The main objectives of a control system may be summarised as:
- Safe plant operation.
- Protection to the building and system components.
- Maintenance of desired conditions.
- Economy in operation.
It is essentially the desire to achieve energy savings that may lead sometimes to a proliferation of controls; nevertheless, such an objective is fundamental to good practice and may include:
- Limiting plant operating periods.
- Economical control of space conditions.
- Efficient plant operation to match the load.
- Monitoring system performance.
Building Regulations
The current Building Regulations make it mandatory that space heating or hot water systems in buildings must be provided with automatic controls, such that:
- Space temperatures are controlled by thermostats.
- The temperature of hot water heating systems is varied according to the outside temperature (weather compensated).
- Systems are provided with a timeswitch (or optimum start control) to ensure that they operate only when the building is occupied.
- Multiple boiler plant is controlled in the most efficient manner.
Elementary components
The nature of heating and air-conditioning systems is such that, for the majority of the period of operation, plant and system capacity will exceed demand and the order of this excess varies with time.
A simple control system comprises a sensing device, to measure the variable, a controller, to compare the measured variable with the desired set-point and to send a signal to the control device, which in turn regulates the input.
For example, a thermostat (sensing device) in the flow pipe from a heat exchanger measures the temperature of the water (controlled variable) and signals the information to the controller. The controller compares the flow temperature with the desired temperature (set point) and passes a signal to the control valve (control device) to open or close, thereby regulating the amount of heat introduced to the heat exchanger. This is an example of closed loop control, where feedback from the controlled variable is used to provide a control action to limit deviation from the set-point. An open loop system has no feedback from the controlled variable.
Sensing devices
Siting of sensing elements is critical to the achievement of good control. In pipework or ductwork, sensors must be so arranged that the active part of the device is immersed fully in the fluid and that the position senses the average conditions.
Temperature: thermal expansion of metal or gas or a change in electrical characteristics due to temperature variation are the common methods of detection.
Electronic sensing elements have no moving parts. The resistance bulb type, normally a coil of nickel, copper or platinum wire around a core, produces a variation in electrical resistance with change in temperature. Thermistors, which are semi-conductor devices, also produce a change in resistance, but inversely with respect to temperature change such that resistance decreases with increase in temperature; the non-linear output may be corrected using linearising resistors in the circuit. Thermo-couples comprise two dissimilar metal wires joined at one end; a voltage proportional to the temperature difference between the junction and the free ends results.
Humidity: fabrics which change dimension with humidity variation, such as hair, nylon or wood, are still in use as measuring elements but are unreliable and require considerable maintenance. Hygroscopic plastic tape is now more common. These media may be used to open or close contacts or to operate a potentiometer.
For electronic applications, use is made of a hygroscopic salt such as lithium chloride, which will provide a change in resistance depending upon the amount of moisture absorbed. These are relatively cheap but are slow to respond to change and are easily damaged. More robust but considerably more expensive are the solid state sensors which use polymer film elements to produce variations in resistance or capacitance.
Pressure: bellows, diaphragms and Bourdon tubes are typical of the sensing devices used and, of these, bellows and diaphragms acting against a spring are the most common. Such equipment can be sensitive to small changes in pressure, typically 10 Pa. The pressure sensing motion may then be transmitted directly to an electric or pneumatic control device.
Flow: there are many methods used to detect fluid rate. In water systems the most common is to detect pressure difference across a restriction to flow, such as an orifice plate or a venturi.
Various devices are available to sense air velocity in ductwork. In larger cross-sections, where the velocity may vary across the duct area, an array of sensing devices is required to establish an average value.
Enthalpy: normally used in air-conditioning plant, temperature and humidity sensors feed signals to a controller, the output from which is a signal proportional to the enthalpy of the air. Control devices are available which accept signals corresponding to the enthalpies of two air streams, typically outside air and exhaust air and, depending upon the relationship between these two values, a control action on dampers or heat exchangers is initiated.
Control devices
The most common components used in the field are control valves, for steam and water, and control dampers for air systems. The selection and sizing requires an understanding of both the devices and of the system characteristics. The system to be controlled and the associated flow rates would be sized at the peak design load, but would operate for most of the time at some partial load. The control device, therefore, has to provide stable control over the full range of operating conditions.
The movement of a valve or damper is determined by an actuator which is the component that responds to the signal from the controller. The actuator characteristics which are of importance are torque (the ability to cause movement of the control device) and stroke period (period of movement between the limiting positions). Selection of the actuator type will depend upon the choice of control system.
Controller modes of operation
There are various ways in which a controller can cause a control device to operate in response to a signal from a sensing device.
The most common modes are:
- Two-position control: a typical application is on/off switching, in which the sensing device provides two signals. The interval between the switching actions, an inherent characteristic of the device, is normally referred to as the differential gap. On/off control would give quite acceptable results where the controlled variable has large thermal inertia, such as a hot water system storage calorifier or an space heated by a mainly radiant source.
- Step-control: it is sometimes necessary to operate a series of switching operations in sequence from one sensing device. For example, when multiple refrigeration compressors have to be started in turn, with increasing cooling load as sensed by a change in chilled water return temperature.
- Proportional control: with this form of control the output signal from the controller is proportional to the input signal from the sensor.
- Floating control: with floating control, there is normally a neutral zone around the set-point, within which no control action occurs, the control device remains in the last controlled position.
- Integral control: seldom used alone, this is an important addition to other forms of control, particularly to the proportional mode. With integral action there is continuous movement whilst deviation from the set-point persists such that the rate of movement is a function of the amount of deviation from the set-point.
- Derivative control: this mode involves a further development of integral action such that the controller output is a function of the rate of change of the controlled variable. This form of control, like the integral mode, would not normally be used alone, but in combination with others.
- Proportional plus integral (PI): this combination gives stable control with zero off-set. So long as there is deviation from the set-point, the controller will continue to signal a change until zero error exists. In addition, this mode is used more generally for applications where close control is required.
- Proportional plus integral plus derivative (PID): this mode of control would be used where there are sudden and significant load changes and where zero off-set from the desired point is required.
Building Management Systems (BMS)
Flexibility in available systems has led to different approaches to application and this, in turn, has given rise to descriptions such as energy management system, building energy management system, building automation system, supervisory and control system...
The basic functions of BMS may include:
- Initiation of systems control functions.
- Continuous monitoring of systems.
- Warning of out of limit conditions (alarm).
- Initiation of emergency sequences.
- Logging of significant parameters.
- Monitoring and recording energy use.
- Condition monitoring and fault analysis.
- Planned maintenance.
- Tenant billing.
Systems may be further enhanced by the use of modern database software, graphics and word processing techniques to provide opportunities for applying BMS to total building management functions. Some of the benefits which can result are:
- Lower energy consumption.
- Improved system reliability.
- Savings from programmed maintenance.
- Reduced number of watch keeping operatives.
- Improved building management.
One of the major difficulties experienced on many projects, particularly the larger and more complex, is the achievement of satisfactory completion including correct commissioning and performance testing.
Using BMS, it is possible for the systems and controls to be finely tuned with the usage patterns of the building and to the actual thermal response of the building elements, and this may be undertaken on site or at a remote location.
The magnitude of potential energy savings arising from the use of a BMS is dependent upon the type and condition of the installations before the addition, but energy savings of up to 40% may be achieved where BMS is introduced into an existing installation which was poorly controlled and maintained. Even so, compared with an efficiently operated system without BMS, the addition may offer potential savings of up 10%-12%.
Running costs
The cost of operation of any system providing space heating, ventilation, air-conditioning or hot water supply will depend upon a number of variables such as:
- Fuel consumption.
- Power consumption.
- Water consumption.
- Maintenance and consumables.
- Labour.
- Insurance and similar on-costs.
- Interest on capital and depreciation.
When selecting systems for a building it is necessary that both the initial cost of the installation and the operating costs be calculated for all the options to establish the most appropriate balance to suit the client´s circumstances.
Part 2 of the CIBSE Energy Code sets down procedures to enable designers to compare calculated energy demands for thermal and electrical consumption with energy targets.
It is essential, in the first place, to analyse energy use: where, how much and in what form it is being expended.
Maintenance
There are various levels of maintenance which may be applied to building services, the two most common being:
- Corrective. The majority of operations are carried out on breakdown or fall-off in performance, backed up sometimes with specific tasks undertaken on a regular basis.
- Preventative. Planned procedures are undertaken at regular intervals related to statistical failure rates of equipment with intend to extend the life of the plant overall to a maximum and to minimise the risk of breakdown. Work is carried out to a predetermined schedule enabling resources and material purchase to be planned in advance.
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