Water-based Indoor Climate Systems, 2007
www.swegon.com
DESIGN
Selection key
Selection data:- Cooling requirement (use Swegon climate calculation software, ProClim)
- Heating requirement (use Swegon climate calculation software, ProClim)
- Air flow required (use Swegon diffuser calculation software, ProAir)
Other factors affecting the system selection:- Maximum permitted air velocity in the occupied zone (use Swegon diffuser calculation software, ProAir)
- Acoustic requirements (use Swegon sound calculation software, ProAc)
- Requirements for directional operative temperature (use Swegon climate calculation software, ProClim)
Calculation procedure
1) Calculate the supply air’s cooling effect [W]
Pl = ql •1.2 • Dtl where ql is the supply air flow [l/s]
Dtl is the temperature difference between the room temperature and the supply air [K]
Guide tables for the primary air’s cooling capacity are also documented with the product’s cooling capacities, see relevant sections. 2) The waters' required cooling effect is obtained by subtracting the cooling effect of the supply air from the total cooling requirement. If the product required only provides water cooling (i.e.: passive beam), go to point 3a. If the product includes supply air (i.e.: active beam), go to point 3b. 3a) Use the tables that show the water cooling effect per chilled beam as a function of the mean temperature difference and select a suitable chilled beam with a capacity corresponding to the calculation in point 2 or the number of chilled beams if one is not sufficient. 3b) Use the tables that show the cooling effect per chilled beam as a function of the mean temperature difference. Select the nozzle configuration that matches the preferred air volume. Select a ceiling unit with the preferred air flow or interpolate between close air flows. Check that the sound level is acceptable. 4) The water flow at the chosen Dt on the cooling water is obtained from the diagrams “Water flow - cooling effect”. 5) The pressure drop on the cooling water circuit in the product is then calculated with the help of the formula Dpk = (qk/kpk)2 where kpk is documented in the table as the cooling capacity. 6) Heating. Use the corresponding method as set out in points 3-5 above.
System design
Cooling system
The cooling system should be constructed as an indoor evaporator. The surplus heat is led away either through outdoor condensers, or via a brine system and cooling fluid grids placed outdoors.
If instead an outdoor cooling appliance is chosen, i.e. the evaporator is placed outside, the placing of indoor auxiliary exchangers is recommended This is in order to avoid an anti-freeze fluid (brine) in the cooling water circuit. There are two reasons why brine should be avoided in the cooling water circuit. The pressure drop is increased by 15-25% depending on the strength of the brine solution. The cooling effect is also reduced by approx. 15% due to the fact, that the heat transmission factor becomes lower on the water side.
The most common system is one with water tube boiler evaporators in which the cooler’s cooling fluid takes up energy from the cooling water circulating in the building. For environmental reasons, this solution must be preferred despite the loss of effect in the exchanger.
Regulation of room units
Chilled beams and perimeter systems are almost exclusively connected through two-way valves. The advantages compared to a three-way union are lower costs and simpler design and adjustment. Overpressure valves are positioned in a few areas of the system to prevent high pressure at low loads. The fact that pressure regulated pumps can now be installed to a reasonable cost, contributes towards the preference for a two-way system.
Condensation protection
On hot days in late summer, the air humidity can be high. The higher the humidity in the air, the higher the dew point temperature for condensation on surfaces. A Mollier diagram (moisture-enthalpy-diagram) will show the relationship, see Figure 34. At, for example, 25°C and 50% relative humidity the dew point is 14°C (applicable to normal atmospheric pressure, 101 kPa), i.e. it starts to condense on surfaces whose temperature is 14°C or lower. On some late summer days after a rain shower the dew point temperature can sometimes rise to 15°C and in extreme cases up to 17°C. In order to avoid condensation problems, measures should be taken to ensure that the system prevents condensation on the room units. The supply air should always be chilled in order to prevent condensation in the cooling battery on the supply air unit. Another method is to use a sensor that measures the relative humidity of the exhaust air, see Figure 33. The shunt group valve is controlled to hold the water temperature above the dew point temperature. In order to ensure the drying of the air at high outdoor temperatures and high relative humidity the supply air temperature should be weather compensated as set out in Figure 32. The engagement point of +5°C can vary a little from installation to installation. See the dashed alternative curve. However, it is important, at outdoor temperatures around + 22°C and above, to achieve dehumidification after the air handling unit so that the supply air’s dew point temperature is lower or equal to the supply temperature of the cool ceiling’s cooling medium.
Figure 31.System solution.
1 = Water tank
2 = Water cooler
3 = Air treatment unit
4 = Controller
5 = Differential pressure valve
6 = Chilled beams
Figure 32.Supply air compensation in relation to outdoor air temperature.
Figure 33.Condensation protection control via the shunt group.
1 = Evaporator/condenser
2 = Circ. pump
3 = Shunt
4 = Exhaust air duct
5 = Chilled beam
RC = Controller
GX = humidity sensor
GT = Temperature sensor
Design of condensation protection
Proposals are given here for suitable pipe system designs with associated control and adjustment valves to provide interaction between the air treatment’s drying of the supply air through condensation and the chilled water temperature to the chilled room unit to counteract condensation.
The system has been selected for the state DUT = +25°C and R = 50% which corresponds to a dew point of +14°C. The cooling medium side’s selected temperature to the cool ceiling is set to +13°C on the supply and +17°C on the return, see Figure 34.
On the air treatment side the cooling battery is selected for +8°C on the supply temperature and +13°C on the return temperature. These are the temperatures that provide good conditions even with district cooling. Here an installation of 1000 m2 with a supply air flow of 1.5 l/s m2 has been assumed.
With district cooling a return temperature preferably higher than +16°C is wanted, which is achieved for installations with combined air and chilled unit conditioning. It is then important to also maintain the return temperature from air treatment so that it does not lower the return temperature to the district system's heat exchanger.
As is evident from the Mollier diagram’s curve for the above selection data an enthalpy difference D i of 16 kJ/kg is obtained.
PTL = qTL · pTL · Di [kW]
PTL = 1.5 · 1.2 · 16 = 28.8kW
PTL = requisite effect for cooling the supply air with associated condensation precipitation at DUT
rTL = density of the supply air in kg/m3
qTL = the supply air’s flow in m3/s
The above gives a selected chilled water flow qw at DtW = 5 K (+8°C to +13°C), and PTL = 28.8 kW
qw = PTL / DtW · cp = 1.72 l/s
qw = 28.8 / 5 · 4.187 = 1.72 l/s
rw = the density of the water kg/m3
cp = the water’s specific heat in kJ/kg °C
4.187 = rw · cp / 1000
In the equivalent way, for the chilled room unit section 1000 m2 · 40 W/m2 = 40 kW is obtained in the required cooling effect.
qwk = 40 / 4 · 4.187 = 1.38 l/s.
Try the bypass flow 0.09 l/s to the three-way valve SV1 past the air cooler. With the help of the separate flows and their temperatures the mixing temperatures in the pipe system’s separate parts are then calculated. From the calculated mixing temperatures it is evident that they interact well in the design instance. The adjustment valves should be measured with the control valve’s port fully open against respective adjustment valves when the established flow for the RV valve is adjusted to the calculated value. The resulting flows and temperatures are evident from Figure 35.
Figure 34.Condensation protection operating instance 1. Change of state for the supply air.
Figure 35. Condensation protection operating instance 1. System principle with flows and temperatures.
Temperatures
The temperatures given below serve merely as a guide.
Differences are likely to occur.
Recommended temperatures
Supply temperature, cooling: | >13°C (see section
Condensation protection) |
Temperature increase
cooling: |
2-4K
|
Ventilation air when cooling: | see Figure 32 |
Room adjustment
Regulation of the room temperature normally takes place individually in each room via a room temperature controller. The room unit controls cooling and (where relevant) the heating valve in sequence so that the room does not receive heating and cooling simultaneously if the building is new.
In older buildings with inferior insulation, it should not be assumed that cooling and heating should always occur in sequence. Here it is recommended that the directed operative temperature is be checked. It may be the case that you need to have heating by the perimeter wall at the same time, as there is a cooling requirement in the internal zones.