According to ASHRAE (1999a) the desirable temperature for swimming pools is 27c , however this will vary from the culture by so much as 5 degree Celsius. .If the geothermal water is higher in temperature then some sort of mixing or cooling by aeration or in a holding pond is required to lower the temperature . If the geothermal water is used directly in the pool , then a flow through process is neccessary to replace the used water on regular basis. In many cases the pool water must be treated with chlorine , therefore it is more economical to used a closed loop system for treatment water and have geothermal water provide heat through heat ex changer . The Water Heating System should be installed in the return line to the pool. Acceptable water circulation level vary from eight hours to six hours for a complete change of water. Heat exchanger must be designed to resist the corrosive effect of the chlorine in the pool water and scaling or corrosion from the geothermal water. This often requires in the case of plate heat exchanger using titanium plates .
Four Factors determine the sizing of the system for temperature and flow rate . These are
i) Conduction through the pool walls
2) convection through the pool surface
3 ) Radiation from the pool surface
4) Evaporation from the pool surface
Conduction is Least significant unless the pool is above ground or in contact with the cold underground water
Convection losses depends on the temperature difference between the pool water and the surrounding air and the wind speed.this substantially low for indoor pool also pool with wind speed breakers.
Radiation losses are greater at night for the outdoor pools , however their will be gain in temperature during daytime. A Floating pool Covers can reduces both radiation and evaporation losses. Evaporation loss constitute the greatest heat loss from pools -50 to 60 % in most cases. The rate of which evaporation occurs is a function of air velocity and pressure difference between the pool water and the water vapor in the air .
As the temperature of the pool water is increased or the relative humidity of the air is decreased evaporation rate increase.
The required Gethermal heating output q can be determined by the following two equations
q1 = Density of Water * Pool heat up *pool Volume * ( Desired Temp - intial Temp ) * Pool heat up time
q2 = Surface heat Transfer Coefficient * pool Surface area * ( Pool Temp - ambient temp)
then Q= q1- q2
if there is no heat up time which is typical for geothermal pools then equations (1) will be zero and only equation 2 will apply. Equation 2 will assume a wind velocity of 5 to 8 Km/h . For Sheltered Pool wind velocity factor less than 5km/h
The neccessary Heat to increase and maintain the temperature of an outdoor pool can be expressed as
H( Total) = h (Surface) + h (heat up)
h (heat up) = Volume *8.34 (lbs/gal) * ( intial Temp - Final Temp) * 1.0 / 72
72 = time required to Rise the temp of pool
h (Surface) = ks * dtw* A
ks = surface heat loss factor
dtw = Temp Difference between the air and surface water in the pool
A = Surface area of the pool