Application
Although no two designs are ever alike, there are basic similarities in all designs, particularly for comfort conditioning. To illustrate the application of automatic control consider the
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Percentage airflow rate |
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Percentage damper rotation (0°-90°) |
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Fig. 13.19 Typical installed damper characteristic. |
Pressure drop across exhaust air dampers = 15 Pa when fully open
R1a
DamperD———————
Motor!
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Recirculated Air Cooler Coil |
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’ State M SP = -15 Pa |
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Supply air |
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R1b ———— Q -+//-/-/ Damper motor Fresh 7 |
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* / |
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Air |
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Pressure drop across recirculated air dampers = 30 Pa when fully open |
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Fig. 13.20 Typical static pressure drops and dampers.
Plant and psychrometry shown in Figures 13.21(a) and 13.21(6). The intention is to control the dry-bulb temperature and relative humidity in a room. For simplicity, the temperature rise through the extract fan is ignored. The mode of control is to be proportional plus integral over both temperature and relative humidity.
In the summer design case, minimum outside air at state Os mixes with maximum recirculated air at state R, to form a mixture state, Afs. The temperature of the air leaving the cooler coil, tw, is sensed by controller Cl a, which regulates the flow of chilled water through the cooler coil by means of the three-port valve, Rla. Since the method of control is proportional plus integral there will be no offset in the value of rw.
When the enthalpy of the outside air exceeds that of the air recirculated from the room it is economical to use as much recirculated air as possible and minimise the amount of fresh air handled. Accordingly, the enthalpy of the outside air is sensed by an enthalpy controller, Clb, located in the fresh air duct. The set point of Clb is the design enthalpy of the air in the room. When the outside air has an enthalpy exceeding the set point, the motorised dampers are moved to positions giving minimum fresh air with maximum recirculated and minimum discharged air. This is achieved by means of damper motors Rib (fresh air), Rlc (recirculated air) and Rid (discharged air). If the enthalpy of the outside air has a value less than that of the recirculated air but more than the enthalpy of state W, the dampers move to the position that gives 100 per cent fresh air and the group of motors, Rib, Rlc and Rid, is controlled in sequence with the three-port valve Rla.
At the same time, room temperature is controlled by a thermostat C2, located in the extract air duct, close to the extract grille, so as to sense a temperature that is representative of the whole of the room. Upon fall in temperature sensed by C2, motorised valve R2, in the LTHW line from the reheater battery, is opened. Since proportional plus integral control is specified for C2, the value of the room temperature sensed will return to the set point, after a short while. There will be no offset.
As the summer passes and winter approaches, the state of the outside air moves from Os towards Ow and, in due course, the dampers move to give 100 per cent fresh air. Less and less cooling will be needed and Cla will progressively open the by-pass port of Rla, closing the port to the cooler coil at the same time. Eventually, all the chilled water is bypassing the cooler coil because the outside air temperature is the same as the set point of Cla, 100 per cent fresh air being handled. Any further reduction in the temperature of the outside air will make Cla operate the damper motors to vary the proportions of fresh and recirculated air in order to give a constant value of temperature, /w (= fmw), without offset, since control is proportional plus integral.
Humidity is controlled by a humidistat, C3, located in the extract air duct. Upon fall in the value of relative humidity motorised valve R3 is opened to inject dry steam into the supply airstream as near as possible to the supply air grille or diffuser. (Sometimes dry steam is injected in the conditioned space itself.) The steam injection process for design winter conditions is seen on the psychrometric chart (Figure 13.21(6)) by the line from B to Sw.
Upon rise in humidity sensed by C3, the control between Cla and Rla is overridden and C3 closes the by-pass port of the chilled water valve, Rla, opening the port to the cooler coil. An interlock is included to ensure that humidifying and dehumidifying cannot occur simultaneously. In this illustration, since C3 exercises proportional plus integral control, there will be no offset in the relative humidity.
In formulating a scheme for the automatic control of an air-conditioning system it is essential that a schematic or flow diagram be prepared, after the fashion of Figure 13.21(a)
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Fw — ^mw («
Fig. 13.21 (a) Schematic of the plant and controls to achieve control over temperature and humidity in a room. (b) Psychrometry for Figure 13.21(a). Subscript s refers to summer design. Subscript w refers to winter design. Proportional plus integral control gives no offset in dry-bulb temperature tw (- *mw)- Temperature rise through the extract fan is ignored for simplicity of illustration.
And that all relevant data be shown on it. Such relevant data would be, for example, all airflow and waterflow rates and all psychrometric states. It is also essential that a schedule of operation of the automatic controls be drawn up. This should include such information as the set points, proportional bands (or differential gaps) and location of all controllers (thermostats, humidistats, etc.), and the flow rates, pressure drops when fully open related to an authority between 0.2 and 0.4, location, size, etc., of all regulators (motorised valves, dampers, etc.). A short description of the action of each regulator on, say, rise in temperature and also on plant shut-down, should be included in the schedule. All controllers and regulators should be denoted in a systematic fashion, such as Cl, Rl, C2, R2 and so on.
It is also necessary that the sequence of operation of the components of the air conditioning plant be stated, due care being paid to safety considerations, e. g. the chilled water pump must start before the refrigeration compressor does.
A system cannot operate unless it is well designed, properly installed and competently commissioned. Commissioning cannot be started unless regulating valves for balancing water flow have been provided in the right places in the pipework, together with appropriate points for pressure tappings and temperature measurement. The designer must supply the commissioning team with adequate details about the system. Hence the necessity of a good schematic, supported by comprehensive controller and regulator schedules, referred to earlier. As well as setting the controls correctly, commissioning involves balancing the airflow rates in the duct systems and the waterflow rates in all the main and subsidiary circuits, adopting the principles of proportional balancing given in the CIBSE Commissioning Codes for air and water (1986).
It follows that operating instructions must also be provided by the designer (particularly if Building Management Systems are to be used). These are best written throughout the development of the design and should always be available for the commissioning team as well as the future occupier of the building.
Posted in Air Conditioning Engineering