bleed-off’ from the water circuit: 1. The make-up ball valve can be set a little high so that some water always goes down the overflow pipe. This is rather difficult to set initially, but is reliable and cannot easily be tampered with. It will work at all times, and so will waste water if the plant is not running. 2. A small bleed-off pipe is taken from the pump discharge, with an adjusting valve, and led to waste. This can be more easily adjusted and

Works only when the condenser is running, but is subject to interference by unauthorized persons.

3. A tundish, having an area possibly 1% of the cross-sectional area of the tower, is located just above the water level and is led to the drain, forming part of the overflow fitting. This will bleed off 1% of the water falling through the tower.

All these methods provide the maximum required rate of bleed-off at all times of the year, and so will waste water at light load conditions. The user should be aware of the essential nature of bleed-off, since cases often occur in dry weather of misguided persons closing off the bleed to ‘save water’.

In some locations, it is necessary to drain the tank frequently to clear other contaminants. With careful control, this can be used as the necessary bleed-off.

Posted in Refrigeration and Air Conditioning