Air Conditioning Heat Load Calculation (Room & Building estimate)

The calculation estimate formulas used here are just rough approximations to simplify the total heat load calculations and quickly select an air conditioning unit capable of providing the necessary cooling of the room or space.


--------------------- QUICK HEAT LOAD CALCULATION ESTIMATE -------------------

Typical Room or Small Office:

Space Heat Load = Cooling Space Dimensions x 4

Space Heat Load = Room Length (ft) x Room Width (ft) x Room Height (ft) x 4

Space Heat Load = L x W x H x 4


Occupant Load = 600 BTU/person


Total Heat Load = Space Heat Load + Occupant Load



------------------- DETAILED HEAT LOAD CALCULATION ESTIMATE ------------------
     
1 Btu (British thermal unit)
1 Btu = energy required to cool or heat 1 pound of water by 1 degree Fahrenheit

Heat Load of People
Heat generated per person = 600 Btu

Units:

1 Btu = 0.3 watt hours
1 Btu = 250 calories
1 Btu = 0.25 kilocalories

1 watt-hour = 3.4 BTU

1 TON of cooling = 12,000 BTU/hour

1 TON of refrigeration = 12000 Btu/hr
1 TON of refrigeration = 200 Btu/min
1 TON of refrigeration = 3.5 KJ/s
1 TON of refrigeration = 3.5 KW  



Total Heat Load:

The total heat load of a room/building is a function of:
1. Size of space to be cooled
2. Area of windows
3. Number of occupants
4. Equipment heat load
5. Lighting heat load

Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu
 
   
Example:
Calculate the Total Heat Load and select an Air Conditioning Unit.

Room dimensions:
Length, L = 15 ft
Width, W = 10 ft
Height, H = 8 ft (2.5 m height from floor to ceiling)

Window dimensions:
L = 4 ft
W = 4 ft

Occupants:
2 persons

Equipment wattage:
TV = 100 watts
Radio = 50 watts
Laptop = 60 watts

Lighting Load:
60 watts light bulb


Air-conditioning Heat Load Estimation Formulas:

Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu

Space Btu = Room Length (ft) x Room Width (ft) x 32

Window Btu = Total Window Area (sq. ft) x 80

Occupant Btu = Number of Occupants x 600

Equipment Btu = Total Equipment Wattage x 3.4

Lighting Btu = Total Lighting Wattage x 4.3


Total Heat Load Calculation:

Space Btu:
Space Btu = Room Length (ft) x Room Width (ft) x 32
Space Btu = L x W x 32
Space Btu = 15 x 10 x 32
Space Btu = 4800 Btu

Window Btu:
Window Btu = Total Window Area (sq. ft) x 80
Window Btu = Window Length (ft) x Window Width (ft) x 80
Window Btu = L x W x 80
Window Btu = 4 x 4 x 80
Window Btu = 1280 Btu

Occupant Btu:
Occupant Btu = Number of Occupants x 600
Occupant Btu = 2 persons x 600
Occupant Btu = 2 x 600
Occupant Btu = 1200 Btu

Equipment Btu:
Equipment Btu = Total Equipment Wattage x 3.4
Equipment Btu = (TV watts + Radio watts + Laptop watts) x 3.4
Equipment Btu = (100 watts + 50 watts + 60 watts) x 3.4
Equipment Btu = (100 + 50 + 60)  x 3.4
Equipment Btu = 714 Btu

Lighting Btu:
Lighting Btu = Total Lighting Wattage x 4.3
Lighting Btu = 60 watts x 4.3
Lighting Btu = 60 x 4.3
Lighting Btu = 258 Btu


Total Heat Load
= Space Btu + Window Btu + Occupant Btu + Equipment Btu + Lighting Btu
= 4800 + 1280 + 1200 + 714 + 258
= 8252 Btu

Select 8000 Btu to 10,000 Btu Air Conditioning Unit


Commercial Air Conditioner Unit Selection & Prices (July 2013):

5000 Btu Room Air Conditioner
150 sq. ft. bedroom (15 ft x 10 ft) or 15 sq. meter (5 m x 3 m)
Price: $100


8000 Btu Room Air Conditioner
300 sq. ft. master bedroom (20 ft x 15 ft) or 35 sq. meter (7 m x 5 m)
Price: $200


10,000 Btu Room Air Conditioner
450 sq. ft. living room (30 ft x 15 ft) or 50 sq. meter (10 m x 5 m)
Price: $400


See also:
1. Basics of Refrigeration and Air Conditioning
*.
*.
*.
*.
*.
*.



#RX#

Why motors have large starting current

Reasons why motors have big starting current or inrush current:

- running motors produce back emf (electromotive force) in their coils

- back emf is caused by magnetic field which resists current (flow of electrons) through the armature winding

- back emf functions like a resistor which prevents current flow

- no back emf exists when motor is not running

- no back emf when motor is at rest

- at start up, resistance is zero (which is a short circuit condition), therefore current is large because no resistance to flow of current

- when a motor is running, the armature windings produce a resisting voltage (pressure) which opposes current flow as long as it is running

- a motor at rest acts like a transformer having secondary winding short circuited because the rotor windings in a squirrel cage induction motor is short circuited (closed circuit)

- starting currents (in-rush currents) for induction motors generally are 8 to 10 times the rated full load current

- a motor at rest has large inertia, therefore larger current is required to start it (this is similar to pushing a cart from stationary position, it's harder to push at the beginning, you need to exert a larger force and power, but as soon as the wheels start rolling, it is easier to push the car once it is moving)