Water-based Indoor Climate Systems, 2007
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COMFORT COOLING
COMFORT COOLING
COMFORT COOLING
The excess heat that must be removed from a building to keep the indoor temperature lower than a predetermined highest permitted temperature is known as the cooling requirement. The climate systems used to actively cool buildings can generally be divided up into three types.- Systems with air based cooling
- Systems with water based cooling
- Combined systems (cooling is supply using both air and water)
It is important to differentiate between the sensible cooling effect requirement and the total cooling effect including wet cooling. The sensible cooling effect refers to the output determined by the temperature difference between the required temperature and the temperature you would have without comfort cooling. The total cooling effect should also include the latent cooling requirement, which includes wet cooling. This refers to the enthalpy difference that must be achieved in order for the supply air to be dehumidified in the ventilation unit’s cooling coil, see Figure 16. If you include the latent cooling requirement, in general the total design cooling requirement increases by over 100%.
System with air based cooling
In these systems the design air flow is determined by the cooling requirement. That’s to say it is the thermal demands and not the demands on the air quality that dictate the design.
In existing buildings it is normally both difficult and expensive to change the duct system. If you cannot transport sufficiently large air flows in the existing ducts to satisfy the cooling requirement, you usually install a water based cooling system during the renovation. The cooling system must have the capacity to take care of variations in the cooling requirement, both during the day and over the year. The two basic types of system with air based cooling are a constant flow system or a system with a variable flow (a combination of the two methods is also available).
System with constant air flow – CAV system
The CAV systems (Constant Air Volume) is characterised by the air flow being constant. The rooms with the greatest cooling requirement normally determine the selection of the supply air temperature prepared in the central air handling unit. In some rooms, such as conference rooms, it may be necessary to reheat the supply air. This is done so that the occupants will not experience a chill because the room has been vacant during a certain period.
Even if a CAV system supplies air at a constant flow sometimes fans with two-speed motors are used, this allows the speed to be decreased when the cooling requirement in the building so permits. The air flow reduces in proportion to the speed.
The supply air temperature in a CAV system can be constant or variable in relation to the outdoor temperature. When temperature control takes place centrally or with a constant supply air temperature, a correction to the right room temperature is made during the winter in individual rooms, for example, using radiators.
Figure 16.The sum of latent and sensible cooling effects give the total cooling effect.
Figure 18.Principle of a CAV system.
Systems with variable air flow – VAV system
In VAV systems (Variable Air Volume) the air flow supplied to each room varies as required, yet the temperature of the supply air is kept constant, i.e. the supply air temperature does not change with a change in load. On the other hand, seasonal control of the supply air temperature takes place, as a function of the outdoor temperature.
The airflow to each room is regulated by means of a damper inside some form of terminal unit in direct connection to the room, while central supply air and extract air fans are controlled by means of guide vane control or speed-controlled fan motors, usually frequency controlled.
Control normally occurs by maintaining constant static pressure with sensors in the farthest branch ducts of the supply air system. The flow varies from max. in the warmest day down to approx. 20 % of max. during the coldest days of the year, when the air only has the task of meeting the demands made on air quality.
Systems with water based cooling
These types of system supply the individual rooms with water based cooling. The air system used only satisfies the demands on air quality.
In a conversion or refurbishment situation it is preferable to use this type of cooling system. When installing the system there is usually space in the existing suspended ceiling to install the required pipes needed for the distribution of chilled water in the building.
Combined systems
Air based and water based cooling can be combined in many different ways. One situation when the systems must be combined is when the air based system does not have sufficient cooling capacity.
It is also possible to combine air based systems so that in specific parts of a building, or in specific rooms, a VAV system is used (by utilising VAV units on which the air flow can be controlled), and in remaining parts of the building a CAV system is used.
Figure 18.Principle of a VAV system.
Figure 19.Principle of water based cooling.
Conventional electrically driven compressor cooling
Cooling generation using a compressor cooler is the “classic” method for cooling. When mechanical cooling for comfort cooling is discussed this is usually considered to be the standard.
Using a compressor driven chiller gives you immense flexibility with regard to the methods of supplying cooling to the building. As previously mentioned it is possible to deliver cooling from chillers either to the cooling battery in an air treatment unit or to cooling equipment placed directly in the room, for example, chilled beams or fan coil units.
Evaporative cooling
Evaporative cooling of air utilises the fact that the air’s temperature drops by making it moist with the help of water evaporation from a wet area that the air passes. Cooling is possible as long as the air is not saturated with water vapour. The lowest temperature the air can take with this type of cooling is limited by the air’s wet temperature, which is sometimes called the air’s cooling limit. Direct evaporative cooling refers to a process where the supply air is moistened and the temperature drops. At the same time the air’s moisture content increases. With indirect evaporative cooling the exhaust air is moistened, whereby the exhaust air’s temperature drops. This is followed by a heat exchange (without moisture transfer) between the exhaust and supply air where the heat from the supply air can be transferred to the exhaust air. The possibility to cool is mainly determined by the current condition of the outdoor air. The more moisture (the higher twet value) it contains the poorer its ability to cool. Consequently, the method is considered to have limited use in offices and other commercial premises.
Sorptive cooling
In order to lower the temperature of the supply air as far as possible, it is beneficial to have the driest possible air when moistening starts. In the sorptive cooling process moistening from the evaporative process is supplemented by drying of the supply air before it is moistened, see Figure 22.
A sorption cooling unit consists of a dehumidifier section that dries the air and a section that cools the air (the evaporative part). The supply air is dehumidified using a moisture absorbing rotor. On the exhaust air side the absorbed water is driven out of the rotor. Heat is used for this. Accordingly a sorptive cooling unit also needs to supply heat.
Figure 20.Principle for direct evaporative cooling.
Figure 21.Principle for indirect evaporative cooling.
Figure 22.Principle for sorptive cooling.
District cooling
It is becoming increasingly common for energy companies to offer their customers district cooling. Depending on the requirements of each individual energy company concerning the production possibilities and the layout and proximity of customers, cooling is produced and distributed in different ways in different districts. In a district cooling system production units can be made up of everything from “free cooling” (e.g. cold sea water that can be utilised directly for cooling purposes), compressor coolers, to heat driven cooling machines.
It is relatively common to utilise cooling from existing heat pumps that are already used to deliver heat to the district heating system. Initially it was usual that users with a relatively large cooling requirement were connected to the district cooling system. This typically could be a hospital area or a shopping centre. However, it is becoming more common that individual properties are being offered the opportunity of being connected.
“Chilled water” is delivered to the customer in a substation, in principle the same way as a subscriber centre for district heating. From here secondary water is distributed to the building or buildings to be cooled as set out in Figure 23.
In the same way as for district heating, it is important to be aware of the conditions provided by the supplier of the district cooling. It is normal for suppliers to state the following conditions:- Temperatures: for example, tsupply - treturn = 6-16 or 7-17, i.e. Dt = 10 K.
- Flow charge: which increases in those cases the temperature difference decreases in relation to that agreed.
Free cooling
There is a possibility with water based cooling systems to use what is known as free cooling. Here it is necessary to install some form of heat exchanger to the outdoor air. This is usually integrated in the comfort cooling unit, see Figure 24. A heat exchanger is connected between the liquid cooler unit’s refrigerant and cooling medium circuits.
In connection with the utilisation of free cooling when using water based cooling it is common that at a predetermined outdoor temperature you allow all water to be cooled against the outdoor air. Accordingly, at lower temperatures than this temperature the cooler is not used. The outdoor temperature at which switching occurs normally lies around 10°C.It is also common to utilize free cooling by installing heat exchangers and reversing valves between the building’s brine system from the liquid cooling unit and the cooling medium system, from the cooling unit’s cooling medium cooler.
Figure 23.Principle for district cooling.
Figure 24.Cooler with free cooling.
1. Condenser
2. Heat exchanger for the cooling medium
3. Outdoor air
4. Cooling medium return
5. Cooling medium supply
6. Evaporator
7. Compressor