05.03 Effect of air leakage
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Introduction
Mass transfer is the simplest form of energy transfer to deal with. If some solid or fluid is transferred from one side of a facade to the other then it will probably undergo some temperature change. The temperature change represents a change in the energy of the substance, and so represents an energy transfer (another way of looking at this is to say that if a volume of conditioned air is lost from a building then it must be replaced with an equal volume of unconditioned air from outside a building - the energy ‘exchange’ is that energy required to condition the air entering the building).
Under steady conditions if air passes through a facade at a mass flow-rate of m' kg/s and has a specific heat capacity of Cp J/kgK then the energy transfer for a DT temperature change is,
DEm = m' Cp DT
If the volumetric flow-rate Q (in m3/s) of the air is known then the mass flow-rate is the volume flow-rate multiplied by the air density
m' = r Q
The air flow-rate may be a function of the pressure difference across the facade, in the case of natural ventilation or air leakage, or may be reasonably steady, in the case of forced ventilation.
Methods of installation of glazings, for instance, should be chosen after taking into account the detrimental effect that air leakage will have, which is far more significant if the glazing is particularly energy-efficient. A common design aim is that all air movements through the facade must be designed for, and incidental air movements avoided.
Although some air movement is required through the facade, for ventilation purposes, a modern view is that the air movement should be controlled. Uncontrolled air ‘leakage’ is generally the result of poor joint design, and may cause discomfort (draughts) and result in poor energy efficiency.
Discomfort
The presence of significant air movements may lead to occupier discomfort in two ways - either the occupier of a room is in the direct line of the moving air and feels cold as a result (the draught air is generally below body temperature even without allowing for ‘wind chill’), or some other lightweight item is in the direct path of the air jet and moves as a result (papers blowing off a desk, curtains billowing over a window or slatted blinds vibrating). The joint and gasket designers should aim to eliminate jets of air moving through a joint.
Draughts are discussed in CIRIA SP87A (1992), which gives a simple chart relating acceptable air velocity (in terms of occupant comfort) to air temperature. The acceptable velocity ranges from about 0.1 m/s for air at 17° C, to about 0.5 m/s for air at 30° C. For the laminar flow of air through a uniform gap between two parallel plane surfaces the mean air velocity is related to the pressure differential across the joint by
v = t2 Dp / 12 m l
v is the mean velocity, in m/s
t is the width of the gap between the surfaces, in m
m is the dynamic viscosity of the air, (about 1.8´ 10-5 Ns/m2)
Dp is the air pressure differential across the joint, in Pa
l is the length of the joint, in m
For a gap 0.1 mm wide and 50 mm long a mean air velocity of 0.1 m/s only requires a pressure difference of 108 Pa. This is the static pressure difference that will occur if a wind of 13 m/s blows against the outside surface of the building. Although this wind velocity may appear to be high it is classed on the Beaufort scale as a fresh to strong breeze. A larger joint width will obviously give a higher air velocity for the same pressure differential.
It should be noted that the air flow-rate through a 1 metre length of this joint is
Q = A v = 10-5 m3 / s
or
q = 3600 A v = 0.036 m3 / hr / metre of joint
The requirements of BS 6375 Part 1 (1989) state that for a 108 Pa pressure differential the acceptable rate of air leakage through a window with an opening light is 2.1 m3/h per metre of opening joint - significantly greater than for the example above.
Energy efficiency
Regardless of the direction of air movement through the facade there is a loss of energy from the building because either the air entering the building has to be conditioned to the interior conditions, or the air leaving the building has already been conditioned. If the flow-rate of air passing through the facade is known then the rate of energy loss due to mass transfer is at least
E = r Cp Q ( Ti - Te )
E is the rate of energy loss, in W
r is the air density, (about 1.23 kg/m3)
Cp is the air specific heat capacity, (about 1008 J/kgK)
Q is the air leakage flow-rate, in m3/s
Ti is the interior air temperature, in K
Te is the exterior air temperature, in K
This equation determines the change in the internal energy of the air as a result of the temperature change. Since the heating or cooling plant is unlikely to be 100% efficient there will be an additional amount of energy required to condition the interior air, and the value given by the formula above should be divided by the overall efficiency of the heating or cooling process to determine the true energy penalty due to air leakage.
Now consider typical figures for air leakage through weather-strips. As stated above BS 6375 Part 1 (1989) allows a leakage of 2.1 m3/h per metre of opening joint for a window with an opening light, at the 108 Pa pressure differential calculated above. If the air temperature inside the building is 20°C and the air temperature outside the building is 0°C then the rate of energy loss per metre of joint is at least
E = 1.23 x 1008 x 2.1 x 20 / 3600 = 14.5 W
For a casement window 900 mm wide by 1250 mm high the estimated total joint length is 4m, which gives a total rate of energy loss of 58 Watts. If the window is a typical unit with low-emissivity glazing its overall U-value will generally be less than 2.5 W/m2K (CWCT, 1995) and the rate of energy loss by heat transfer is
E = U A ( Ti - Te )
U is the overall U-value of the window, in W/m2K
A is the overall projected area of the window, in m2
which gives
E = 2.5 x ( 0.9 x 1.25 ) x 20 = 56.3 W
The rate of energy loss by air leakage (58 W) is greater than the normal heat transfer through the window (56.3W).
Further energy losses occur if the moisture content of the air entering and leaving the building is modified. The energy required to change the moisture content of a given flow-rate of air is
E = r hfg Q ( gi - ge )
E is the energy required, in W
r is the air density, (about 1.23 kg/m3)
hfg is the latent heat of evaporation of water, (about 2450´ 103 J/kg)
Q is the air leakage flow-rate, in m3/s
gi is the interior air moisture content, in kg of water per kg of dry air
ge is the exterior air moisture content, in kg of water per kg of dry air
Again there may be some inefficiency associated with the conditioning process and the value given by the formula above must be divided by the overall efficiency of the humidification/de-humidification process to determine the true energy penalty.
As an example, considering the window described above, if the room air is at 40% relative humidity (at 20°C this gives gi= 0.0058 kg/kg) and the outside air is at 90% relative humidity (at 0°C this gives ge= 0.0034 kg/kg) then the energy loss per metre of joint is
E = 1.23 x 2450000 x 2.1 x ( 0.0058 - 0.0034 ) = 4.2 W
The rate of energy loss due to changes in moisture content is about 25% of the rate of energy loss due to changes in the air temperature alone.
It is apparent that the energy efficiency of a building will be greatly reduced if the leakage air flow-rate Q is not controlled. However, an air-tight building must have some ventilation if the occupants are to be comfortable (ventilation also helps to reduce the risk of condensation and is now a requirement of the Building Regulations Approved Document F (1995)). This apparent conflict of interest in requiring that leakage must be stopped and ventilation then introduced is not a problem if it is remembered that the aim is to provide sufficient ventilation - not a surfeit.