PRINCIPLES OF VENTILATION DESIGN

PRINCIPLES OF VENTILATION DESIGN

1. Calculate heat or cooling load, including sensible and latent heat

2. Calculate necessary air shifts according the number of occupants and their activity or any other special process in the rooms

3. Calculate air supply temperature

4. Calculate circulated mass of air

5. Calculate temperature loss in ducts

6. Calculate the outputs of components - heaters, coolers, washers, humidifiers

7. Calculate boiler or heater size

8. 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 may be suitable

For cooling where the inlets are near occupied zones - 6 - 8oC below room temperature

4. Calculate Air Quantity

Air Heating

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

qh = Hh / cp (ts - tr) (1)whereqh = volume of air for heating (m3/s)Hh =heat load (W)cp = specific heat capacity of air (J/kg K)ts = supply temperature (oC)tr = room temperature (oC) = density of air (kg/m3)Air Cooling

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

qc = Hc / cp (to - tr) (2)where:qc = volume of air for cooling (m3/s)Hc =cooling load (W)to = outlet temperature (oC) where to = tr if the air in the room is mixedExample - heating load:

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/hMoisture

If it is necessary to humidify the indoor air, the amount of supply air needed may be calculated as:

qmh = Qh / (x2 - x1) (3)whereqm = volume of air for humidifying (m3/s)Qh = moisture to be supplied (kg/s) = density of air (kg/m3)x2 = humidity of room air (kg/kg)x1 = humidity of supply air (kg/kg)Dehumidifying

If it is necessary to dehumidify the indoor air, the amount of supply air needed may be calculated as:

qmd = Qd / (x1 - x2) (4)whereqmd = volume of air for dehumidifying (m3/s)Qd = moisture to be dehumified (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/sAlternatively 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 expressed as:

H = A k ( (t1 + t2) / (2 - tr) ) (5)whereH = 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 = q cp (t1 - t2) (5b)whereq = mass of air flowing (kg/s)cp = specific heat capacity of air (kJ/kg K)(5) and (5b) can be combined to

H = A k ((t1 + t2) / 2 - tr)) = q cp (t1 - t2) (5c)For large temperature drops should logarithmic mean temperatures 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 manufactures catalogues.

7. Boiler

The boiler rating can be expressed as:

B = H (1 + x) (6)whereB = 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 0.2Boiler with correct rating must be selected from manufacturer catalogues.

8. Sizing Ducts

Air speed in a duct can be expressed as:

v = Q / A (7)wherev = air velocity (m/s)Q = air volume (m3/s)A = cross section of duct (m2)Overall pressure loss in ducts can be expressed as:

dpt = dpf + dps + dpc (8)wheredpt = 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 expressed as

dpf = R l (9)whereR = 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 expressed as

R = / dh ( v2 / 2) (10)whereR = pressure loss (Pa, N/m2) = friction coefficientdh = hydraulic diameter (m)Summer Conditions

For summer the values below may be used as an indication of acceptable conditions.

Optimum temperature20oC to 22oC

Optimum relative humidity40% to 65%

Preferred indoor conditions for exposures less than 3 hours:

Outside dry bulbtemperatureInside air conditions withdew point constant at 14oC

oCoC maxoC min%

352718.544

322618.046

292517.852

272417.551

242317.257

212217.057

Some common used thermal properties for water:

Maximum density at 4 oC - 1,000 kg/m3, 1.940 slugs/ft3 Specific Weight at 4 oC - 9.807 kN/m3 Freezing temperature - 0 oC (Official Ice at 0 oC) Boiling temperature - 100 oC Latent heat of melting - 334 kJ/kg Latent heat of evaporation - 2,270 kJ/kg Critical temperature - 380 oC - 386 oC Critical pressure - 221.2 bar, 22.1 MPa (MN/m2) Specific heat capacity water - 4.187 kJ/kgK Specific heat capacity ice - 2.108 kJ/kgK Specific heat capacity water vapor - 1.996 kJ/kgK Thermal expansion from 4 oC to 100 oC - 4.2x10-2 Bulk modulus elasticity - 2.15 x 109 (Pa, N/m2)Thermal properties of water:

Temperature - t -Absolute pressure- p -Density- - Specific volume - v -Specific Heat- cp -Specific entropy- e -

(oC)(kN/m2)(kg/m3)10-3 (m3/kg)(kJ/kgK)(kJ/kgK)

0 (Ice)916.8

0.010.6999.81.004.2100

4 (max.density) 0.91000.0

50.91000.01.004.2040.075

101.2999.81.004.1930.150

151.7999.21.004.1860.223

202.3998.31.004.1830.296

253.2997.11.004.1810.367

304.3995.71.004.1790.438

355.6994.11.014.1780.505

Relative humidity < 40 - 60%Air shifts (recommended) = 5 10 airshifts/hour for electrical rooms and warehouses

10 15 airshifts/hour factory buildings, fumes & moisture

2 4 airshifts/hour generally industrial areas

Reccomended temperature 15-27 deg.C (with dew point constant at 14 deg.C)

To avoid damage of products, or to achieve proper process conditions, it is often important to keep the environment and the indoor climate within certain temperature and humidity limits.

Low relative humidity may dry up the product, or high relative humidity may increase the water activity growing mould in the production process lines.

The table below can be used as a guidance to recommended Relative Humidity - RH - in some common production and process environments.

Production and Process EnvironmentRecommended Relative Humidity - RH -(%)

Breweries35 - 45%

Transformer Winding15 - 30%

Semiconductors30 - 50%

Paper Storage35 - 45%

Preventing Rust and Corrosionbelow 55%,< 40% for no rust generation

Spray Paint30 - 50%

Laboratory electronics45 - 60%

Plastic Pallets5 - 30%

Computer Peripherals50 - 60%

Rust ResistanceBelow 40%

Powder Storage30 - 45%

Wood Drying25 - 35%

Normal Storage50 - 55%

Chemical Laboratory30 - 45%

Ventilation systems - air intakes and outlets and rules of thumbs

Intakes

Intakes should be at least 0.15 m above the terrain. In areas with traffic the intake should be at least 5 m above the terrain.

The distance between intake, firewalls and surrounding buildings must be according the rules of the local authorities.

Short cutting intake air with used outlet air must be avoided.

An air intake should be located in a position where the wind influence on the pressure conditions within the system is limited.

Air velocities in the intake openings should not exceed 2.5 m/sOutlets

Outlets should go direct out in unrestricted area.

Short cuts with intake air, window openings and residence areas must be avoided.

The distance between outlet, firewalls and surrounding buildings must be according the rules of the local authorities.

An air outlet should be located in a position where the wind influence on the pressure conditions within the system is limited.

The air velocity through the outlet should not exceed 3 - 15 m/s