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# Design of Ventilation Systems

## Design procedure for ventilation systems - air flow rates, heat and cooling loads, air shifts according occupants, air supply principles

The procedure below can be used to design ventilation systems:

• Calculate heat or cooling load, including sensible and latent heat
• Calculate necessary air shifts according the number of occupants and their activity or any other special process in the rooms
• Calculate air supply temperature
• Calculate circulated mass of air
• Calculate temperature loss in ducts
• Calculate the outputs of components - heaters, coolers, washers, humidifiers
• Calculate boiler or heater size
• Design and calculate the duct system

### 1. Calculate Heat and Cooling Loads

Calculate heat and cooling loads by

• Calculating indoor heat or cooling loads
• Calculating surrounding heat or cooling loads

### 2. Calculate Air Shifts according the Occupants or any Processes

Calculate the pollution created by persons and their activity and processes.

### 3. Calculate Air Supply Temperature

Calculate air supply temperature. Common guidelines:

• For heating, 38 - 50oC (100 - 120oF) may be suitable
• For cooling where the inlets are near occupied zones , 6 - 8oC (10 - 15oF) below room temperature may be suitable
• For cooling where high velocity diffusing jets are used, 17oC (30oF) below room temperature may be suitable

### 4. Calculate Air Quantity

#### Air Heating

If air is used for heating, the needed air flow rate may be expressed as

qh = Hh / (ρ cp (ts - tr))                                  (1)

where

qh = volume of air for heating (m3/s)

cpspecific heat air (J/kg K)

ts = supply temperature (oC)

tr = room temperature (oC)

ρ = density of air (kg/m3)

#### Air Cooling

If air is used for cooling, the needed air flow rate may be expressed as

qc = Hc / (ρ cp (to - tr))                                  (2)

where

qc = volume of air for cooling (m3/s)

to = outlet temperature (oC) where to = tr if the air in the room is mixed

If the heat load is Hh = 400 W, supply temperature ts = 30 oC and the room temperature tr = 22 oC, the air flow rate can be calculated as:

qh = (400 W) / ((1.2 kg/m3) (1005 J/kg K) ((30 oC) - (22 oC)))

= 0.041 m3/s

= 149 m3/h

#### Humidifying

If the outside air is more humid than the indoor air - then the indoor air can be humidified by supplying air from the outside. The amount of supply air can be calculated as

qmh = Qh / (ρ (x1 - x2))                                 (3)

where

qmh = volume of air for humidifying (m3/s)

Qh = moisture to be supplied (kg/s)

ρ = density of air (kg/m3)

x2 = humidity of room air (kg/kg)

x1 = humidity of supply air (kg/kg)

#### Dehumidifying

If the outside air is less humid than the indoor air - then the indoor air can be dehumidified by supplying air from the outside. The amount of supply air can be calculated as

qmd = Qd / (ρ (x2 - x1))                               (4)

where

qmd = volume of air for dehumidifying (m3/s)

Qd = moisture to be dehumidified (kg/s)

#### Example - Humidifying

If added moisture Qh = 0.003 kg/s, room humidity x1 = 0.001 kg/kg and supply air humidity x2 = 0.008 kg/kg, the amount of air can expressed as:

qmh = (0.003 kg/s) / ((1.2 kg/m3) ((0.008 kg/kg)- (0.001 kg/kg)))

= 0.36 m3/s

Alternatively the air quantity is determined by the requirements of occupants or processes.

### 5. Temperature Loss in Ducts

The heat loss from a duct can be calculated as

H = A k ((t1 + t2) / 2 - tr )                                        (5)

where

H = heat loss (W)

A = area of duct walls (m2)

t1 = initial temperature in duct (oC)

t2 = final temperature in duct (oC)

k = heat loss coefficient of duct walls (W/m2 K) (5.68  W/m2 K for sheet metal ducts, 2.3 W/m2 K for insulated ducts)

tr = surrounding room temperature (oC)

The heat loss in the air flow can be expressed as

H = 1000 q cp (t1 - t2)                                  (5b)

where

q = mass of air flowing (kg/s)

cp = specific heat air (kJ/kg K)

(5) and (5b) can be combined to

H = A k ((t1 + t2) / 2 - tr)) = 1000 q cp (t1 - t2)                                      (5c)

Note that for larger temperature drops logarithmic mean temperatures should be used.

### 6. Selecting Heaters, Washers, Humidifiers and Coolers

Units as heaters, filters etc. must on basis of of air quantity and capacity be selected from manufacture catalogs.

### 7. Boiler

Boiler rating can be expressed as

B = H (1 + x)                                            (6)

where

B = boiler rating (kW)

H = total heat load of all heater units in system (kW)

x = margin for heating up the system, it is common to use values 0.1 to 0.2

Boiler with correct rating must be selected from manufacture catalogs.

### 8. Sizing Ducts

Air speed in a duct can be expressed as:

v = Q / A                                       (7)

where

v = air velocity (m/s)

Q = air volume (m3/s)

A = cross section of duct (m2)

Overall pressure loss in ducts can be calculated as

dpt = dpf + dps + dpc                                           (8)

where

dpt = total pressure loss in system (Pa, N/m2)

dpf = major pressure loss in ducts due to friction (Pa, N/m2)

dps = minor pressure loss in fittings, bends etc. (Pa, N/m2)

dpc = minor pressure loss in components as filters, heaters etc. (Pa, N/m2)

Major pressure loss in ducts due to friction can be calculated as

dpf = R l                                    (9)

where

R = duct friction resistance per unit length (Pa, N/m2 per m duct)

l = length of duct (m)

Duct friction resistance per unit length can be calculated as

R = λ / dh (ρ v2 / 2)                                   (10)

where

R = pressure loss (Pa, N/m2)

λ = friction coefficient

dh = hydraulic diameter (m)

## Related Topics

• Ventilation - Systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more

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Temperature

oC
oF

Length

m
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in
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yards
miles
naut miles

Area

m2
km2
in2
ft2
miles2
acres

Volume

m3
liters
in3
ft3
us gal

Weight

kgf
N
lbf

Velocity

m/s
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ft/min
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mph
knots

Pressure

Pa (N/m2)
bar
mm H2O
kg/cm2
psi
inches H2O

Flow

m3/s
m3/h
US gpm
cfm

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