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# 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 (t o - t i ) (1)

where

H = heat (W)

ρ = air density (1.2 kg/m3 )

cp = specific heat of air (1.0 J/kg oC)

q v = volume flow air (m3 /s)

t o = 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)

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 (1000 kg/m3 )

c pw = specific heat of water (4.2 J/kg oC)

q w = water volume flow (m3 /s)

t wi = temperature water in to the heater (oC)

t wo = 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 = q s h e (4)

where

q s = steam flow (kg/s)

h e = 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:

H c = ρ cp qv (t i - t o ) (5)

where

H c = 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)

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 the enthalpy of air - the sensible and latent heat - must be used in the calculations:

H c = ρ qv (h i - h o ) (6)

where

h o = enthalpy in moist air out from the cooler (kJ/kg)

h i = 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

• ### 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.

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## Citation

• The Engineering ToolBox (2003). Heaters and Coolers in Ventilation Systems. [online] Available at: https://www.engineeringtoolbox.com/heater-coolers-ventilation-systems-d_200.html [Accessed Day Month Year].

Modify the access date according your visit.

6.3.17

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