HVAC -CRASH COURSE

 HVAC is the branch of science that deals with the Heating, Ventilation and Air Conditioning System. It has become attractive to many engineers and has made their living with it. 

There are many simple but complex at the same time logics which feels very tiresome to the beginners if they don't have proper guidance. So, here are basic rules I hope anyone, from fresher to experts, can get their head on. 

1. Heat Load Calculation

Thumb Rule: 120 sq.ft/Ton→ Very Hot Zone

                        150 sq.ft/Ton→ Normal

                        180 sq. ft/ton→ cool Climate like Kathmandu

For the information, the wall facing north lacks light so is cooler. The southern is very hot as it gets light all day from morning to evening. The west gets the worst as it get the intense afternoon light till night. 

2. Fresh Air: Each person needs 7 CFM. And allocate 0.6 CFM per sq. mtr of floor Area. 

3. For Duct Sizing→ Say, you need 1000 CFM to the flow-through duct. 500x200 (500x2=1000) will work fine. Or you can provide 400x250 (400x2.5=10). Just don't use it. Use ductulator. It is used for judgment. 

4. For Grille and Diffuser Sizing: Assume 500 FPM at the terminal or Grilles. Say you need 3200 CFM to flow and you have three Grilles positions. Now, For 3200→ 3200/500= 6.4 sq.ft area Grille total of three is needed. 2.1 sq.ft each.  suppose you chose 8 inch  height i.e. .66 ft, then width = 2.1/.66 =3.1 ft (i.e. 930 mm ) of Grille length for 8 inch (200 mm) width. So, 1000x200 Grille size 3 pcs is selected for 3200 CFM. 

5. Dampers have high  Pressure losses. Almost 15mm of H2O is lost in Damper. So, don't place the damper more than two points between terminals and Machine Unit. 

6. 1 mm of Water = 9.81 Pascal (Normally AHU ESP ranges from 50-75 mm of Water: 1 damper =15 mm of water, So 2 dampers will reach 30 mm of H2O )

    1 inch of water= 249 pascal

Calculate the internal Static pressure in AHU

1. Pressure Drop for Filter

    Fine Filters: 0.0981 inch/wg

    Pre Filters: 0.19-.75 inch/wg

    HEPA Filter: 0.98-2.95 inch/wg

2. Pressure Drop for Coil

    2 Row Coil: .11 inch of Wg- 8 Fins

    4 Row Coil: 0.22 inch of wg-8 Fins

https://www.nrc.gov/docs/ML0916/ML091610395.pdf

HEPA Filter Pressure Losses The HEPA filters in the HEPA filter plenum, shown in Appendix B, have pressure losses as follows: • The 1st stage HEPA filter in the HEPA filter plenum has a pressure loss of 3 in. w.g. • The 2nd stage HEPA filter in the HEPA filter plenum has a pressure loss of 2 in. w.g.