Heaters and Coolers in Ventilation Systems
Basic equations for heat transfer - selecting criteria for heaters and coolers in ventilations systems.
Classification of heaters and coolers
It is often common to classify both heaters and coolers by the medium used to provide or remove energy:
- water heated
- steam heated
- electrical heated
- water cooled
- brine cooled
- evaporation medium cooled
Basic Heating Equations
Heating Air
The basic equation to express the heating process of air in a heater is
H = ρ cp qv (to - ti ) (1)
where
H = heat (W)
ρ = air density (1.2 kg/m3)
cp = specific heat of air (1.0 J/kg oC)
qv = volume flow air (m3/s)
to = temperature out of heater (oC)
t i = air temperature in to heater (oC)
Heat Transfer through Exchanger Surface
The heat transfer through a heat exchanger surface can be expressed as
H = A dtm U (2)
where
A = heating surface (m2)
dtm = logarithmic mean temperature difference (oC)
U = heat transmission coefficient (W/m2K)
The heat transmission coefficient - U - depends on the air speed and the water (fluid) speed through the heater.
Water Heater
The heating process on the water (fluid) side of the heat exchanger can be expressed as
H = ρw cpw qw (twi - two ) (3)
where
ρw = density water (1000 kg/m3)
cpw = specific heat of water (4.2 J/kg oC)
qw = water volume flow (m3/s)
twi = temperature water in to the heater (oC)
two = temperature water out of the heater (oC)
Steam Heater
If saturated steam is used to heat air the heating process can be expressed as
H = qs he (4)
where
qs = steam flow (kg/s)
he = evaporating enthalpy of water (J/kg)
The evaporating enthalpy of water depends on the steam pressure. At one bar absolute pressure (100 oC) the evaporating enthalpy is 2258 kJ/kg (539 kcal/kg) . At ten bar absolute pressure (180 oC) the evaporating enthalpy is 2014 kJ/kg (481 kcal/kg) .
Selecting Heaters Criteria
Important design criteria when selecting heaters:
Water Heated Heaters
- increase of temperature (oC)
- air volume through the heater (m3/s)
- air speed through the heater (front area) between 2 to 5 m/s
- water inlet temperature (oC)
- water speed (m/s). Copper pipes 0.2 - 1.5 m/s. Steel pipes 0.2 - 3 m/s.
- maximum running and test pressure (Pa)
- maximum running temperature (oC)
- material properties for pipes and lamellas
- horizontal or vertical installation
Steam Heated Heaters
- increase of temperature (oC)
- air volume through the heater (m3/s)
- air speed through the heater (front area) between 2 to 5 m/s
- steam temperature (oC)
- steam pressure (Pa)
- steam properties (overheated or saturated)
- maximum running and test pressure (Pa)
- maximum running temperature (oC)
- material properties for pipes and lamellas
- horizontal or vertical installation (Note! Remember condensate drain)
Electrical Heated Heaters
- increase of temperature (oC)
- air volume through the heater (m3/s)
- air speed through the heater (front area) according manufactures recommendation
- voltage (V)
- control system (on/off, step or modulating)
- safety switches according local rules (in general one safety switch at 110 oC for manual reset and one safety switch for automatic reset at 65 oC)
- material properties for pipes and lamellas
- horizontal or vertical installation
Cooling Air
When coolers are calculated, it is important to know if the surfaces are dry or wet.
- If the cooling medium has a temperature below the dew point temperature of the air - the surface may be wet
- If the cooling medium has a temperature above the dew point temperature of the air - the surface is always dry
Dry Surface Cooling
For a cooler with a dry surface the same equation used for the heating process (1) may be modified and used:
Hc = ρ cp qv (ti - to ) (5)
where
Hc = heat removed from air (W)
ρ = air density (kg/m3)
cp = specific heat of air (J/kg oC)
q v = volume flow air (m3/s)
to = temperature out of the cooler (oC)
t i = air temperature in to the cooler (oC)
Wet Surface Cooling
For coolers with wet surfaces, the water vapor in the air is condensed and the enthalpy of air - the sensible and latent heat - must be used in the calculations:
Hc = ρ qv (hi - ho ) (6)
where
ho = enthalpy in moist air out from the cooler (kJ/kg)
hi = enthalpy in moist air into the cooler (kJ/kg)
Water (fluid) Flow through the Cooler
The water flow cooling process can be calculated with a modified formula similar as for the heating process.
Selecting Coolers Criteria
Important design criteria when selecting coolers:
- decrease of air temperature and enthalpy in cooler ( oC, kJ/kg)
- air volume through cooler (m3/s)
- air speed through the cooler (front area) - velocity should be between 2 to 3 m/s - with velocities above 3 m/s drop separators should be installed
- water inlet temperature (oC) or evaporating temperature for the cooling medium
- water speed (m/s) - maximum velocities in copper pipes should not exceed 0.2 - 2.0 m/s
- properties for the cooling medium
- maximum running and test pressure (Pa)
- maximum running temperature (oC)
- material properties for pipes and lamellas
- horizontal or vertical installation - Note! Wet surfaces must be drained
Related Topics
-
Ventilation Systems
Design of systems for ventilation and air handling - air change rates, ducts and pressure drops, charts and diagrams and more.
Related Documents
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Arithmetic and Logarithmic Mean Temperature Difference
Arithmetic Mean Temperature Difference in Heat Exchangers - AMTD - and Logarithmic Mean Temperature Difference - LMTD - formulas with examples - Online Mean Temperature Calculator. -
Design of Ventilation Systems
Design procedure for ventilation systems - air flow rates, heat and cooling loads, air shifts according occupants, air supply principles.