How to calculate de-humiditfication for indoor swimming pool?

Cooling Load Design Procedure in Brief with a few hint for dehumidification of swimming pools.

1.       Summing up of cooling load estimates of a zone referring to the walls, glasses, skylights, internal heat gains, latent heat loads, appliances, computers, utility equipment load, infiltration load, factory of safety allowance etc.

2.       We determine the total heat gains of the zone in terms of sensible and latent heat values.

3.       Compute the Sensible heat ratio with the relation : Sensible heat/(Sensible heat + Total heat)

4.       On the Psychometric chart extend the slope line from the SHR inclined line graduations found on the right hand side of the chart, by selecting the calculated SHR value.(Can be done using Protractor available in Ashrae Psychometric chart)

5.       This line of slope represents the supply air leaving the cooling coil, to offset the sensible heat gain inside the Zone (Conditioned space). In this line lies two points, one is the temperature of supply air leaving the cooling coil and the other is the temperature of the space after the cooling of the space taken place, which is set by the designer which is also called room dry bulb temperature. Corresponding wet bulb temperature at those two points can be read from the psychometric chart.

6.       We have the space conditioned air total cooing sensible and latent, the reference temperatures and the computed temperature difference(TD), with the air density in hand we need to calculate the design air flow rate using sensible cooling value and the formula : CFM = Sensible heat (Btu/hr)/(TD* Air factor (1.08).

7.       The phenomenon of Latent heat is that gains up in this process which shall be offset by condensation in the cooling coil when the moist air reaches near to dew point.

8.       Then calculating the gain in specific humidity Δω using the formula: Latent heat (Btu/hr)/ (CFM*60*0.075*1059), where 60 is min/hr, 0.075 is air density, lb/ft3 (standard air), & 1059 is the latent heat of vaporization at 600 F. Btu/lb.

9.       While using Chilled water cooling system the phenomenon and representation of cooling and dehumidification SHR slope line stays upper in the chart, and for DX package air conditioning the line is placed lower means at lower humidity.

10.   Thus for DX package unit dehumidification process done by the cooling coil is at best performance.

11.   Dehumidication of Swimming pool with a recirculation system is best operational with DX package unit.

12.   Instead of dehumidifying the swimming it is recommended to use a 100% fresh air handling unit to air condition the pool space and discharge the displaceable air to outside (atmosphere), if I am not wrong.

13.   Or to use the same Air conditioner using the space for the dehumidification as few designers does it.

14.   The evaporation of water in an indoor swimming pool shall be calculated as below:

Rate of evaporation of water in a swimming pool:

E = PAW/T+459.67

Where E is the rate of evaporation in Gallons/day

A is the surface area of the pool in ft2

W is the wind speed above pool mph

P is water’s vapour pressure (mmHG) at ambient temperature (Pool space temperature).

T is temperature in 0F.

15.   The process of removing the evaporated water out of the pool shall be dehumidification from the cooling coil, by using DX package unit.

16.   Δω is going to be increased abnormally, the CFM selected to be of medium speed air flow, and the SHR line slope shall be steep predominantly which increases the capacity of the AC equipment. With selecting the capacities in multi stages for redundancy. The application engineer from the manufacturer selects the equipment by observing the designed datas.

17.   The ADP is designed to have a much lower temperature at 400 F, and lowering the supply air temperature which is not acceptable for human comfort.

18.   Hence multistage reheat panels to be installed either with the equipment or separately as Duct heaters to raise the temperature of the supply air to human comfort level. The maximum capacity shall be the equivalent energy used to dehumidify the whole evaporated water of the pool.

19.   Electrical Control and DDC panels, temperature and humidity sensors to be included in the system design for the successful operation of the Airconditioning and dehumification of the swimming pool.

20.   An alternate remedy for dehumidification of pool is to set up a standalone dehumification system exclusive for the evaporation of water inside the swimming pool, besides the air conditioning system.

to calculate dehumidification load we have to calculate evaporation from indoor pool surface ,moisture from outside ventilation air and moisture from occupants .

1-from ashrae handbook can take evaporation rate  from activity factor (AF) depend on application residential or hotel or,.......etc 

2- evaporation rate factor (ERF) depend on pool water temperature and air temperature     (lb/hr.sq ft)

3- determine pool area in (sq .ft)

4- then  can calculate evaporation rate (lb/hr) = ERF lb/hr. sq ft X AF X pool area

5- ventilation moisture = [CFM oa x (grains oa - grains return)]/

6- recommended ACH 4-8 for outside air

7- for one person calculate btu/ hr latent heat load and calculate enthalpy h(lb/hr)

Use the calculation formulas below to determine the dehumidification and ventilation needs for your indoor pool or spa room

A. Moisture load from the water surface

1. Look up the lbs. per ft² per hour evaporation rate from the WATER EVAPORATION CHART using the desired pool water temperature, air temperature and relative humidity (RH) %______ lbs. of moisture load/ft²/hr x ____ ft² area of pool surface = ______ lbs./hr moisture load
2. Covered Pool (pools that are uncovered less than 2 hours/6 hours______ lbs./hr moisture load x _____hrs/6 hours pool uncovered/ 6 hrs/day = _______ lbs. hr (average)

B. Exhaust ventilation to prevent moisture penetration of the structure and remove chemicals

1. Calculate the natural air leakage
   1. Volume of pool space _______ ft length x ______ ft width x _______ ft ceiling height = _______ ft³ pool room volume
   2. Estimate natural leakage, air changes per hour (ACH)_______ ft³ pool room volume x _______ ACH natural leakage /60 min/hr = _______ ft³/min natural air leakage
2. Exhaust air flow required to prevent moist air penetration of the pool room insulation.Recommended air exhaust rate 1.5 (minimum) times greater than the natural leakage rate of the structure.

   _______ ft³/min. natural air leakage x _______ (1.5) = _______ft³/min. recommended exhaust air flow rate to depressurize the pool room (prevents moisture penetration of the insulation)

3. Indoor Pool Room Moisture Removed by VentilationLook up the lbs. of moisture removed per hour per 100 CFM of exhaust ventilation using the VENTILATION CHART at the desired room conditions.

   _______ lbs. moisture removed/100 CFM per hr of ventilation x _______ CFM of exhaust ventilation/100 CFM = _______ lbs. of moisture per hour removed by ventilation

C.  Determining the amount of dehumidification needed

1. First method (uncovered pool or spa)Requires that the moisture removal rate equals the moisture load.  The RH is maintained with the exception of during times of high activity and pool splashing.  This may generate two times the humidity load.

   _______ lbs./hr moisture load _______ lbs./hr moisture removed by ventilation = _______ lbs./hr of dehumidifier capacity

2. Second method (covered pool with low usage – less than 2 hours per 12 hours)Allows a rise in the RH levels when water is uncovered by providing a return to normal RH levels 2-3 hours after the water is covered.

   _______ lbs./hr average moisture load _______ lbs./hr moisture removed by ventilation = _______ lbs./hr, estimated dehumidifier capacity

www.michiganair.com/newsletters/2012-3/section1.pdf

Please download the following link and read it

DEHUMIDIFICATION CAPACITY = (1.3 : 1.4) KG WATER / T.R