Ventilation Principles

Some commonly used ventilation principles - the short cut, mixed air, displacement and piston principle

Ventilation systems may be classified according their way to supply and withdraw air from the rooms ventilated. It is common to differentiate in the

• short cut principle
• mixed principle
• displacement principle
• piston principle

Short Cut Principle

The ventilation system is "short cut" when the make up air is withdrawn from the room before it has been in the zones where people operates.

A "short cut" reduces the efficiency of the ventilation system, has no mission, and is in general avoided.

Mixed Principle

With a ventilation system based on the mixed principle, make up air is supplied to the room with high speed, and/or local fans are used to mix the air in the room to an homogenous mass.

The mixed principle is

• suited in general for ventilation, cooling and heating systems
• where homogeneous temperatures in the room are required
• where homogeneous pollution concentrations in the room are required

Displacement Principle

With the displacement principle heat and pollution is transferred from the residence zone close to the floor, up to the ceiling where it's evacuated through the outlet system.

Make up air is supplied with low velocity very close to the floor. The supply air is normally colder than the average air in the residence zone. The evacuated air close to the ceiling is warmer than the average air in the residence zone.

Activities in the room, people and machines, creates convective air flows from the floor to the ceiling:

• heat and pollutions are transferred out of the residence zone
• heat (lights) supplied under the ceiling have limited influence on the temperatures in the room
• the cooling temperatures of the air supplies are limited to some degrees under the temperature in the residence zone
• the pollution concentration in the residence zone is limited

The displacement ventilation system is suited for ventilation and cooling systems. The system is not suited for heating.

Piston Principle

In a ventilation system based on the piston principle, the supply air moves through the rooms as a piston. It may often be regarded as an extreme variant of the displacement system with a minimum of turbulence in the air flow through the room.

• used in special cases as clean rooms, operating theatres etc.

Ventilation Efficiency

The efficiency of a ventilation system may be related to temperature or pollution concentration.

Temperature Efficiency

The temperature efficiency of a ventilation system may be expressed as:

μ_t = (t_o - t_s) / (t_rz - t_s) (1)

where

μ_t = temperature efficiency

t_o = outlet air temperature (^oC)

t_s = supply air temperature (^oC)

t_rz = room temperature in residence zone, mean value (^oC)

Pollution Efficiency

Pollution efficiency may expressed as:

μ_v = c_o / c_rz (2)

where

μ_v = ventilation efficiency

c_o = pollution concentration in the outlet air (kg/m³)

c_rz = pollution concentration in the air in the residence zone, mean value (kg/m³)

The pollution concentration in the outlet air can be expressed as

c_o = q_s / q_v (3)

where

q_s = supply of pollution (kg/s)

q_v = outlet air flow (m³/s)

The mean pollution concentration in the room can be expressed as:

c_rz = G / V (4)

where

c_rz = pollution concentration in the room (kg/m³)

G = total pollution in the room (kg)

V = volume of the room (m³)

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