Ventilation Efficiency

The efficiency of ventilation systems are in general related to

• temperatures, and/or
• pollution and contaminants concentrations

in supply air, room air and outlet air.

Temperature Efficiency

Temperature efficiency of a ventilation system can be calculated as

μ _t = (t _o - t _s ) / (t _rz - t _s ) (1)

where

μ _t = temperature efficiency

t _o = outlet air temperature ( ^oC, ^oF)

t _s = supply air temperature ( ^oC, ^oF)

t _rz = room temperature in the residence zone - mean value ( ^oC, ^oF)

The potential efficiency of a system depends on the ventilation principle used.

Short Cut Ventilation

With "short cuts" the outlet temperature is close to the supply temperature and the ventilation efficiency approaches to  zero.

Mixed Ventilation

With mixed ventilation the outlet temperature is close to the room temperature in the residential zone and the ventilation efficiency approaches to one.

Displacement Ventilation

With displacement ventilation the outlet temperature is higher than the temperature in the residence zone and in the supply air. The ventilation efficiency is higher (often much higher depending on the actual conditions) than one.

Piston Ventilation

Piston ventilation is an extreme variant of displacement ventilation. The efficiency is always higher than one.

Pollution Efficiency

Pollution or contaminant efficiency may calculated as

μ_v = c _o / c _rz (2)

where

μ_v = ventilation efficiency

c _o = pollution concentration in the outlet air (kg/m³, lb_m /ft³ )

c _rz = pollution concentration in the air in the residence zone, mean value (kg/m³, lb_m /ft³ )

The pollution or contaminant concentration in the outlet air can be expressed as

c _o = q _s / q_v (3)

where

q _s = pollution flow in the air (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 )

Short Cut Ventilation

With "short cuts" the pollution concentration in the outlet is less than in the air in the residential zone and the ventilation efficiency will approach to zero.

Mixed Ventilation

With mixed ventilation the outlet pollution concentration is close to the pollution concentration in the residential zone and the ventilation efficiency will approach to one.

Displacement Ventilation

With displacement ventilation the outlet pollution concentration is higher than the pollution concentration in the residence zone. The ventilation efficiency is higher (often much higher depending on the actual conditions) than one.

Piston Ventilation

Piston ventilation is an extreme variant of the displacement ventilation . The efficiency is always higher than one.