Commercial applications generally need to condition high quantities of ventilation air, and it is efficient to combine a desiccant rotor after a pre-cooler and use the waste heat to dry the desiccant, which creates a much lower dew-point and free reheat effect. The air can be dried sufficiently lower than the room set-point to allow for internal latent moisture loads.
SELECTION
Example. For 10,000m3 /h Chilled water DOAS (Dedicated Outside Air) unit needs 5C off coil and a Desiccant-DOAS system requires 12C and 84kW Condenser heat/hot water to maintain 23C 50%rh assuming 7.5l/s and 60W person . This saves 130kWR and the increase in water temperature of 4-5C improves an air cooled chiller COP by 10%.
STEP1 Number of building occupants?
- Local codes have a minimum OA quantity
- USA 15cfm per person
- AUSTRALIA 7.5 l/s per person
- Eg 400people x 7.5l/s pp = 3000l/s (10000cmh)
STEP 2 OA design condition
Important to know for chilled water capacity check eg35CDB 24CWB
STEP 3 Room humidity ratio
- 50%rh 23C library 8.8g/kg
- 60%rh 23C commercial building
- 50%rh 23C (or other) Supermarket 8.8g/kg
- 60%rh at 28C swimming pool
STEP 4 People x nominal load
100g/h pp Eg 400 x 100g/h = 40kg/h NOTE 40kg/h x .683 = 28 kW latent
STEP 5 Internal infiltration rate
Best to take extra consideration as over design may occur in commercial projects. This can be data provided by consultant (design institute), including external duct work, 1% leakage rate or from customer on large systems, or some estimate based on DST How much OR Munters guide for small rooms
- Tight 0.1 cfm/ft2 A(8.8cmh/m2).
- Average 0.3 cfm/ft2(25cmh/m2)
- Loose 0.6 cfm/ft2 (50cmh/m2)
Moisture Load
Room 20 x 20 x 3 (A= 20×20=400m2) = cmh/m2 x m2 x 1.2 x Design day g/kg – Room g/kg
=8.8 x 400 x 1.2 x (14-8.8)/1000
=23kg/h (USA btu/hr= .68 x cfm/ft2 x (design day HR – space HR)
NOTE: DST how much selection factor 3 equated to 1200cmh
It is advisable to make selection based on moisture from people alone, if it is below
STEP 6 Calculate Humidity ration Δg/kg
Fresh air delivered this much dryer than room dew-point
= internal load (Q4+Q5kg/h) / (OA m3/h x ρ kg/m3) x1000
= (40+23)/(10,000x 1.2) =5.25g/kg (NOTE 3.3g/kg on moisture from people alone)
STEP 7 : REQUIRED OA Supply HR g/kg
SA HR = Room HR(step3) – Δg/kg(step6)
ie Supermarket : SA HR= 8.8-3.5=5.3g/kg OR 4.5C dew-point (Cant be done reliably by chilled water or DX alone.) Usually desiccant duty around 3.5g/kg so off pre cooling coil using desiccant 8.8g/kg or 12Cdp.
Desiccant dehumidification has had substantial application into commercial buildings in humid climates due to operating and compliance requirements of the case manufacturers, and the inability of conventional HVAC to dehumidify effectively
FAQ: Dehumidification Requirements and Solutions
How to calculate dehumidification requirements?
To calculate dehumidification requirements, determine the number of occupants and the local codes for minimum outdoor air (OA) quantity. Next, identify the outdoor air design conditions and the target room humidity ratio for the specific application. Calculate the moisture load from occupants and internal infiltration rates, and then determine how much dryer the fresh air needs to be compared to the room dew-point. Finally, calculate the required supply air humidity ratio.
What is the formula for calculating a dehumidifier?
The formula for calculating the capacity of a dehumidifier is:
Dehumidifier Capacity (kg/h)=Internal Load(Q4+Q5 kg/h)OA m3/h×ρ kg/m3×1000\text{Dehumidifier Capacity (kg/h)} = \frac{\text{Internal Load} (\text{Q4} + \text{Q5 kg/h})}{\text{OA m}^3/\text{h} \times \rho \text{ kg/m}^3} \times v1000Dehumidifier Capacity (kg/h)=OA m3/h×ρ kg/m3Internal Load(Q4+Q5 kg/h)×1000
This formula helps determine the dehumidifier capacity needed by considering the internal load, outdoor air volume, and air density.
How do I choose a dehumidifier capacity?
To choose a dehumidifier capacity, calculate the total moisture load from occupants, internal sources, and infiltration. Assess the required airflow to maintain the desired humidity levels and match these requirements to a dehumidifier’s specifications. Ensure the selected dehumidifier can handle the calculated moisture load and airflow requirements effectively.
What is the most efficient method of dehumidification?
The most efficient method of dehumidification often involves using desiccant dehumidifiers, especially in commercial settings. Combining desiccant rotors with pre-coolers and utilizing waste heat for drying the desiccant creates a lower dew-point and improves efficiency. This method is highly effective in handling low dew-point requirements and high moisture loads, making it more efficient than traditional HVAC systems.