Constant Volume Air Handling

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Introduction:

Figure 1 shows the general arrangement for the four-channel control of an air handling system serving one zone in which the four channels control, in sequence, heating → heat recovery → fresh air (‘free’ cooling) → chilled water-based cooling.

Note that the heat recovery is optional and would only tend to be used in this application when the minimum fresh air in total air handled is high due, for instance, to high occupancy density in the zones served by the air handling system. Note also that the controlled condition becomes supply air temperature in situations where the air handling system’s function is to provide pre-conditioned air to rooms or zones which themselves have local sequenced heating and cooling control.

 

Operation:

 The controlled condition is the space temperature, θr. It is usual to measure this in the return air duct, provided that there are no intervening heat gains, such as light fittings, in the return air path. The return air will be well-mixed and the temperature sensor at the higher in-duct air velocities will tend to be more responsive than in the comparatively still air conditions of the space. For space or room temperature control, the manipulated variables will be:

•  the flow rate of heating water by means of diverting control valve, VH;

•  the proportion of direct and bypass air at the heat recovery heat exchanger, HRX, by means of face and bypass dampers, DHR;

Figure 1 Control for constant-volume air handling.

•  the proportion of fresh and recirculating air at the mixing dampers, DFA; and

•  the flow rate of chilled water by means of diverting control valve, VC.

During very warm summer weather, the fresh air damper will be fully open and the heat recovery face and bypass dampers positioned for full bypass in order to achieve ‘free cooling’. The limit to free cooling is reached when the fresh air intake enthalpy exceeds the enthalpy of the return air at a point downstream of the return air fan, a condition that takes place during the peak summer season in the UK though will often be short-lived. At this condition, it is necessary to signal to the fresh air damper a return to its minimum position and for the heat recovery face and bypass dampers to re position so as to allow full heat recovery (which will now, in effect, provide pre-cooling of the fresh air from the cool building exhaust air). Enthalpy sensors he and hf provide this and most control system manufacturers will permit additional plug-in modules for their multi-channel controllers to achieve these summer reset conditions. In the arrangement shown, when he – hf produces a negative signal, the reset conditions described are enacted.

To provide frost protection, there are two features. Firstly, the heat recovery heat exchanger (if present) is protected from icing during very cold weather by an upstream frost coil on the fresh air side. Because of the highly temporal nature of calls on the frost coil, a simple thermostatic control through a two-position two-port control valve is usually sufficient. Frost thermostat FTH provides for this. Secondly, should any primary heating plant such as central boilers or pumps fail, all coils in the arrangement are vulnerable to freezing of their respective water contents during very cold weather, especially the frost coil itself. To provide last-line protection against this, frost thermostat FTF acts to trip both fans (a further precaution might be to provide a two-position isolating damper at the fresh air intake which coincidentally closes in these conditions thereby eliminating any natural drought through the plant when idle). Typically, FTH would have a set point of 5°C, and FTF, 3°C.

 

Sequencing:

 Figure 2 shows the sequencing of this arrangement. Essentially, sequencing ensures that all control actions are mutually exclusive. From a very low controlled condition value, heating modulates as heat recovery is sustained at maximum, fresh air is a minimum and cooling is off. With the controlled condition value rising, the heating eventually reaches its off position. Heat recovery is then allowed to modulate with minimum fresh air and cooling off, and free cooling is allowed to proceed when the heat recovery reaches its off position. Only when maximum free cooling is effected is the chilled water cooling allowed to modulate at the high controlled condition value. By this sequence of events, minimum energy use is assured.

Dead zones may be programmed into the settings of each control channel, since each channel will be allocated a band of the control variable value within which it is active. A dead zone for instance between a heating and cooling action can help to ensure not only the mutual exclusivity of heating and cooling, but also that a gap exists between winter heating set point and summer cooling set point thereby minimizing energy use throughout the seasons.

Figure 2 Sequencing for constant-volume air handling.

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