Heater and Coolers in Ventilation Systems

Engineering ToolBox > Heater and Coolers in Ventilation Systems

Basic equations of heat transfer and selecting criteria of heaters and coolers

Classification of heater and coolers

It is often common to classify both heaters and coolers by the medium used to provide or remove the energy:

water heated
steam heated
electrical heated
water cooled
brine cooled
evaporative medium cooled

Basic Heating Equations

Heating Air

The basic equation expressing the heating process of the air in an heater can be expressed like

H = ρ c_p q_v (t_o - t_i) (1)

where

H = heat (W)

ρ = air density (kg/m³)

c_p = air specific capacity (J/kg^oC)

q_v = volume flow air (m³/s)

t_o = temperature out of the heater (^oC)

t_i = air temperature into the heater (^oC)

Heat Transfer through Exchanger Surface

The heat transfer through a heat exchanger surface can be expressed as:

H = A dt_m U (2)

where

A = heat surface (m²)

dt_m = 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 = ρ_w c_pw q_w (t_wi - t_wo) (3)

where

ρ_w = density water (kg/m³)

c_pw = water specific capacity (J/kg^oC)

q_w = water flow (kg/s)

t_wi = temperature water to the heater (^oC)

t_wo = temperature water from the heater (^oC)

Steam Heater

If saturated steam is used to heat the air, the heating process can be expressed as:

H = q_s h_e (4)

where

q_s = steam flow (kg/s)

h_e = evaporating enthalpy 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:

H_c = ρ c_p q_v (t_i - t_o) (5)

where

H_c = heat removed from air (W)

ρ = air density (kg/m³)

c_p = air specific capacity (J/kg^oC)

q_v = volume flow air (m³/s)

t_o = 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 it is necessary to use the enthalpy of air in the calculations:

H_c = ρ q_v (h_i - h_o) (6)

where

h_o = enthalpy in the air out from the cooler (kJ/kg)

h_i = enthalpy in the 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