Previous (Central heating and cooling system)   |   Start   |   Next (Electrical system)

What input is needed?

Ventilation system

Air handling units

Air handling components

In the default configuration, the air handling system consists of the following components (figure below): supply air temperature setpoint controller (1), exhaust fan (2), heat exchanger (3), heating coil (4), cooling coil (5), supply fan (6), schedule (7) for operation of both fans and a schedule for the operation of the heat exchanger (8). The unit provides temperature-controlled air at a given pressure. Some key parameters of individual components are presented in the form; open them to edit.

The supply air temperature setpoint is connected to the heat exchanger and to both coils. All three components have separate ideal control circuits, which independently strive to maintain the setpoint. After the coils, the supply fan raises the supply air temperature further by either a fixed number of degrees (default) or by depositing motor and drive losses to the air stream.

In the setpoint controller, three methods are provided for setpoint selection. In the default AHU, the setpoint is set to constant, 16 °C. The second alternative is to let the setpoint vary with time according to a schedule. Thirdly, an option is available where the setpoint is calculated as a (user-defined) function of outdoor air temperature. Note that if for example the chiller (that supplies the cooling coil from the plant) has been turned off or has insufficient capacity, the supply air will not be cooled to the setpoint.

Default air handling unit

The heating coil (4) has two important parameters: the air side temperature effectiveness and the desired water side temperature reduction. Capacity control is achieved by adapting the actual effectiveness up to the given maximum level. The necessary water flow is calculated and the water temperature is reduced, if possible, by the desired number of degrees. There is no bypass on the liquid side; control is achieved by restricting the water flow.

In the default configuration, the temperature effectiveness is set at 1.0. There are basically two situations when it may be desirable to change this to a more realistic value: when sizing the actual coil by means of simulation experiments, or when making energy calculations in cases where the heat generation efficiency is dependent on temperature conditions (nearly always for the ESBO Plant.) In addition, the simplest and quickest way of removing the entire coil is to set the effectiveness to zero.

The cooling coil (5) works in the same way as the heating coil, but is mathematically more complicated because air dehumidification is calculated. For wet operation, the given effectiveness is defined as (1 - ”bypass factor”), according to ASHRAE’s nomenclature. Physically, this means that the state of the cooled air in the psychrometric chart lies somewhere on a straight line between the state for the incoming air and the apparatus dew point temperature on the saturation curve. In the model, the average temperature of the liquid side defines the apparatus dew point. On this line, the given efficiency indicates the status: 0 is no cooling whatsoever, and 1 indicates maximum cooling, which also means that the air at most is chilled to (the arithmetic) mean value of the liquid incoming and outgoing temperatures.

Similarly, the air-to-air heat exchanger (“3” in figure above) is controlled by adapting the actual effectiveness selected by the model, up to the maximum limit set by the user, so that the setpoint for the supply air temperature is reached (if enough heat is available). The temperature of the (often chilled) exhaust air, which may not fall below a certain level (TEXHOUTMIN parameter), sets another limit. This is to avoid freezing. Note that for rotating heat exchangers, it is usually possible and desirable to cool the exhaust air below freezing. A Capacity parameter for the rated air flow may optionally be provided in the heat exchanger. If this parameter is provided, the actual effectiveness will be adjusted to compensate for lower air flows as well as for unbalanced flow on the supply and exhaust sides.

The heat-exchanger will utilize the return air stream both to heat and to cool the supply air.

The heat exchanger takes into consideration condensation on both the supply and exhaust sides. During wet processes, the given efficiency is interpreted as (1 - ”bypass factor”), in the same way as for the cooling coil, but the apparatus dew point for the heat exchanger is defined as the incoming temperature for the opposite medium.

The fans have ideal pressure control with given setpoints and by default constant efficiencies, i.e. they supply a fixed pressure head. In most cases, both these parameters only have significance for calculating fan electricity consumption. By default, the user gives the increase in air temperature by the fan (and the system). As an option, the temperature rise may be computed automatically with a given percentage of motor and drive losses being deposited into the air.

For modeling of CAV systems, the fan pressure rise and efficiency at the intended operating point should be entered (pressure can also be given in terms of specific fan power, SFP). For VAV systems, on the other hand, the performance should be adapted for flows below the design point. ASHRAE Standard 90.1 prescribes a way to do this that has been implemented in the fan model. This part load efficiency reduction is activated selecting something else than <unlimited> in the drop box. A rated flow must also be provided for this option.

The fans operational schedule (7) is connected to both the supply and exhaust fans. When the control signal is zero, the fans supply a very low pressure head (for numerical reasons, greater than zero). The fan schedule is also connected to all the air terminals in the zones. When the fans switch off, all terminals are closed. This is to avoid a spontaneous flow through the system caused by the chimney effect.

The fan schedule, like other schedules, gives normally values of 0  and 1  as output signals. However, it is meaningful for the fan schedule to sometimes give other values, thereby forcing ventilation flow. For example, if a value of 1.2 is given, all terminals will supply a 20% higher flow than that selected locally in the zone. This applies to both CAV and VAV systems. In the same way, a value of 0.5 results in half the flow.

The local terminal connection to the central schedule has no counterpart in actual systems, but has been introduced partly to avoid unintentional spontaneous flow in the system, and partly to permit forcing the flow in case of CAV.

The heat exchanger is similarly controlled by a schedule (8). To turn it off, the schedule should give the value 0. A value 1 turns on the heat exchanger. Any value between 0 and 1 will be interpreted as using a corresponding fraction of the heat exchange surface.

Custom AHU modelling

Previous (Central heating and cooling system)   |   Start   |   Next (Electrical system)