Heat Recovery
Ventilation and heatrecovery calculations, sensible and latent heat  online calculators  imperial units.
A heatrecovery system captures building exhaust air and reuse some of the energy to precondition the makeup air before supplying it to the building.
Airtoair heatrecovery systems for comfort applications can in general be categorized as
 sensible heat (dry bulb only) systems
 total heat (wet bulb  sensible heat plus latent heat) systems
Sensible HeatRecovery Systems
A sensible heatrecovery systems will transfer sensible (drybulb) heat from the exhaust air to the supply air entering the building. Rotary wheel heat exchangers and cross flow heat exchangers are some of the common designs used in typical comforttocomfort sensible heatrecovery applications.
The amount of heat transferred can be expressed as
H = 1.08 q (t _{ 2 }  t _{ 1 } ) η (1)
where
H = heat transferred (Btu/h)
q = quantity of airflow (cfm)
t _{ 1 } = air temperature of the exhaust air before the heat recovery unit ( ^{ o } F)t _{ 2 } = air temperature of outside air after the heat recovery unit ( ^{ o } F)
η = heatrecover efficiency
Example  A Sensible Heat Recovery System
The 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 heatrecovery can be expressed as
H = 1.08 (40000 cfm) ((90 ^{ o } F)  (75 ^{ o } F)) 0.73
= 473040 (Btu/h)
A Sensible Heat Recovery Calculator
The calculator below can be used to calculate the sensible heat recovery where the air flow, outside and inside temperature and recovery efficiency are known.
Total Enthalpy Heat Recovery System
For total enthalpy heatrecovery systems the heat recovered can be expressed as
H = 4.5 q (h _{ 2 }  h _{ 1 } ) η (2)
where
4.5 = (0.075 lb _{ m } /ft ^{ 3 } ) (60 min/h)
h _{ 1 } = air enthalpy of the exhaust air before the heat recovery unit (Btu/lb)
h _{ 2 } = air enthalpy of outside air after the heat recovery unit (Btu/lb)
Related Mobile Apps from The Engineering ToolBox
 free apps for offline use on mobile devices.
Example  Total Enthalpy Heat Recovery System
The outside air wetbulb temperature is 70ºF ( enthalpy 34 Btu/lb ) and the exhaustair wetbulb temperature is 60ºF ( enthalpy 26.4 Btu/lb ). The system operates at 40,000 cfm at 73 percent efficiency.
The total heatrecovery can be expressed like
H = 4.5 (40000 cfm) ((34 Btu/lb)  (26.4 Btu/lb)) 0. 73
= 998640 (Btu/h)
A Total Heat Recovery Calculator
The calculator below can be used to calculate the total (latent and sensible) heat recovery where the air flow, enthalpy of outside and inside air and the recovery efficiency are known.
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