Water-based Indoor Climate Systems, 2007
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HEAT TRANSFER
HEAT TRANSFER
An individual’s heat exchange with the surroundings primarily occurs in three ways, these are:- heat emission through radiation to surrounding areas or to free space.
- heat emission through convection to the surrounding air.
- heat emission through the evaporation of fluid, which takers place mainly through us perspiring.
A fourth form of heat exchange can also occur through conducting to fixed or floating objects in direct contact with the body. However, in normal cases this is so small it is completely negligible.
Radiant heat
Radiant heat is constantly emitted from warmer to colder surfaces and increases with the temperature difference between these. The radiant heat exchange is primarily dependent on the following factors:- the size and placement of the surfaces = solid angle relation to each other
- the individual temperatures of the surfaces
- the character of the surfaces, which determines the emission and absorption factors, i.e.. the ability to emit and receive radiant heat
We make a distinction between two types of radiant heat:- High temperature radiation from bodies with temperatures above approximately +500°C
- Low temperature radiation from bodies with temperatures below approximately +250°C
The temperature limits above are only approximate.
As the room surfaces and conventional heaters have, relatively low temperatures, a person’s heat transfer with surrounding surfaces takes place indoors in the form of long wave, low temperature radiation. The structure and colour of the surfaces have virtually no significance for low temperature radiation as regards its capacity to emit and absorb thermal radiation with the exception of untreated metal surfaces. Examples of sources of low temperature radiation are panel radiators as well as ceiling and floor heating systems.
When calculating the indoor climate within the temperature range -50 to + 100°C the heat transfer between the room’s surfaces is made up of invisible long wave, low temperature radiation. You can also completely disregard the radiation transfer through emission and absorption from and to the room air. Accordingly, with room thermal balance calculations only the low temperature radiation between the room’s different surfaces, walls, floor, ceiling, furniture, heater etc should be taken into consideration and any incoming high temperature radiation from the sun.
Heat transfer via radiation between separate room surfaces is normally expressed with the following equation:
PS = aS· A1 · (t1 - tm) [W]
PS = transfer of the heating effect in W between the surface A1 in m2 that has the temperature t1 with all other room surfaces which combined have a mean surface temperature tm
aS = thermal exchange constant for radiation in W/m2 K
With good accuracy, (deviation less than approximately 2.5 %) aS within the temperature interval 0 to + 100°C for t1 and tm can be noted.
aS= e0 · (4.38 + 0.034 · (t1 + tm)) [W/m2 K]e0 = emissivity
Figure 4.Radiant transfer occurs between all surfaces that have different temperatures irrespective of the direction.
Convection
If a surface is warmer than the room air it emits heat to the room air. In the same way, the room air emits heat to a surface that is colder than the room air. This form of heat transfer is called convection and is divided into:- Natural convection
- Forced convection
Natural convection is obtained through the differences in density in the different layers of air, which are created by the temperature differences between the air and the different bodies that the air flows against or around. The air closest to the body surface has another temperature than the air outside of its boundary layer, which in doing so gives the separate layers a different density and types of flow occur with regard to the body’s form, temperature deviations and location.
Forced convection is obtained, for example, with delivery using a fan, i.e. the fan determines the air’s flow.
In this case the air is driven around the surfaces of the bodies by forces other than differences in density.
In summary, the convected heat transfer is affected by the air’s flow along the body surface, the size of the surface and the temperature difference between the body and air.
The heat transfer per unit area increases with the increased air velocity, reduced size of the surface and increased temperature difference between the body surface and the air.
Induction
Induction is a form of forced convection that occurs when an air jet passes stationary air at a high speed and the air is then drawn along with the air jet. The air jet then grows in volume.
The induction principle is utilized in active climate beams (climate beams with integrated supply air) as well as induction units, see the section entitled COOLING THE PREMISES.
Evaporation
Evaporation heat is expended when a liquid changes to a gas state. When a person perspires this evaporation heat is mainly taken from the body surface which then cools. People give off heat through evaporation. Heat transfer through evaporation and convection also takes place during breathing.
Heat emissions due to evaporation are dependent on the room air’s relative humidity. At normal temperatures between approximately +18°C to +25°C and normal relative humidity about 20-50% this effect is minimal.
If the moisture level is not kept at an acceptable level, typically 45% at 25°C, but when rising to 60% RH and above the skin surface becomes clammy. When this occurs it makes evaporation difficult. It is therefore important that the climate system is selected to maintain a moisture content in the building that is not too high. This is done by equipping the supply air units and comfort cooling system with sufficient cooling capacity to be able to dehumidify the supply air before it is supplied to the rooms, also see the section Condensation protection.
Figure 5.Heat emission from the body with different types of work.
A = Convection (green)
B = Radiation (red)
C = Evaporative emission (yellow)
1 = Total rest
2 = Light office work
3 = Normal office work
4 = Computer work
5 = Light physical work
6 = Slow walk
7 = Painting
Figure 6.Forced convection.