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 |
![]() |
|||
|
|
||
|
|||
|
|||
![]() |
|||
|
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