recovery
The system shown in Fig. 5-6 depicts a suitable concept for indoor swimming pools. The relatively slow control response of the large air volume can be handled well with a room/supply air temperature cascade control (see chapter 0). The room temperature controller (6, PI-controller) compensates the setpoint of the supply air temperature controller (5, PI-controller) between a minimum and maximum value, which at the same time serves as min/max-control for the supply air temperature.
The aim of this system strategy is not so much to maintain a comfortable level of indoor humidity as to prevent the formation of condensation on walls and window surfaces. This means that the relative indoor humidity must be reduced to a level at which the dew point temperature of the indoor air is always lower than the lowest surface temperature of the enclosing surfaces (θWi), i.e. walls, windows etc. of the space concerned. However, since this surface temperature varies as the outdoor
temperature changes, it makes sense (to avoid wasting energy) to compensate the room humidity controller setpoint based on the lowest wall or window surface temperature. There are special surface temperature sensors (window temperature sensors) available for this purpose.
The internal surface temperature calculation is based on the principles of heat transfer and thermal transmittance (see “Physical principles” in BT_0100_EN Introduction to HVAC and building
technolgy). The lowest internal wall or window surface temperature θWi can be calculated simply with the following formula:
θWi = θIDA -Dθi
provided that the following information is known:
· Room temperature θIDA
· Outdoor air temperature θODA
· Heat transfer coefficient, indoor air Þ wall ai (ai = 8 W/(m2K) for still air)
· Lowest U-value of external wall (this is normally the U-value of the windows)
Fig. 5-6 Indoor swimming pool air temperature and humidity control with compensation of the relative humidity setpoint depending on window surface temperature
1 Room temperature sensor 7 Humidity controller
2 Humidity sensor 8 Shift controller
3 Surface temperature sensor 9 Frost protection thermostat 4 Supply air temperature sensor 10 Frost protection sensor 5 Supply air temperature controller 11 Frost protection controller 6 Room temperature controller 12 Minimum priority selector
The temperature differentialDθi to be calculated is determined using the following formula:
∆θi = U∙(θIDA- θODA)∙1
αi [K]
If, as the result of a low outdoor temperature, the surface temperature of the external wall or the inside of the window falls to a level approaching the dew point of the indoor air (resulting from room temperature θIDA and relative room humidityj IDA), this is detected by the surface temperature sensor (3) and the room humidity setpoint is reduced in linear fashion via the shift controller (8) to a level low enough to prevent condensation on wall and window surfaces.
In larger indoor swimming halls, where a bigger proportion of outdoor air needs to be mixed with the recirculated air, it is advisable to install a combined circulation system for heat recovery (extract air duct is located remotely from the supply air conditioning unit). The required frost protection controller (10,11) can intervene with the supply air temperature controller’s output signal (5) for the minimum priority selector (12) for the heat recovery valve.
Example
In a system, the relative indoor humidity setpoint is to be compensated as a function of the
temperature of the internal window surface. The room temperature θIDA is to be controlled at 28 °C.
The (glass) windows have a U-value of 2 W/(m2K), and a valueai of 8 W/(m2K) can be assumed for the internal heat transfer value.
Dp
The first task is to determine the maximum permitted indoor humidity setpoint when the outdoor temperature θODA is at its lowest. This temperature depends on the location of the plant and can be seen in the plant planning documents. For the purposes of our example, the lowest temperature to be taken into consideration is - 15 °C.
It is now possible to determine the surface temperature associated with this outdoor temperature.
Under the conditions described above, this is calculated as follows (see further above for an explanation):
This information can now be used to determine the allowable indoor humidity setpoint from the psychrometric chart (see Fig. 5-8). It is apparent that the room humidity setpointj IDA must be kept below 50 % RH to prevent the formation of condensation on the window surface.
Other operating points (e.g. at θODA = + 5 °C) can now be investigated and the associated permitted indoor humidity levels can be calculated. In Fig. 5-7 the dotted line (φMAX) indicates the applicable limit as a function of outdoor air temperatures. To avoid the formation of condensation on walls and windows, the indoor humidity must be maintained below 70 % at a temperature of + 5 °C, and below 50 % at a temperature of - 15 °C. In practice, the actual setpoint compensation value is set slightly below the limit curve (see Fig. 5-7, wj).
Fig. 5-7 Linear set point compensation of room relative humidity based on the lowest wall or window inner surface temperature
θWi Lowest wall or window inner surface temperature θODA Outdoor temperature
φMAX Maximum relative room air humidity (condensation limit) wj Set point compensation for prevention of condensation
60
Fig. 5-8 Using the psychrometric chart to determine the maximum permissible room relative humidity based on the room temperature and the lowest wall or window surface temperature
θWi Lowest wall or window inner surface temperature θIDA Room temperature
jMAX Maximum relative room air humidity (condensation limit)) -10
Water content x in g/kg
Barometric pressure:
1013 mbar 0 a.s.l.
1 2
Relative humidityj in % 40
Densityrin kg/m3
Partial air conditioning system with adiabatic cooling
The goal of an adiabatic cooling system is to cool the air without a chiller or to be able to reduce the necessary cooling output, in order to save energy. The cooling is achieved by first humidifying the warm extract air to a maximum, thereby the extract air is cooled and can serve as a heat sink in a heat exchanger, recovering heat from the outdoor air which is supplied to the room.
The cooling potential increases, the drier the extract air is (thereby maximizing the cooling potential).
Per gram water, which is added to the extract air, the air temperature will sink 2.5 K.
Whether the adiabatic cooling is enough depends on the outdoor air humidity and the required cooling output.
Adiabatic cooling can help save electrical energy and reduce greenhouse gas emissions and emmisisons from refrigerant leakages.