Primary air volume compensation

The throttling of the air outlets via the individual room or zone control-lers gives rise to an increase in the static pressure in the distribution duct if the fan speed remains constant. The pressure rise corresponds to the slope of the fan characteristic and can be prevented by control-ling the static pressure (Fig. 8-19) in the respective distribution duct.

The most effective positioning action of the pressure controller in this case is the speed control of the fans in the central air treatment plant.

Fig. 8-19 Static pressure control in the air ducts of VAV systems 1 Duct pressure sensor

2 Minimum value selection of the pressure measurement signals 3 Central supply air duct pressure controller

4 Fan speed controllers

The positioning of the duct pressure sensors must be especially well planned. They should be placed at a location where 50 to 70 % of the respective duct resistance can be acquired at the full load air volume.

The pressure sensors (1) transmit the measured value to the pressure controller (3) via the minimum value selection (2). The controller com-pares this value with the defined setpoint and changes the speed of the supply air fan in case of deviation. The speed of the extract air fan is also varied synchronously via a slave control. This solution assumes that the supply air ducting and extract air ducting have approximately the same pressure loss characteristic. If this is not the case, and if the de-mands regarding stable air conditions in the rooms are higher than ave-rage, extract air volume control with compensation for the supply air vo-8.5.4 Static pressure control

in the air ducts

Fan coil systems are ideal air heating and cooling systems for hotel rooms. During heating operation, a central heating system (floor hea-ting) controlled according to the outside temperature provides the base load heating, i.e. the room temperature is maintained at approximately 15 °C in economy mode. On changeover to comfort mode, the fan coil unit achieves the desired comfort temperature within a few minutes. In all other rooms, the fan coil units remain in economy mode or off.

The fan coil unit (Fig. 8-20) is installed on a suitable wall of the room and is connected to the cold and hot water piping and to the electrical supply. If the device is installed on an outside wall, a small quantity of outside air can be drawn in via a manually adjusted damper (5) and mixed with the recirculated air.

Fig. 8-20 a) Fan coil unit and its components b) Fan coil unit with outside air box 1 Control elements

2 Finned tube heat exchanger 3 Fan

4 Adjustable supply air grille 5 Outside air box with damper

6 Hot or cold water circulation (two-pipe system)

The heating or cooling load in the room or room zone is basically cove-red by the hot and cold water circulation in a two-pipe or four-pipe system. The two-pipe system can only be centrally switched between heating and cooling operation (changeover system), whereas the four-pipe system provides for individual heating or cooling in each room.

Depending on fan coil unit design, air or water side control elements are available for room temperature control. Fig. 8-21 shows the control of a two-pipe fan coil unit with a water-side control element.

The room temperature controller (2) compares the room temperature measured by the sensor (1) against the setpoint. In case of deviation, it adjusts the valve (3). Depending whether hot or cold water is circulating in the supply, the changeover thermostat (4) switches the direction of control action to heating or cooling. If the current direction of control action is heating, the controller opens the valve when the room tempe-rature falls below the setpoint; if the direction of control action is coo-ling, the controller opens the valve when the room temperature rises above the setpoint (see function diagram, 9). It is possible and useful to 8.6 Fan coil systems

Room devices also offer the possibility of manual or automatic (pre-sence control) switchover between comfort and economy mode. In economy mode, the heating setpoint is approximately 5 K lower, and the cooling actuation signal is locked out. Additionally, the fan is switched off when the valve is closed.

Fig. 8-21 Water-side control of a fan coil unit (two-pipe system)

1 Room unit with temperature sensor and speed selector switch 2 Room temperature controller

3 Actuator for air heating/cooling coil 4 Changeover thermostat

5 Fan speed controller 6 Recirculated air 7 Outside air 8 Supply air

9 Room temperature controller function diagram

In large buildings (hotels), the fan coil units are operated with digital controllers which are managed via bus by a building automation and control system.

This system provides the following functions:

• Definition of normal setpoints for the comfort and economy modes

• Feedforward of setpoint adjustment influences

• Definition of all control parameters

• Comfort mode lockout

• Control function monitoring

Instead of a fan, induction units contain a noise-absorbing primary air chamber with close-coupled plastic nozzles through which the primary air is blown at high velocity into a mixing chamber, where a negative pressure is created (Fig. 8-22). The negative pressure draws in (induces) room air (so-called secondary air) via a finned tube heat exchanger, where it is heated or cooled as required.

Fig. 8-22 Principle of operation of an induction system 1 Central primary air treatment

2 Primary air duct (high-velocity system) 3 Boiler

4 Water chiller 5 Room

6 Induction unit (two-pipe system)

Fig. 8-23 shows the principle design of an induction unit. The primary air is injected at high velocity into the pressure chamber (4) where the dynamic pressure is converted to static pressure. The static pressure causes the primary air to be distributed evenly to all nozzles and forced into the mixing chamber at high velocity. The injection effect of these air flows gives rise to a negative pressure with respect to the room air, which is drawn in (induced) via the finned tube heat exchanger (6) as secondary air.

Fig. 8-23 Principle design of an induction unit (two-pipe system) 1 Primary air (treated outside air)

2 Secondary air (room air)