1 Air Handling Units In ICE

Transcription

1 1 Air Handling Units In ICE From version 3.0, there is a library of common air handling units (AHU) in the distribution. A simulated building can have several AHUs. The following limitations apply: - every AHU should serve at least one zone - a zone may must always be connected to an AHU (but the flow may be permanently very close to zero) An additional AHU can be added from the building form by pressing the New AHUbutton. The list of available AHUs is opened and the user should select one, give it a name and click OK, then the new AHU will appear in the HVAC systems list. In the zone form one selects the AHU to serve the zone. An existing AHU can be replaced: Select the the AHU and press the replace button. Select a suitable AHU and click OK. 1.1 Standard AHU The Standard AHU is automatically inserted, when a new simulation case is created. It is described in some detail in Chapter 6 of the manual. Figure 1.1. Standard AHU Mika Vuolle Page

2 1.2 ASHRAE Toolkit AHU In all pre-built AHUs except ASHRAE Toolkit AHU, coils are described with efficiencies, thus they can serve 1 or zones without any changes of parameters. The ASHRAE Toolkit AHU has heating and cooling coils with limited capacity. The AHU can be used to design and study sizes of heating and cooling coils. The coil models are taken from the ASHRAE Secondary Toolkit library and have been translated into NMF (the modeling language used by IDA). Most other component models from the ASHRAE secondary toolkit are also available in NMF. The documentation is available from ASHRAE. Figure 1.2. ASHRAE Toolkit AHU By double-clicking on the left of the two empty boxes in the AHU form (Figure 1.2), the form (Figure 1.3) is opened, where a rated operating point should be given. From these values the actual UA-value and other parameters of the coils are calculated by the models. Mika Vuolle Page

3 Figure 1.3. Coil macro In the coil macro, the following components complement the coil model: - a static sensor in the air stream to measure outgoing air temperature - a PI-controller - a valve - an adder-model to create a small deviation (0.02 C) between setpoints of heating and cooling coils to avoid simultaneous heating and cooling. The setpoint values are set outside of the coil macros in the same manner as in the default AHU. 1.3 Control of Exhaust Air Temperature AHU The AHU tries to keep the temperature of the exhaust air constant by changing the supply air temperature with a PI controller. Desired exhaust temperature, minimum and maximum supply temperatures are given as parameters (Figure 1.4). Mika Vuolle Page

4 Figure 1.4. Control of Exhaust Temperature AHU In addition to standard components, there are: - a static sensor in the exhaust stream to measure temperature - a PI-controller to compute a control signal [0,1] - a linear transformation model, which calculates temperature setpoint values from control signal for heat exchanger and coils 1.4 Extra Heating Coil AHU This unit has an extra heating coil. This kind of setup is needed to dehumidify supply air. Mika Vuolle Page

5 Figure 1.5. Extra Heating Coil AHU The cooling coil setpoint is given separately as a parameter in the form. Both heating coils receive their setpoints from the Setpoint for supply air temperature box. In addition to standard components, there are:?? water merge and split models to serve the two coils 1.5 Indirect & Direct Evaporative Cooling AHU This unit has evaporative humidifiers in both air streams. The humidifier in the exhaust side is in operation when the ambient temperature is higher than a given parameter. The supply side humidifier can be controlled by: - the maximum relative humidity of air stream leaving humidifier - a schedule - a setpoint temperature The control signal from the schedule has On/Off-influence on the supply side humidifier. When it is On, the relative humidity and temperature setpoint controls are valid and the minimum factor of them is used. Thus leaving air stream has never lower temperature than setpoint or more relative humidity than given allowed maximum value. By default the maximum relative humidity value is 99%. Mika Vuolle Page

6 Parameters at a rated point may be given to the evaporative humidifier model by double clicking on the symbol, thus limited capacity can be modeled as well. By default, the humidifier is quite large. Figure 1.6. Indirect & Direct Evaporative Cooling AHU 1.6 Indirect Evaporative Cooling AHU The unit only has an evaporative humidifier in the exhaust air stream. It is in operation the when ambient temperature is higher than a given parameter. A rated point may be given as parameters, as discussed in the previous section. Mika Vuolle Page

7 Figure 1.7. Indirect Evaporative Cooling AHU 1.7 Recycling AHU Here, the heat exchanger has been replaced with a mixing box. A minimum fresh air flow is given in the AHU form and it may be modulated by a schedule. The mixing box tries to reach the supply air temperature setpoint by controlling the amount of return air mixed into the air stream. A bound on the minimum amount of fresh air that will enter into the system is kept in this process. The actual minimum amount of fresh air is given by the value of the minimum fresh air parameter times the schedule value. For example, while heating the building in the morning before occupancy, the minimum fresh air could be set to zero by setting the schedule to zero during this period. In the same manner, minimum fresh air stream can be increased. Mika Vuolle Page

8 Figure 1.8. Recycling AHU 1.8 Separate Setpoints AHU In this unit, all heat exchangers have separate setpoints. In the current version, all setpoints are constant (given as parameters). It is useful for cases when heating or cooling should be limited to extreme temperatures. This AHU is used when a system is generated from the IDA Room wizard. Mika Vuolle Page

9 Figure 1.9. Separate Setpoints AHU 1.9 Steam Humidifier AHU In this AHU, there is a steam humidifier in the supply air stream. It attempts to control the relative humidity of the exhaust stream by a PI controller. There is an outdoor temperature limit, under which the device is turned off. The steam humidifier has an internal limit that leaving air will never have a relative humidity above 90 %. This can be changed by opening the humidifier component. The steam humidifier will not cool the supply air stream but only add moisture and heat to it. Mika Vuolle Page

10 Figure Steam Humidifier AHU 1.10 Supply Temperature Function of Exhaust AHU In this AHU, there is a static sensor in the exhaust air stream to measure exhaust air temperature. Temperature setpoints of all the devices in the supply air stream are calculated as a user-specified function of the exhaust air temperature. Figure Supply Temperature Function of Exhaust AHU Mika Vuolle Page

11 1.11 Additional AHUs Additional AHUs can be built by the user using the basic commands that are discussed in Chapter 8 of the manual. If you have created an AHU of your own and you want to add it into the AHU list, you should copy.idc and.idm files to lib/ice/ahu. Please contact your distributor for help in constructing customized AHUs. Mika Vuolle Page