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Ventilation Efficiency

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

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, ^oF)

t_s = supply air temperature (^oC, ^oF)

t_rz = room temperature in 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 zero.

Mixed Ventilation

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

Pollution Efficiency

Pollution or contaminant 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, 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, and often much higher depending on the actual conditions, than one.