Heat-Recovery

Calculating ventilation heat-recovery processes

A heat-recovery system will capture a building exhaust air and reuse the energy to precondition the outside air before supplied to the building.

In comfort-to-comfort applications, the energy recovery process is reversible - the enthalpy of the building supply air is lowered during warm weather and raised during cold weather.

Air-to-air heat-recovery systems for comfort-to-comfort applications fall into two general categories:

• sensible heat (dry bulb only) systems
• total heat (wet bulb, sensible heat plus latent heat) systems

Sensible Heat-Recovery Systems

A sensible heat-recovery systems transfer sensible (dry-bulb) heat between the exhaust air leaving the building and supply air entering the building.

Rotary wheel heat exchangers are common used in typical comfort-to-comfort sensible heat-recovery applications.

The amount of heat transferred can be expressed as:

H = 1.08 q dt η (1)

where

H = heat transferred (Btuh)

q = quantity of airflow (cfm)

1.08 = a constant for sensible heat equations

dt = the dry-bulb temperature difference between the airstreams (^oF)

η = heat-recovery efficiency

Example - Sensible Heat Recovery System

The average summer outside air temperature is 90ºF and the exhaust air temperature is 75ºF. The system operates at 40,000 cfm at 73 percent efficiency.

The sensible heat-recovery can be expressed as:

H = 1.08 40,000 ( cfm) (90 - 75)(ºF) 0.73

= 443,040 Btuh

Total (enthalpy) Heat-Recovery System

For total (enthalpy) heat-recovery systems, the heat recovered can be expressed as

H = 4.5 q dh η (2)

where

4.5 = a constant for total heat equations, and

dh = the enthalpy (total heat) difference between the airstreams (from wet bulb temperatures)

Example - Total (enthalpy) Heat-Recovery System

Outside air wet-bulb temperature is 70ºF (enthalpy 34.1 Btu/lbºF) and the exhaust-air wet-bulb temperature is 60ºF (enthalpy 26.5 Btu/lbºF). The system operates at 40,000 cfm at 73 percent efficiency.

The total heat-recovery can be expressed as:

H = 4.5 40,000 ( cfm) (34.1 - 26.5)(Btu/lbºF) 0. 73

= 998,640 Btuh

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