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Heaters and Coolers in Ventilation Systems

The basic equations for heat transfer and the selecting criteria for heaters and coolers in ventilations systems

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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/kgoC)

qv = volume flow air (m3/s)

to = temperature out of heater (oC)

ti = 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/kgoC)

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 (100oC) the evaporating enthalpy is 2258 kJ/kg (539 kcal/kg). At ten bar absolute pressure (180oC) 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 110oC for manual reset and one safety switch for automatic reset at 65oC)
  • 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/kgoC)

qv = volume flow air (m3/s)

to = temperature out of the cooler (oC)

ti = 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)

hienthalpy 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
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