Steam Cycles

The Rankine cycle

Although the Carnot cycle is the most efficient cycle, its work ratio is low. Further, there are practical difficulties in following it.

Consider the Carnot cycle for steam as shown:

2 to 3: heat is supplied at constant temperature and pressure.
3 to 4: the vapour expands isentropically from the high pressure and temperature to the low pressure. In doing so, it does work on the surroundings, which is the purpose of the cycle.
4 to 1: the vapour, which is wet at 4, has to be cooled to state point 1 such that isentropic compression from 1 will return the vapour to its original state at 2. From 2 the cycle is repeated.

The four processes are physically very different from each other and they each require particular equipment:
- The heat supply, 2-3, can be made in a boiler.
- The work output, 3-4, can be obtained by expanding vapour thorough a turbine.
- The vapour is condensed, 4-1, in a condenser,
- and to raise the pressure of the wet vapour, 1-2, requires a pump or compressor.

At state 1 the steam is wet but it is difficult to stop condensation at this point and the compress it just to state 2. It is more convenient to allow the condensation process to proceed to completion, as shown:

The working fluid is water at the new state point 3, and it can be conveniently pumped to boiler pressure at state 5. The pump has much smaller dimensions than it would have if it had to pump a wet vapour, the compression process is carried out more efficiently, and the equipment required is simpler and less expensive.

This ideal cycle, which is more suitable as a criterion for actual steam cycles than the Carnot cycle, is called the Rankine cycle.

Rankine cycle with superheat

The average temperature at which heat is supplied in the boiler can be increased by superheating the steam. Usually the dry saturated steam from the boiler drum is passed through a second bank of smaller bore tubes within the boiler. This bank is situated such that it is heated by the hot gases from the furnace until the steam reaches the required temperature.

The Rankine cycle with superheat includes a steam receiver which can receive steam from other boilers. In modern plants a receiver is used with one boiler and is placed between the boiler and the turbine. Since the quantity of feedwater varies with different demands on the boiler, it is necessary to provide a storage of condensate between the condensate and boiler feed pumps. This storage may be either a surge tank or hot well.

The reheat cycle

It is desirable to increase the average temperature at which heat is supplied to the steam, and also to keep the steam as dry as possible in the lower pressure stages of the turbine.

The wetness at exhaust should be no greater than 10%. High boiler pressures are required for high efficiency, but that expansion in one stage can result in exhaust steam which is wet.

This is a condition which is improved by superheating the steam. The exhaust steam condition can be improved most effectively by reheating the steam, the expansions being carried out in two stages.

1´-2 represents isentropic expansion in the high-pressure turbine, and 2´-3 represents isentropic expansion in the low-pressure turbine. The steam is reheated at constant pressure in process 2-2´. The reheat can be carried out by returning the steam to the boiler, and passing it through a special bank of tubes. Alternatively, the reheat may take place in a separate reheater situated near the turbine; this arrangement reduces the amount of pipe work required.

The regenerative cycle

In order to achieve the Carnot efficiency it is necessary to supply and reject heat at single fixed temperatures. One method of doing this, and at the same time having a work ratio comparable to the Rankine cycle, is by raising the feedwater to the saturation temperature corresponding to the boiler pressure before it enters the boiler.

This method is not a practical proposition but this regenerative cycle has an efficiency equal to the Carnot cycle, since the heat supplied and rejected externally is done at constant temperature.

However, the Rankine efficiency can be improved upon in practice by bleeding off some of the steam at an intermediate pressure during the expansion, and mixing this steam with feedwater which has been pumped to the same pressure.

This mixing process is carried out in a open feed heater. The steam expands from condition 5 through the turbine. At the pressure corresponding to point 6, a quantity of steam, is bled off for feed heating purposes. The rest of the steam completes the expansion and is exhausted at state 7. This amount of steam is then condensed and pumped to the same pressure as the bleed steam. The bleed steam and the feedwater are mixed in the feed heater and then being pumped in a second feed pump.

Because of the number of feed pumps required, the heating of the feed water by mixing is dispensed with and closed heaters are used.

Underfloor Heating

Underfloor Heating System (UFH) is generally defined as that which warms the floor structure, including the floor surface, in order to heat the room or space above.

The method to heat transfer from the heated floor surface is circa 55% by radiation plus 45% by convection depending on the surface to air temperature differential. Because the majority of the heat is radiant, it creates occupancy comfort at lower air temperatures than would be expected from radiators, convectors and warm air systems.

Properly designed and installed underfloor heating systems produce a uniform room temperature with almost no stratification. The absence of heat stratification benefits fuel economy appreciably and, in addition, years of experience has proved that when an occupant´s feet are slightly warmer than the head, optimum human comfort conditions are assured.

Warm water underfloor heating systems operate by circulating a fluid through pipes of mostly plastic or metal/plastic composite materials embedded in floor structures. Therefore flooring components need to be conductive or have conductive metal elements provided in the floor to transfer the heat from the pipes to the floor surface.

Because heated floor surfaces are unusually large when compared to the size of a steel panel radiator, the floor surface temperature required is very low, indeed only a few degrees more than room temperature. However, it should be at or below 29ºC in all occupied areas so as to achieve an acceptable degree of foot comfort. Lower temperature limits, such as 27ºC for timber floors, are primarily required for protecting delicate structures or surfaces finishes.

Any boiler or heat generator or heat source can be used with underfloor heating but direct connection to Condensing Boilers and Heat Pumps improve efficiency resulting from the lower flow and return water temperatures normally used in underfloor heating systems.

Most rooms in modern well-insulated properties have a required design heat flow density in the range of 50-70 W/m2, well within the limitations of UFH systems.

Floor surface temperature is critical to comfort, as well as to heat output. New well insulated houses very often require only 26ºC floor surface temperature from UFH systems; this equates to a UFH output of around 66 W/m2 (based on 10.8 W/m2 per degree of temperature difference between the mean floor surface temperature and test standard room air temperature of 20ºC). BS EN 1264 Floor Heating, Systems and Components, Part 2. Determination of Thermal Output gives guidelines on the maximum values for floor surface temperatures.

It is normally recommended that the floor finish covering resistance R does not exceed 0.15 m2 K/W.

Distribution manifolds or headers are normally constructed from a non-ferrous metal, but engineering plastic versions are also available. All are configured with a flow barrel, and a return barrel, with individual circuit isolating valves, circuit regulating valves, drain cocks, air-vents and wall brackets.

Manifold positions need to be located strategically to minimise the amount of uncontrolled energy and the length of the circuits.

Pipe materials should be restricted to those specified within BS EN 1264 and BS 7291, namely Polybutylene (PB), Cross-linked Polyethylene (PE-X), PE-RT Polypropylene (PPC-2), Multilayer Plastic/Aluminium composite or Soft Annealed Copper Tube (BS EN 1057).

It is recommended the addition of a suitable corrosion inhibitor to the water in circulation to protect the appliance and other fittings during use.

Ventilation Systems

Ventilation - a means of changing the air in an enclosed space. The volume of air necessary to provide for human occupancy may be considered under the following principal headings:

- Provide fresh air for respiration; approx. 0.1 to 0.2 l/s per person.
- Preserve the correct level of oxygen in the air; approx. 21%.
- Control carbon dioxide content to no more than 0.1%.
- Control moisture; relative humidity of 30% to 70% is acceptable.
- Remove excess heat from machinery, people, lighting...
- Dispose of odours, smoke, dust and other atmospherics contaminants.
- Relieve stagnation and provide a sense of freshness; air movement of 0.15 to 0.5 m/s is adequate.

Measures for control:

- Health and Safety at Work.
- The Factories Act.
- Offices, Shops and Railway Premises Act.
- Building Regulations, Approved Document F - Ventilation.
- BS 5925: Code of practice for ventilation principles and designing for natural ventilation.

The statutes provide the Health and Safety Executive with authority to ensure buildings have suitably controlled internal environments. The Building Regulations and the British Standard provide measures for application.

Air changes per hour or ventilation rate is the preferred criteria for system design. This is calculated by dividing the quantity of air by the room volume and multiplying by the occupancy.

Some examples of guiding to ventilation rates (air changes per hour) are:
- Boiler plant rooms: 10-30.
- Canteens: 8-12.
- Cinema/theatre: 6-10.
- Hospital wards: 6-10.
- Hospital operating theatres: 10-20.
- Libraries: 2-4.
- Offices: 2-6.

Natural ventilation - Passive Stack Ventilation (PSV)

Natural ventilation is an economic mean of providing air changes in a building. It uses integral components with construction such as air bricks or openable windows. The sources for natural ventilation are wind effect/pressure and stack effect/pressure.

PSV consists of vertical or near vertical ducts of 100 to 150mm diameter, extending from grilles set at ceiling level to terminals above the ridge of a roof.

PSV is energy efficient and environmentally friendly with no running costs. It works by combining stack effect with air movement and wind passing over the roof. It is self-regulating, responding to a temperature differential when internal and external temperatures vary.


Mechanically Assisted Extract Ventilation Systems (MAVS or MEV)

MAVS may be applied to dwellings and commercial premises where PSV is considered inadequate or impractical. A low powered silent running fan is normally located within the roof structure. It runs continuously and may be boosted by manual control when the level of cooking or bathing activity increases. Humidity sensors can also be used to automatically increase air flow.

MAVS are acceptable to Approved Document F1 of the Building Regulations as an alternative to the use of mechanical fans in each room. However, both PSV and MAVS are subject to the spread of fire regulations (Approved Document B). Ducting passing through a fire resistant wall. floor or ceiling must be fire protected with fire resistant materials and be fitted with a fusible link automatic damper.


Mechanical Ventilation with Heat Recovery (MVHR)

MVHR is a development of MAVS to include energy recovery form the warmth in fan extracted moist air. The heat recovery unit contains an extract fan for the stale air, a fresh air supply fan and a heat exchanger. This provides a balanced continuous ventilation system. Apart from natural leakage through the building and air movement from people opening and closing external doors, the building is sealed to maximise energy efficiency. Up to 70% of the heat energy in stale air can be recovered.

Mechanical ventilation

There are three categories of mechanical ventilation systems:

- Natural inlet and mechanical extract.
- Mechanical inlet and natural extract.
- Mechanical inlet and mechanical extract.

Some noise will be apparent from the fan and air turbulence in ducting. This can be reduced by fitting sound attenuators and splitters.

Internal sanitary accommodation must be provided with a shunt duct to prevent smells or other contaminants passing between rooms. In public buildings, duplicated fans with automatic changeover are also required in event of failure of the duty fan.

Basement car parks require at least 6 air changes per hour and at exits and ramps where queueing occurs, local ventilation of at least 10 air changes per hour must be provided.

Ductwork in all systems should be insulated to prevent heat losses from processed air and to prevent surface condensation.

For efficient distribution of air, the uniformity of circular ducting is preferred for the following reasons:

- Less opportunity for turbulence.
- Less resistance to friction.
- Inherent rigidity.
- Lower heat losses or gains.
- Sound transfer generally less.
- Less potential for air leaks.

Where space is restricted under floors or in suspended ceilings, rectangular ducting of high aspect ratio may be required for practical reasons.

Galvanised sheet steel is the most common material used for ventilation and air conditioning ducting.

Flexible ducts are useful for short connections from air distribution boxes or plenums to several diffusers within close proximity. Flexible connections to fans will help to reduce vibration and sound.

Sound attenuation in ducting can be achieved by continuously lining the duct with a fire resistant, sound absorbing material. Where this is impractical, strategically located attenuators/silencers composed of perforated metal inserts or a honeycomb of sound absorbent material can be very effective.

Air velocity within a room or workplace should be between 0.15 and 0.5 m/s, depending on the amount of activity. Sedentary tasks such as desk work will fall into the range of 0.15 to 0.3 m/s, whilst more active assembly work, workshop and manufacturing between 0.3 and 0.5 m/s.

Estimation of duct size and fan rating can be achieved by simple calculations and application to design charts.


Types of Air Filters

Cell or panel: flat or in a vee formation to increase the surface contact area. Available in dry or wet (viscous) composition in disposable format for simple fitting within the ductwork. A rigid outer frame is necessary to prevent flanking leakage of dirty air. Dry filters can be vacuum cleaned to extend their life, but in time will be replaced. The viscous filter is coated with an odourless, non-toxic, non-flammable oil. These can be cleaned in hot soapy water and recoated with oil.

Absolute: a type of dry cell filter produced from dense glass paper. The paper is folded into deep pleats to create a series of vee formations arranged parallel to the air flow to increase surface contact.

Bag: a form of filtration material providing a large air contact area. When the fan is inactive the bag will hang limply unless wire reinforced. It will resume a horizontal profile during normal system operation. Fabric bags can be washed periodically and replaced.

Roller: operated manually or by pressure sensitive switch. As the filter becomes less efficient, resistance to air flow increases. The pressure effects a detector which engages a motor to bring down clean fabric from the top spool.

Viscous: these have a high dust retention capacity and are often specified for application to industrial situations. An improvement on the panel type has close spaced corrugated metal plates continuously sprayed with oil.

Electrostatic unit: this has an ionising area which gives suspended dust particles a positive electrostatic charge. These are conveyed in the air stream through metal plates which are alternatively charged positive and negative. The unit can have supplementary, preliminary and final filters giving an overall efficiency of about 99%.

Activated carbon: otherwise known as activated charcoal. A disposable filter composed of carbon particles resembling pieces of coconut shell and arranged to provide a large surface area. A glass fibre matting is often used to contain the carbon shells. This type of filter is used specifically in commercial cooker hoods and in other greasy, odorous atmospheres, as the carbon is extremely absorbent.



Controlled Environment in Hospitals

There has been a growing concern in the medical community regarding the hazardous effects of poor indoor air quality on the heath of individuals which leads to increased incidence of health related symptoms like headache, dizziness, eye and throat infection, fatigue, memory loss etc. The terminology 'Indoor Air Quality' refers to the nature of the conditioned (heated/cooled) air that circulates throughout the space (2). This refers not only to the comfort, which is affected by temperature, humidity, odour, but also to the harmful chemical and biological contaminants present in the conditioned space.

The basic differences between controlled environment by air conditioning for hospitals to take care of the above mentioned factors and that for other building types stem from :

1. The need to restrict air movement in and between the various departments.
2. The specific requirements for ventilation and filtration to dilute and remove contamination in the form of odour, air-borne microorganisms and viruses, and hazardous chemical and radioactive substances.
3. The different temperature and humidity requirements for various areas; and
4. The design sophistication needed to permit accurate control of environmental conditions.

Air filteration: To prevent the flow of air containing infectious particulates, air filtration is provided in Air Handling Units which filters particles, pathogens and water droplets carried into the air, either from the coils and humidifiers or through leaks in the low-pressure side of the unit. For critical care areas like operation theatres, ICU, emergency and recovery areas normally three-stage filtration is provided.

- Pre-Filters (BS-6540) : These are first stage filters having efficiency 70% down to 10 Microns. These filters are cleanable and washable and installed at inlet of airstream.

- Fine filters (BS-6540-part-I) : Second stage filters having efficiency 99% down to 5 Microns. The pressure drop in dirty conditions should not exceed 20mm WG and the initial drop should be between 6.5 to 8.5mm WG. These filters are washable.

- Hepa filters: With efficiency 99.97% down to 0.3 Microns used for operating rooms and ICU's. These are special high flow types with more media to handle higher

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.