HVAC LOAD CALCULATIONS
DUCT SIZING
DUCT LAYOUT
CLASSICAL APPROACH
(Calculation of Sensible & Lastent Loads)
Heat Gains by Thermal Conduction
Heat Gains across Fenestration by mode of Radiation
Occupancy Loads
Illumination Loads
Electrical Appliance Loads
IT Loads
Psychrometry Loads
VENTILATION RATE
( C o n d i t i o n e d S p a c e Vo l u m e s & F l o o r A re a C a l c u l at i o n s )
SENSIBLE & LATENT LOADS
Sensible Heat Gains:
H e a t Tr a n s f e r a c r o s s B l d g E n v e l o p e b y T h e r m a l
Conduction, Solar Radiation through Fenestration
S y s t e m s , H e a t G a i n s d u e t o O c c u p a n c y, H e a t g a i n s d u e t o
Lighting systems, electric motors (fans), Electrical
Appliances such as coffee & tea dispensers, Electronic
e q p t s u c h a s L a p t o p s , P C ’s , L C D p a n e l s , H o m e T h e a t e r
systems, Psychrometric loads etc.
Latent Heat Gains:
Humans, pets, potted plants, indoor water bodies, food,
air infiltration, wall moisture, etc.
Calculation: Breathing Zone Air Flow Rate
Average Fresh air requirement per person per second is 8 ltrs (Standard breathing
rate)
Based on ASHRAE standards, ideal ventilation rates for building space is expressed as:
Design airflow rate per unit area (cfm per square foot)
Design airflow rate based on occupancy (cfm per person)
Vbz = Rp · Pz + Ra · Az
Vbz – Air Vol requirements for breathing zone
Az = zone floor area (the net occupiable floor area of the ventilation zone in sqft/sqm)
Pz = zone population
Rp = outdoor airflow rate required per person as determined from ASHRAE Table 6-1
Ra = outdoor airflow rate required per unit area as determined from ASHRAETable 6-1
Example
Consider
A Restaurant (Byuilding Type)
of Floor Area 5000 sqft
Design Occupancy of 200 pers
As per ASHRAE Table 6.1
The minimum ventilation rate for a restaurant is 0.18 cfm/sqft and 7.5 cfm/pers.
Area component = 0.18 cfm/sq.ft x 4,500 sq.ft = 810 cfm
Occupancy component = 10 cfm/person x 300 persons = 3,000 cfm
Total Airflow = 810 cfm + 3,000 cfm = 3810 cfm
This value is called the Breathing Zone Outdoor Airflow.
It is then divided by the Zone Air Distribution Effectiveness, to obtain the Zone
Outdoor Airflow that must be supplied by the ventilation system. If the distribution
effectiveness for the example above is 0.8,
the zone outdoor airflow must be 3810 cfm / 0.8 = 4762.5 cfm.
Cooling Tons
Assuming that
The Floor to ceiling height is in the range of 8 to 10 feet
1 cfm is the conditioned air requirement per sqft of floor area.
This is the average air quantity required for a room or an entire building.
This number is based upon an averaged heat load calculation for comfort
conditioning.
1 TR = 400 cfm for Centralized air conditioned spaces
1 TR = 300 cfm for Non Centralised spaces
Following the Guideline
1 cfm will cater for 1 sqft area of conditioned space.
Therefore, 1 TR = 400 cfm will cool 400 sqft area of conditioned space.
Duct Sizing
Duct Type & Air Velocities
Primary/Main Ducts
Q (Supply Plenum)– 6 to 8
m/s
Branches – 4 to 5 m/s
Air Distribution Ducts –
2 m/s
Trunk & Branch Supply & Exhaust Grilles
Duct System
– 1 to 3 m/s
Reflected Ceiling Plan (Ducts)
Illumination Load
Q lighting = Wattage value x Use factor x Allowance factor
Taken as 1.25 to account for
Modular Lighting
power usage of ballasts/drivers
Unit
in FLs & LEDs
Use Factor is the ratio of actual wattage in use to rated wattage.
(For residences, offices & shops – 1, For Industrial setups – 0.5)
Electrical Motors
For fans & Motors (Food processors, Juicers, Mixers)
For heating devices (Hot plates, snack stations etc.)
Q = Motor rating in KW x Load Factor/Efficiency
eqpt
(Load factor is 1 if usage at max rating & Efficiency ~ 80%)
Ref to charts for sensible & latent load values of Kitchen appliances
Occupancy Load
Work out the average no of occupants of the design space.
If Heat Gains (SL+LL) for Men is x,
Heat gains for Women is 0.85 x
& Children is 0.75 x
Nature of Activity Where Sensible (Watts) Latent (Watts)
Seated (at rest) Cinema, 53 44
Libraries
Seated (light work) Offices 57 67
Standing, light work, walking Malls 58 88
slowly
Moderate Heavy Work SHG’s, SSU’s 88 205
Heavy Work Shop Floors 136 288
Conduction Gains
0.48 0.81 0.024 0.81 0.48 K-Value in W/m.K
k1 k2 k3 k4 k5
Q
Tе Tі
d1 d2 d3 d4 d5
0.013 0.230 0.115 0.115 0.013 Layer thickness in m
Solar Heat Gain through Fenestration systems
G Factor
Emitted Gains (Q2)
Sol Zenith ()
Sol Alt ()
Absorbptance ()
U Factor
Conductive Gains (Q3)
Ventilation Load
Assume a scenario of 25 guests in a non smoking coffee shop.
Fresh air requirement/person = 0.5 m³/min.
Identify the purpose of the conditioned space to fix
fresh outside air content.
(Residence-0.3, Office-0.2, Cinemas-0.3,Hospitals-0.9, Bar-0.8, Restaurant-0.7,
Washrooms-0.9, Banks-0.4, Kitchens-0.9, Classrooms-0.3)
Fix Recirculation value
(If Ventillation is set to 50 %, Outside/Fresh Air inducted = 12.5 m³/min & Recirculated Air = 12.5
m³/min.
OASH (Outside Air Sensible Heat) & OALH (Outside Air Latent Heat) will now be calculated for Air
Volume of 5 m³/min. )
OASH = 0.02044 . v . (T₂-T₁) Watts.
OALH is given as = 50 . v . (W₁-W₂) Watts.
T2 (Outside air temp), W1 (humidity of outside air at entry),
T1 (Conditioned air temp), W2 (humidity of air after conditioning)
Qvent = OASH + OALH
