Skip to main content

Design of Ventilation Systems

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)
Hh = heat load (W)
cp = specific 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)
Hc = cooling load (W)
to = outlet temperature (oC) where to = tr if the air in the room is mixed

Example - 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/h

Moisture

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)

Comments

Post a Comment

Popular posts from this blog

Fire Hydrant System

A fire hydrant, (also known as fire pumps, hydrant boosters, fire water pumps)is a connection point by which firefighters can tap into a water supply. These are high pressure water pumps designed to increase the fire fighting capacity of a building by boosting the pressure in the hydrant service when mains is not enough, or when tank fed. A Fire Hydrant with fully-on valve , releasing pressure water. Safe Operating Procedure- Fire Hydrant The user attaches a hose to the fire hydrant, then opens a valve on the hydrant to provide a powerful flow of water.  Most fire hydrant valves are not designed to throttle the water flow; they are designed to be operated either full-on or full-off. Attaching Hose with Fire Hydrant When a firefighter is operating a hydrant, he or she typically wears appropriate personal protective equipment, such as gloves and a helmet with face shield worn. High-pressure water coursing through a potentially aging and corroding hydrant coul...
Post a Comment
Read more

Ergonomics in the Office-1

Millions of people go to work every day by sitting down in front of a computer. These jobs certainly don’t come with the same dangers as working in a factory or with heavy equipment but the office job can pose very real risks to health. Neck and back pain, shoulder stiffness and wrist pain are a few associated long-term hazards. Fortunately, learning a few things about proper ergonomics in the office can go a long way toward mitigating these risks. Here are a few tips to keep in mind:   Have a Well Designed Chair:  If sitting all day is unavoidable, then having a good chair to do it in is a must. Look for a chair that has firm cushioning and easy height adjustments. Lower back and well-placed arm support are mandatory. Good Posture:  This seems obvious, but the best chair will not do any good if the person sitting in it is slouched over all day. The spine should be straight, keeping the torso perpendicular to the floor. Arms should come straig...
Post a Comment
Read more

Ergonomic Home Design for the Disabled

How difficult is it to open a jar without the full use of one’s hands? How safe is it to step into the shower when one has arthritis or a knee injury? Is getting into a car always as simple as one-two-three, or could it take more planning for someone with an injured back? Asking these kinds of questions – and many more – is part of a new and growing dimension of design. The trend in making products – and information – more accessible to those with any kind of disability is gathering momentum. Interestingly, seeking design solutions that meet the needs of the disabled results in a better overall design, benefitting both the able and disabled. New terminology has been coined to describe more inclusive design processes, including terms such as accessible design, barrier-free design and assistive technology. Universal design is a relatively new approach that has emerged from these models and describes the design elements of buildings, products and environments that allow fo...
Post a Comment
Read more