Electrical loads in modern buildings

Modern buildings contain an increasing amount of electrical apparatus and this technological explosion is increasing the demand for electrical power to support the equipment.

In order for planning to proceed the design engineer must be able to confidently produce load estimates.

An evaluation of the main electrical services requirements in a building should begin with an estimate of the likely load requirements. This is usually based upon a unit loading on a square metre basis.

When applying unit loads on a W/m2 basis for initial design calculations, it is suggested that the gross area of the building be utilised, subtracting the known areas of lifts, shaft ways... Reducing areas to take account of the thicknesses of exterior walls or columns is an unnecessary level of precision for estimating loads.

The electrical load within most commercial buildings can be arranged into the following broad categories:

- Lighting.
- Small power.
- HVAC equipment.
- Lifts and escalators.

Lighting: The design of lighting systems is provided in the CIBSE Code for lighting and CIBSE/SLL Lighting Guides but for broad planning purposes, taken over the whole of a large office building,a unit load of 12-20 W/m2 is reasonable.

Small power: Usually consists of items which are plugged into socket outlets or permanently connected. The small power requirements vary widely throughout a building, from some areas having virtually no small power loads to other areas, such as computer rooms, which have a relatively high unit loading. It is of extreme importance that the engineer obtains details of all the connected equipment and includes an appropriate allowance for future expansion or load increase.

HVAC equipment: In modern buildings the load required for HVAC systems can represent 40-50% of the total building load. Such loads are affected by the nature of the building fabric, fresh air requirements and internal heat gains from lights, people and equipment. Typically, the electrical load resulting from a mechanically ventilated and cooled building could be in order of 40-50 W/m2, but requirements must be assessed for each project.

Lifts: The evaluation of lift requirements must be undertaken by a lift specialist. A large, tall building may have several hundred kilowatts of lift equipment installed. Of particular interest to the electrical engineer is the fact that peak lift loads can be considered short time loads and their impact on overall building demand discounted but not ignored.

It must be taken in consideration when designing, the harmonics that can be generated in an electrical system.
Harmonics are typically generated by inductive luminaries, computers, rotating machines, electronic speed controllers and uninterruptible power supplies.

Harmonic currents may cause distortion of the voltage wave form (notching) and high neutral currents. The notching effect on the voltage wave form can be particularly troublesome on small distribution systems and this can cause equipment malfunction due to line voltage conditions being outside accepted tolerances.

Automatic controls in Building Management Systems

There has been a very considerable development in the application of controls since the introduction of computer technology. This has been particularly marked in the case of building and energy management systems, where significant benefits in the conduct of plant operation, maintenance and economy in running are available.

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.

Electrical Protection for Safety

ELECTRICAL PROTECTION FOR SAFETY

Part 4 of the 17th Edition details the methods and applications of protection for safety, and consideration of these details must be made as part of the design procedure.

Areas that the designer needs to address are:

- Protection against electric shock.
- Protection against thermal effects.
- Protection against overcurrent.
- Protection against overload.
- Protection against fault current.
- Protection against undervoltage.
- Protection against overvoltage.
- Requirements for isolation and switching.

Protection against electric shock

There are two ways that persons or livestock may be exposed to the effects of electric shock, these are:

1. By touching live parts of electrical equipment.

2. By touching exposed-conductive parts of electrical equipment or systems, which have been made live by fault.

One method used to protect against contact with live parts is to insulate them in enclosures and/or place them behind barriers. In order to ensure that such protection will be satisfactory, the enclosures/barriers must conform to BS EN 60529, commonly referred to as the Index of Protection (IP) code. This details the amount of protection an enclosure can offer to the ingress of mechanical objects, foreign solid bodies and moisture. Also, protection for wiring systems against external mechanical impact needs to considered. Reference should be made to BS EN 62262, the IK code.

Protective earthing, protective equipotential bonding and automatic disconnection in case of a fault is the most common method of providing Fault protection.
The disconnection times for final circuits not exceeding 32A is 0.4 s and for distribution circuits and final circuits over 32A is 5 s. For TT systems these times are 0.2 s and 1 s.
The low impedance path for fault currents, the earth fault loop path, comprises that part of the system external to the installation, i.e. the impedance of the supply transformer, distributor and service cables Ze, and the resistance of the line conductor R1 and circuit protective conductor (cpc) R2, of the circuit concerned.
The total value of loop impedance Zs is therefore the sum of these values:

                                                        Zs = Ze + (R1+R2) Ω.

It must be noted that the actual value of (R1+R2) is determined from:


Tabulated value of (R1+R2) x Circuit length x Multiplier
                              1000

The multiplier  corrects the resistance at 20°C to the value at conductor operating temperature.

The designer obviously has some measure of control over the values of R1 and R2 but the value of Ze can present a problem when the premises, and hence the installation within it, are at drawing-board stage. Clearly Ze cannot be measured, and although a test made in an adjacent installation would give some indication of a likely value, the only recourse would either be to request supply network details from the Distribution Network Operator (DNO) and calculate the value of Ze, or use the maximum likely values quoted by the DNOs, which are:

TT system: 21 Ω.
TN-S system: 0.8 Ω.
TN-C-S system: 0.35 Ω.

But these values are pessimistically high and may cause difficulty in even beginning a design calculation.

Supplementary bonding is used as Additional protection to Fault protection and required under the following conditions:

1. When the requirements for loop impedance and associated disconnection times cannot be met (RCDs may be installed as an alternative).

2. The location is an area of increased risk such as detailed in Part 7 of the Regulations, e.g. bathrooms, swimming pools..

Protection against thermal effects

The provision of such protection requires, in the main, a commonsense approach. Basically, ensure that electrical equipment that generates heat is so placed as to avoid harmful effects on surrounding combustible material. Terminate or join all alive conductors in approved enclosures, and where electrical equipment contains in excess of 25 litres of flammable liquid, make provision to prevent the spread of such liquid, for example a retaining wall round and oil-filled transformer.

Protection against overcurrent

The term overcurrent may be sub-divided into:

1. Overload current.

2. Fault current. This is further sub-divided into:
    a) Short-circuit current (between live conductors)
    b) Earth fault current (between line and earth)

Overloads are overcurrents occurring in healthy circuits and caused by for example, motor starting, inrush currents, motor stalling, connection of more loads to a circuit than it is designed for..

Fault currents, on the other hand, typically occur when there is mechanical damage to circuits and/or accessories causing insulation failure or breakdown.

Clearly, significant overcurrents should not be allowed to persist for any length of time, as damage will occur to conductors and insulation.

Protection against overload

Protection devices used for this purpose have to be selected to conform with the following requirements:

1. The nominal setting of the device In must be greater than or equal to the design current Ib: In ≥ Ib

2. The current-carrying capacity of the conductors Iz must be greater than or equal to the nominal setting of the device In: Iz ≥ In

3. The current causing operation of the operation of the device I2 must be less than or equal to 1.45 times the current-carrying capacity of the conductors Iz: I2 ≤  1.45 x Iz

Overload devices should be located at points in a circuit where there is a reduction in conductor size.

Protection against fault current

Short-circuit current

When a "bridge" of negligible impedance occurs between live conductors the short-circuit current that could flow is known as the "prospective short-circuit current" (PSCC), and any device installed to protect against such a current must be able to break and in the case of a circuit breaker, make the PSCC at the point at which it is installed without the scattering of hot particles or damage to surrounding materials and equipment. It is clearly important therefore to select protective devices that can meet this requirement.

When an installation is being designed, the PSCC at each relevant point in the installation has to be determined, unless the breaking capacity of the lowest rated fuse in the system is greater than the PSCC at the intake position. For supplies up to 100A the supply authorities quote a value of PSCC, at the point at which the service cable is joined to the distributor cable, of 16kA. This value will decrease significantly over only a short length of service cable.

Earth fault current

We have already discussed this topic with regard to shock risk, and although the protective device may operate fast enough to prevent shock, it has to be ascertained that the duration of the fault, however small, is such that no damage to conductors or insulation will result. This may be verified in two ways:

1. If the protective conductor conforms to the requirements of Table 54.7 (IEE Regulations).

2. The c.s.a (conductor sectional area) of the protective conductor is not less than that calculated by using the formula:

S =	Ö(I²t)
	k

where	S  is the minimum protective conductor cross-sectional area (mm2)
	I  is the fault current (A)
	t   is the opening time of the protective device (s)
	k  is a factor depending on the conductor material and insulation, and the initial and maximum insulation temperatures.

Discrimination

It is clearly important that, in the event of an overcurrent, the protection associated with the circuit in question should operate, and not other devices upstream. It is enough to simply assume that a device one size lower will automatically discriminate with one a size higher. All depends on the "let-through" energy of the devices. If the total "let-through" energy of the lower rated device does not exceed the pre-arcing "let-through" energy of the higher rated device, then discrimination is achieved.

Protection against undervoltage

In the event of a loss of or significant drop in voltage, protection should be available to prevent either damage or danger when the supply is restored.

This situation is most commonly encountered in motor circuits, and in this case the protection is provided by the contactor coil via the control circuit.

If there is likely to be damage or danger due to undervoltage, standby supplies could be installed and, in the case of computer systems, uninterruptible power supplies (UPS). Switching on of very large loads can have the effect of causing such undervoltages.

Protection against overvoltage

Overvoltages can be caused by:

- Transient overvoltages of atmospheric origin.

- Switching surges within the installation.

Requirements for isolation and switching

An isolator is, by definition, a mechanical switching device which provides the function of cutting off, for reasons of safety, the supply to all or parts of an installation, from every source.

A switch is a mechanical switching device which is capable of making, carrying and breaking normal load current, and some overcurrents. It may not break short-circuit currents.

Isolators are off-load devices and switches are on-load devices.

Common devices are circuit breakers, RCDs, isolating switches, plugs and socket outlets, plug fuses..

CHP Technologies

CHP Technologies

CHP (Combined heat and power) is a specific form of distributed generation (DG), which refers to the strategic placement of electric power generating units. CHP enhances the advantages of DG by the simultaneous production of useful thermal and power output, thereby increasing the overall efficiency.

Summary of CHP Technologies:

- Gas turbines: gas turbines are typically available in sizes ranging from 500kW to 250MW and can operate on a variety of fuels such as natural gas, synthetic gas, landfill gas and fuel oils.

- Microturbines: which are small electricity generators. Microturbines use the fuel to create high-speed rotation that turns an electrical generator to produce electricity. In CHP operation, a heat exchanger referred to as the exhaust gas heat exchanger, transfers thermal energy from the microturbine exhaust to a hot water system HWS. Available models range in sizes from 30kW to 250kW.

- Reciprocating engines: spark ignition (SI) and compression ignition (CI) are the most common. SI engines use spark plugs with a high-intensity spark of timed duration to ignite a compressed fuel-air mixture within the cylinder. SI engines are available in sizes up to 5MW. Diesel engines, also called CI engines, are among the most efficient simple-cycle power generation options in the market. Reciprocating engines start quickly, follow load well, have good part-load efficiencies, and generally have high reliabilities.

- Steam turbines: that generate electricity from the heat produced in a boiler. The energy produced in the boiler is transferred to the turbine through high-pressure steam that in turn powers the turbine and generator. The capacity of commercially available steam turbine typically ranges between 50kW to over 250MW.

- Fuel cells: use an electrochemical or battery-like process to convert the chemical energy of hydrogen into hot water and electricity. There are currently five types of fuel cells under development. These include phosphoric acid (PAFC), proton exchange membrane (PEMFC), molten carbonate (MCFC), solid oxide (SOFC), and alkaline (AFC).

DHC District Heating and Cooling

DISTRICT AND COMMUNITY HEATING

DHC is an integrative technology that can make significant contributions to reducing emissions of carbon dioxide and air pollution and to increasing energy security.

The fundamental idea of DHC is simple but powerful: connect multiple thermal energy users through a piping network to environmentally optimum energy sources, such as CHP, industrial waste heat and renewable energy sources such as biomass, geothermal and natural sources of heating and cooling.

District heating is traditionally divided into two types namely that serving flats, apartments and housing, and that serving a variety of building owners that can include commercial, retail, industrial, residential and local authority. The former is normally called community heating and the latter district heating.

Both district and community heating in recent years have been associated with power generation in the form of combined heat and power plant (CHP) where the electrical power generated is used locally and any excess power supplied to the national grid.

In one community heating system using CHP, blocks of flats are heated from a central plant and the lifts, entrances, corridors and landings are powered and lit from the electricity generated. Waste incineration is used to generate heat and power in district heating as also is biomass.

The focus for district and community heating schemes is clearly heating, so, the heat load for the scheme will be the major component.

The effect on overall efficiency, when the CHP option is used, is to raise it from 30% to as high as 75%. It is necessary however for the generator to find a customer for the heat generated from the CHP plant. Community and district heating can also benefit from the application of CHP technology as electricity can be used to power the plant with the surplus sold to the national grid.

The two main investments in a district heating system are the heat production plant and the network. The heat production plant is a single investment; the cost of it depends on the total annual amount of heat load of the area. The investment in the pipe system, otherwise, is a question of the length of the pipe network within the area of the heat supply and therefore is dependent on two dimensions: thermal length and thermal width. Therefore it follows that the costs of the distribution network can vary appreciably for different network geometries and type of systems.

Factors for consideration:

The feasibility study for a new scheme will need to investigate and research a number of areas before and initial decision can be reached.

- Type and concentration of consumers: the greater the variety the better, and could include commercial, industrial, retail, residential, schools, libraries, museums, leisure centres...

- Base load consumer: this would provide a fairly constant and substantial requirement for heating throughout the year ensuring continuous use of plant. Another base load usually available is the need for hot water supply particularly in residential sector, hotels, hospitals, campings and sport centres.

- Consumer acceptance: a major factor is the charge for the service which needs to be at least 10% below the cost for running and maintaining individual boiler plant. Another factor would be the security of heat supply.

- Environmental factors: ideally the heat and power generating plant needs to be located near. However, this mitigates against the noise disturbance and pollution which may result.

- Energy utilization: a district heating proposal shows a clear advantage in its use of energy from fossil fuel, waste and renewable sources over local heat generation.

- Maintenance philosophy: one of the lessons learnt in early district heating schemes is the importance of getting the maintenance of plant and external pipework right. A sure way to achieve this is to engage the contractors responsible for the installation of the distribution mains, for example, in a maintenance contract, following completion.

CHOICE OF SYSTEM AND OPERATING PARAMETERS

Energy sources available: These would include the most likely energy sources for the scheme - fossil fuel available, type of waste and type of renewable energy.

Heat supply ratios: Assessment of approximate ratios of process, heating, hot water supply loads and power loads are needed for the feasibility study. This will have a direct bearing on the choice of plant.

Heating medium: The heating medium needs consideration. It is invariably water because of its high specific heat capacity and may be LTHW, MTHW or HTHW. Steam might be considered at the central plant because of its high latent heat.

Heat exchange: If high temperatures are employed from the central plant it will be necessary to reduce the temperature using heat exchangers at substations. Industrial consumers may want high temperature distribution for their process and space heating.

Consumer supply: The distribution pipework layout should seek to ensure a ring main arrangement, so that, if a consumer's supply fails, a temporary reconnection can be made quickly.

Basis for charging: There are three ways in which the consumer can be charged for energy use:
    - Flat rate, which avoids the use of metering equipment and hence its initial cost and maintenance (not recommended).
    - Service charge plus charge for energy actually used.
    - Charge for energy use that includes the service charge.
It is important for avoiding consumer concerns and providing a good and reliable service that the instrumentation installed must be carefully selected both in the accuracy of the metering equipment as in the recording process.

Future requirements: It needs to take in consideration future extensions to the scheme and make a strategic plan.

Diversity: The feasibility study will address the diversity factor that will be applied to the plant for sizing purposes. Clearly the central plant is not sized on the total net load when all consumers are on line. As a general rule, the lower the number and diversity of consumers connected to the scheme the higher will be the diversity factor. The CIBSE Guide book A recommends a diversity factor of 0.7 for district heating schemes that have a wide spread and high number of consumers.

BEMS Building Energy Management System

BUILDING ENERGY MANAGEMENT SYSTEM - BEMS

PLANT CONNECTIONS AND CONTROLS:

Single or multiple boilers should always be connected to either a mixing header or separate flow and returns headers. This allows heating circuits to be independently connected to the boiler plant and facilitates the maintenance and breakdown of individual circuits and boilers. It is for this reason and indicator of good engineering practice.

Heating systems with CVVT (Constant Volume Variable Temperature) control are particularly subject to low return temperatures in mild weather. This can have a detrimental effect on the boiler by inducing condensation in the flue gas. Low return temperatures can also cause differential expansion within some boilers and consequent stress in the boiler metal. A by-pass protection is necessary so when the thermostat senses low temperature water in the boiler return, it energizes the pump and cuts it out on a predetermined higher temperature.

Condensing boilers are specifically designed to operate on low return water temperature. In the process, further sensible and latent heat of condensation from the flue gas are given up, therefore increasing the thermal efficiency of the boiler.

Temperature controls

In HVAC systems the heat emitter must be sized to meet the peak heating load (at design conditions) of the room. This involves determining the design steady-state heat loss rate and applying a plant ratio factor to allow for the thermal capacitance of the structure where intermittent heating is used.

The emitter will be required to provide full heat output at start-up, but solar gains and internal gains due to occupants, lighting and equipment will quickly reduce the heat load.

It is essential that the output rate of the emitter can be efficiently controlled both to ensure thermal comfort and to reduce energy consumption.

The output rate of any heat emitter can be achieved in one of two ways: modulating the water flow rate whilst maintaining the flow water temperature or modulating the flow water temperature whilst maintaining the water flow rate.

Modulation of water flow rate:

This is usually achieved by means of a two-port or three-port valve which throttles the water flow rate in response to a room temperature sensor. For radiators and radiant panels, thermostatic radiator valves (TRV) are invariably used. These are low cost and self-actuating valves requiring no wiring. Be aware that the response curve of the system is highly non-linear and such non-linearity requires a valve characteristics which will produce a large reduction in flow rate for a small valve stem movement, but they are relatively expensive.

Flow temperature modulation:

Flow temperature modulation is achieved by the blending of return water with the flow using a three-port mixing valve. The response of this control is very nearly linear, so good control is achieved. Unfortunately, such valves and control systems are relatively expensive so this system is used for controlling large groups of emitters.

Combined flow rate and temperature modulation:

Good control is achieved by combining the two methods. Flow temperature modulation can be carried out centrally by scheduling it to outdoor air temperature. Local two-port valves, controlled by the local room temperature, then modulate the water flow rate to each emitter in response to changing solar and internal heat gains. This combination of central and local control is specified in the Building Regulations.

Control systems in building energy management systems

Traditionally many building services systems are controlled using either pneumatics or electric/electronic and mechanical devices such as the five elements in CVVT control: valve body, valve actuator, immersion thermostat, outdoor detector and controller.

Direct digital control and supervisory control can be more user-friendly and can give the user more control over the building services systems either locally or remotely via a modem to a building energy management system (BEMS). The capital costs and advantages of a BEMS depend upon whether the user has the time and commitment to use this facility and take full advantage of the technology.

The local area network (LAN) might include BEMS outstations or original equipment manufacturer's (OEM) outstations. This would be linked to a modem if the final control and monitoring location is remote. Software is generated and dedicated to operate the controls and relay system conditions such as temperature, relative humidity, pressure, pressure drop, and status such as duty plant operation, standby plant operation. These conditions can be called up on a visual display unit (VDU) or monitor, and will include system logic diagrams. The way a BEMS is connected to a LAN is called the topology, of which there are basically three: bus, star and ring.

The keyboard and central processing unit (CPU) complete with visual display unit (VDU), collectively called a HMI Central Station, are connected to the LAN via modem.

Outstation functions:

These are split in three levels:

1. High Level: remote communication, user interface, optimizer control, cascade control, maintain trend logs, maintain event logs.

2. Mid Level: proportional plus integral plus differential control (PID), main data control (MD), alarm check, alarm communication, calendar/clock control, program defined interlocks.

3. Low level: hard-wired interlocks, timer control, check input limits, sensor interface, scan inputs.

Interlock systems:

Interlocks can be considered as "don't until" statements, and are of importance in defining the control strategy and in detailing the schematic.

An example of a system of interlocks of plant operation might be:

- Don't start primary heating pump until the time is right.
- Don't start the lead boiler unless primary pump is energized.
- Don't start secondary pump on zone 1 until boiler primary circuit is at 80 degrees Celsius ...

Supervisor station and central station functions:

- Plant supervision.
- Maintenance supervision.
- Security supervision.
- Energy monitoring.
- Environmental monitoring.
- System development.
- Plant executive control.
- Reporting.
- Data archival.
- Design evaluation.

Control strategies for heating systems:

These are a few recommendations relating to controls for heating systems:

1. The boiler primary circuit should be pumped at constant volume and be hydraulically independent of the secondary circuits.

2. Domestic hot water (DHW) should be provided by a separate system.

3. The temperature to each zone should be compensated according to outdoor temperature (clearly this cannot apply to fan convectors or unit heaters).

4. The zones themselves should be selected on the basis of:

- Solar heat gain (building orientation).
- Building exposure (multi-storey buildings, horizontal zoning..).
- Occupancy times.
- Building thermal response.
- Types of heating appliance.

5. Frost protection during off periods.

6. Sequence control of multiple boilers.

7. Control of demand.

8. Summer exercising of pumps.

9. Flue gas monitoring and alarm.

10. Graphic display of operating times and off times.

System operation method statement:

An important part of the schematic or logic diagram control is a written statement that explains how you intend the systems to operate.

The Method Statement will include:

- Details of zoning by time scheduling and control of temperature.
- Description of plant and circuits, which would include space heating, ventilation, hot water supply.
- Description of interlocks.
- Client interface (how the client can use the system).
- Specialist interface (adjust/monitor/refine and maintain).

Further reading and information in:

CIBSE Guide H: Building control systems
CIBSE Guide B1 Heating

PLC CONTROL SYSTEMS

PLC CONTROL SYSTEMS

The development of low cost computer has brought the Programmable Logic Controller (PLC). PLC´s have gained popularity on the factory floor and industrial control process, most of this is because of the advantages they offer:

- Cost effective for controlling complex systems.
- Flexible and can be reapplied to control other systems quickly and easily.
- Computational abilities allow more sophisticated control.
- Programming is easier and reduce downtime.
- Reliable components make these likely to operate for years before failure.

There are different types of control such as continuous and discrete controls.

In the continuous control systems the values to be controlled change smoothly and they can be linear or non linear. Linear means that can be described with a simple differential equation (e.g. PID), while in the non-linear systems the mathematics become much more complex (e.g. MRAC or FUZZY LOGIC).

In the discrete control systems, they could be conditional or/and sequential. Conditional or logical means that the value to be controlled are described, with BOOLEAN and EXPERT SYSTEMS, as on-off. The sequential control is a logical control that will keep track of time and previous events, event and temporal based using TIMERS and COUNTERS.

Logical and sequential control is preferred for system design. These systems are more stable, and often lower cost. Most continuous systems can be controlled logically.

When a process is controlled by a PLC it uses inputs from sensors to make decisions and update outputs to drive actuators. That means that the process will change over time because the actuators will drive the system to new states or modes of operation.

The control loop is a continuous cycle of the PLC reading inputs, solving the ladder logic, and then changing the outputs. Like any other computer this does not happen instantly. When power is turned on initially the PLC does a quick sanity check to ensure that the hardware is working properly. If there is a problem the PLC will halt and indicate there is an error. If the PLC passes the sanity check it will then scan all the inputs. After the inputs are stored in memory, the ladder logic will be solved using the stored values (not the current values). When the ladder logic is complete the outputs will be scanned and changed. After this the system goes back to do the sanity check and the loop continues indefinitely.

Many PLC configurations are available such as rack, mini, micro, but the most essential components of all them are:

Power Supply: this can be built into the PLC or can be an external unit. Common voltages required by the PLC are 24 Vdc and 220 Vac.

CPU: Central Processing Unit, this is the brain where ladder logic is stored and processed.

I/O Units: These are the input/output cards provided so that the PLC can monitor the process and initiate actions.
Input cards can be digital or analog, and common input sensors can be proximity switches (using inductance, capacitance, light or mechanical mechanisms), temperature sensors PTC´s, potentiometers for measuring angular positions...
Output cards have typically 8 to 16 outputs and these outputs can be relays, transistors or triacs. Relays are the most flexible but they are slower and they will wear out after millions of cycles, relay outputs are often called dry contacts. Transistors are limited to DC outputs, and Triacs are limited to AC outputs. Popular actuators are solenoid valves, lights, motor starters, servomotors...

Indicator lights: These indicate the status of the PLC including power on, program running and a fault.