Separate and Combined Systems

A separate drainage system is one were the foul water and the surface water are always kept separate. This is shown in the two previous diagrams. When a separate system is used then the sewerage treatment plant will not get overloaded in periods of wet weather.

A combined system is no longer used and joins some or all of the surface water into the foul water drainage system. This means that both surface water and foul water will discharge into the sewerage treatment plant. To avoid the treatment plant being overloaded, it may be possible to extract some foul water at various points in the drainage network. This can be achieved if the surface water is less dense than the foul water and tends to flow at the top in a drain. A separating device can be used to divert surface water into a storm water channel or drain.

It is generally agreed that the installation and running costs of sewerage treatment plant can be minimised if a separate system is adopted. For this reason the separate system is favoured by local authorities.A typical combined system is shown below but not recommended.

Two-Storey Dwellings

It is good practice to provide a vent for foul water drains.

Any smells or pressure may be relieved at the vent.

This may be achieved by continuing the foul water drain to high level above windows in a building.

In a two-storey dwelling the bathroom is normally upstairs so the foul water drainage system will be partly vertical, as shown below.

Combined System is not recommended. The Separate system as shown on the previous page is now used.

The vent is shown on a gable end, this can also be at the rear of a house or situated internally.

The system shown above is a single pipe system where there is one vertical soil and vent pipe. In some installations the vents may be connected at each appliance (wash basin, urinal, etc.). This is shown in the DESIGN POINTS section.

NOTE: Single pipe and two-pipe systems are not to be confused with separate and combined systems as discussed on the previous page.

Design Points

There are some points to note when designing any drainage scheme, these are:

Foul Water

1. Foul water is soil water from toilets and waste water from basins, baths, showers, etc.

2. The one-pipe system is favoured over the two-pipe system because there are fewer pipes and it is more hygienic.

 

3. The two-pipe system uses a

separate vent from each sanitary appliance, which are then joined into a combined vent stack, whereas the single-stack system is simplified.

4. All systems are vented and trapped to exclude smells and foul air.

Traps are devices, which contain a water-seal of about 50mm to 75mm to prevent gases escaping into sanitary fittings like wash basins, water closets, sinks, baths, showers, etc.

Foul water pipes exceeding 6.4 metres long are usually required to be vented.

5. If the waste pipe from a wash basin is at too steep a gradient, self-siphonage may occur. This is where the contents of the trap are sucked out into the waste pipe because the water flows away too quickly thus emptying the trap.

6. Induced siphonage can occur if a suction pressure develops in the drainage system. A suction pressure of 500 N/m2 (50mm water gauge) will reduce the water level in a basin trap by 25mm.

 

7. In badly designed systems backpressure can also occur which is sufficient to remove water from a trap.

 

8. Waste pipes from appliances which discharge into larger pipes avoids siphonage problems because the larger pipes do not normally run full.

For example, a 32mm waste from a wash hand basin is connected to a 100mm diameter Soil and Vent pipe.

9. Waste pipes from appliances which discharge into pipes of the same diameter have limitations on lengths, number of bends and gradients to minimise siphonage problems.

10.Self-siphonage is not normally a problem for sinks, baths and showers because of the near flat base of each appliance allowing the trap to re-fill should it empty.

 

11.The horizontal length of soil pipe from a WC is limited to 6m (Building Regulations U.K.).

 

12.Soil and Vent stacks should have no waste branch close to the connection of the WC.

 

13.Sometimes it is not possible to prevent pressure fluctuations in pipework in which case separate vent pipes should be installed. It may not be possible to limit the length of branches or provide reasonable gradients in some installations.

 

14.A velocity of flow of 0.6 to 0.75 m/s should prevent stranding of solid matter in horizontal pipes.

 

15.Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities.

 

16.A range of 4 lavatory basins, the traps from which discharge into a straight run of 50mm waste pipe not more than 4m long, with a fall of 1-21/2

o, will give rise to a need for venting. (reference British Standard No. 5572)

 

17.It is normal practice to connect a ground floor water closet straight into a manhole. Self-siphonage and induced siphonage will not occur

because of the large pipe from a W.C. diameter (100mm) and because the drain is vented.

 

18.Access points should be sited:

(a) At a bend or change indirection

(b) At a junction, unless each run can be cleared from an access point.

(c) On or near the head of each drain run.

(d) On long runs

(e) At a change of pipe size.

 

Sizing

19. The soil & vent stack or branch to which at least one WC is connected must have an internal diameter of at least 100mm.

Outlets from wash basins have a 32mm minimum diameter branch pipe and sinks and baths have branch discharge pipes of 40mm diameter.

For large drainage installations pipe can be sized using discharge units and appropriate graphs.

 

20. Drains should be laid at a depth of 900mm (minimum) under roads and at least 600mm below fields and gardens.

Drainage SchemesMedical Centre

The drainage scheme below shows a typical layout of a separate drainage system. The building is a two-storey medical centre.Can you identify the various drains and fittings?

 

The drawing below shows the previous drainage scheme with some details added.

 Nursing Home

The drawing below shows the plan of a single storey Nursing Home.There are two separate buildings.The yellow circles indicate foul water pipes exiting the building through the floor. The pink squares show roof downpipes and gullies from the surface water drainage system. The drawing below shows the proposed foul water below ground pipe layout. It may be possible to reduce the number of manholes i.e. No.6 and No.7 could be combined. The next step would be to size all pipework between manholes. Then pipe gradients, manhole depths and invert levels could be calculated if surface levels were known.

When you examine the above drawing you will notice that there are a large number of 100mm diameter underground pipes from sanitary appliances to manholes.It may be possible to reduce the number of these underground pipes and have fewer connections to manholes.This can be achieved by a variety of methods as follows;

1. Join some sanitary appliances drainage pipework above ground outside as shown below.This method is not as neat as when all pipes are underground but the important aspect of access is achieved with the cleaning eye.

2. Join some sanitary appliances drainage pipework above ground inside as shown below.This method has the advantage that footpaths outside are not obstructed.

Sizing Main Foul Water Sewer

Appliance No. off

Discharge Units each

Type of application

Public

TotalDischarge

units

Basin 5 3 15Bath 2 12 24Sink (large) 2 8 16WC (9.0 litre) 5 10 50Bidet 2 8 16Washing Machine 1 8 8Shower 5 8 40Dishwasher 1 8 8

TOTAL = 177

From graph 150 mm pipe is suitable.

Pipe Gradients

Above ground and below ground horizontal drainage pipes should be laid to an adequate gradient.

Gradients from 1 in 40 to 1 in 110 will normally give adequate flow velocities.

A gradient of 1 in 80 is suitable for commencing calculations for pipe schemes.

If a gradient is too steep i.e. steeper than 1 in 40, the liquid may run faster than the solids in the sloping foul water pipe thus leaving the solids stranded, which could then block the pipe.

If the gradient is not steep enough, i.e. less than 1 in 110, then the pipe could still block if the solids slow down and become stranded.

The fall in a pipe may be defined as the vertical amount by which the pipe drops over a distance. The distance can be between sections of pipe or between manholes. The diagram below show pipe fall and distance.

A gradient may be defined as fall divided by distance.

GRADIENT = FALL / DISTANCE

For example is a 24 metre section of drainage pipe has a fall of 0.30 metres, calculate the gradient.

Gradient = 0.30 / 24

Gradient = 0.0125

This can be converted into a gradient written as a ratio or 1: some number.

Gradient = 1 / 0.0125 = 80

Gradient = 1 in 80

The above formula may be rearranged for Fall if the gradient is known:

FALL = GRADIENT X DISTANCE

For example, calculate the fall in a 50 metre section of foul water pipework if the gradient is to be 1 in 80.

A gradient of 1 in 80 is converted to a number instead of a ratio.

1 / 80 = 0.0125

Fall = Gradient x Distance

Fall = 0.0125 x 50

Fall = 0.625 metres or 625mm.

The previous diagram may be completed by adding a pipe gradient.

Invert Levels

The Invert Level of a pipe is the level taken from the bottom of the inside of the pipe as shown below.

The level at the crown of the pipe is the Invert level plus the internal diameter of the pipe plus the pipe wall thickness. It may be necessary to use this in calculations when level measurements are taken from the crown of a pipe.

ManholesA manhole or access chamber is required to gain access to a drainage system for un-blocking, cleaning, rodding or inspection. A typical manhole is shown below.

Manholes may be manufactured from masonry or precast concrete. Sometimes several precast concrete rings are used to form a manhole which speeds up the on-site construction process. Normally deep manholes below 1.0 metre in depth require step irons to assist access for a workman. Manholes and access chambers are also manufactured in PVC. An access chamber is not usually large enough to admit a person but is suitable for access by cleaning rods or hose and they are used for domestic applications, a common size of plastic access chamber is 450mm diameter. For the domestic market plastic, fibreglass or galvanised steel lids may be used but cast iron lids are required where traffic crosses.

A back drop manhole is used in areas where the surface level slopes as shown below.

If the undergroung sewer pipe is to stay below ground it must follow the average gradient of the slope. This invariably means that the pipe gradient becomes too steep , resulting in the solids being left stranded in the pipe therefore causing a blockage.To overcome this problem the back drop manhole was developed, as shown below.

An easier way to construct a back drop manhole is to use an internal vertical section of pipe as shown below.

Drainage Pipe SizingFoul Water Pipe Sizing

The following method is one way of sizing pipework.

1. Choose a minimum gradient for all pipes, say 1:802. Use the table below to calculate the total number of discharge units in

pipe.

No. Appliance No. of units

Total units

WC 14basin 3bath 7

shower 4sink 6

washing machine 4dish washer 4

3. Size section from pipe manufacturers’ graphs.

An example of a pipe-sizing graph is shown below.

Example

Size the foul water pipework for 12 houses from the DATA below in the table.

1. Use a minimum gradient of 1:80 for all pipes.

2. Discharge units from each house:

No. Appliance No. of units

Total units

2 WC 14 282 basin 3 61 bath 7 71 shower 4 41 sink 6 60 washing machine 4 00 dish washer 4 0

Total 51

 

12 houses x 51 = 612 discharge units.

Flow graph gives 150mm-dia. foul drain since the convergence of the two lines on the graph is between the pipe size 100mm diameter and 150mm diameter.

Surface Water Pipe Sizing

The following method is one way of sizing pipework.

1. Choose a minimum gradient for all pipes, say 1:802. Use the table below to calculate the flow rate in each section.

SURFACE TYPE AREA

(A) m2

IMPERMEABILITY FACTOR (f)

TOTAL

(A x f)Road or

pavement0.90

Roof 0.95

Path 0.75

Garden 0.25

Access road, parking

0.90

Total

3. The area of each surface is calculated from drawings.4. The impermeability factor allows for water, which runs off each surface.5. The flow rate (Q) for each house can be calculated from:

Q = area drained x rainfall intensity x impermeability factor

6. If Rainfall intensity = 50mm/hr. m2, then Q becomes:

Q = A x 50 x f

Q = (A x f) 50 ((litres/hour)

7. Divide Q by 3600 to get value in litres/second.8. Multiply Q by number of houses to get Total Q.9. Estimate pipe size from Pipe Sizing graph.

For example, size the pipework for 12 houses from the drawing

Example

Size the surface water pipework for 12 houses using the DATA below in the table.

1. Choose a minimum gradient for all pipes, say 1:80

2. Surface water flow from each house.

SURFACE TYPE AREA

(A) m2

IMPERMEABILITY FACTOR (f)

TOTAL

(A x f)Road or pavement 20 0.90 18.00

Roof 40 0.95 38.00

Path 15 0.75 11.25

Garden 68 0.25 17.00

Access road, parking 25 0.90 22.50

Total 192 Total 103.50

3. Rainfall intensity 50mm/hr.

Q = area drained x rainfall intensity x impermeability factor

Q = A x 50 x f

Q = (A x f) 50

Q = 103.50 x 50 = 5175 litres/hour

Q = 1.438 litres/second per house X 12 houses.

Q = 17.25 litres/second.

Flow graph gives 150mm dia. surface water drain since the point on the graph lies between 100mm and 150mm

Septic Tanks

Introduction

A septic tank treats domestic sewage that is; the outlets from basins, baths, W.C.’s, showers, sinks and other sanitary and domestic appliances.

In septic tanks the solids in the sewage settle to the bottom to form sludge.Relatively clear liquid is left which forms a layer of scum on its surface. Bacteria feed on this liquid and digest some of the matter in it. The liquid then either passes into another settlement tank before passing to a watercourse or is discharged underground through a network of pipes to filter through the soil in a soakaway system.The solids that build up at the bottom of the tank need to be removed about once a year.    

History

In 1860 a French man called Mouras built a masonry septic tank for a house in France.

After a dozen years, the tank was opened and found, contrary to all expectations, to be almost free from solids. Mouras was able to patent his invention on 2 September, 1881. It is believed that the septic tank was first introduced to the USA in 1883, to England in 1895 and to South Africa (by the British military) in 1898.

Digestion

Sewage is allowed to rest in the septic tank for about 16 to 48 hours.

The process of digestion in the septic tank is done by bacteria. These bacteria can be killed by certain chemicals. The process of breaking down the organic matter in sewage is called anaerobic digestion since it is largely outside the presence of air.

The digestion reduces the amount of sludge and makes the contents of the septic tank less smelly. Normally it would take about two months to break down all the sludge in the tank so a normally used septic tank will only partially break down the contents.

Too much bleach, detergents and other household chemicals may destroy the useful bacteria. As a result the sewage will not be treated fully and may cause pollution problems. Emptying the septic tank regularly will ensure the septic tank keeps working properly. If possible use biodegradable 'septic safe' detergents.

Flow of Effluent

The concept is that effluent from the building should enter the tank at one end, be retained in the tank for a period and discharged at the opposite end to enter the soakaway drain.

The septic tank soon fills and as more effluent enters it automatically displaces the same amount out into the soakaway drain.

Inside the tank, flotsam is called the scum layer, and anything that sinks to the bottom forms the sludge layer. In between there is a fairly clear liquid layer. This clear liquor will overflow as new flows come in.

The process of anaerobic decomposition occurs in the tank which reduces the amount of solid matter and provides some treatment of the waste.

The soakaway drain, or percolation trench, is a method of discharging the tank effluent into the surrounding soil. The effluent from a septic tank is by no means fit for discharge into a water course.

Some solids, such as soap scum or fat, will float to the top of the tank to form the scum layer. Heavier solids, such as human and kitchen wastes, settle to the bottom of the tank as sludge.

Construction

Septic tanks can be block/brick built or made with glass reinforced plastic (GRP).

Access covers should be of durable quality to resist corrosion and must be secured to prevent easy removal. Septic tanks should prevent leakage of the contents and ingress of subsoil water and should be ventilated. Ventilation should be kept away from buildings.

Discharge and Soakaway

The water is discharged into a soakaway or ‘leaching field’ which consists of metres of perforated pipes laid under the soil. To allow the waste water to drain away efficiently a sizeable area is preferred and a soil type which actually allows the water to soak away. For this reason the siting of a septic tank in heavy clay soil may not be suitable. Free draining sand and gravel’s offer the best conditions.

The fall of distribution drains from the outlet should be as shallow as possible, i.e. 1 in 100 to 1in 200 to allow slow percolation. The bottom of the trench of the perforated pipe should be 900mm above the seasonally high water table, or bedrock if possible. If the water table is closer to the surface than 900mm then it may be possible to run the soakaway drain also closer to the surface ensuring that water does not come up to ground level.

The trench in which the discharge perforated pipe runs can be backfilled with aggregate to assist in percolation. The aggregate can be laid inside a wrap of geotextile material to impede the silting up of the soakaway with silt from the surrounding trench.

The soakaway drain should be long enough to allow the water to percolate into the sub-soil. Typical evaluation of the permeability of the soil will include a 'percolation test' to see how quickly liquid will disappear into the soil. Clay soils will be less absorbent than coarser sandier soils.

Notes:

A soakaway should not be constructed where the ground water table is close to surface.

In fine soil, the penetration distance of bacteria may be around 3m from the soakaway. Coarser soils will enable greater penetration. Coliforms (gut bacteria) reportedly can survive for as much as a month if they reach a source of groundwater.

Limestone substrata will most probably be fissured, enabling septic tank effluent to flow away too freely into the water table below.

Boggy or peaty ground is also unsuitable since the percolation rate is very slow.

It is almost inevitable that the soakaway will eventually clog, so it is worth positioning the tank and soakaway so that an alternative soakaway drain can be excavated in future.

BRE digest 151 Soakaways, details construction and sizing of soakaways.

Capacity

The size of the septic tank depends on the quantity of liquid being discharged to it which is dependant on the number of people in the dwelling.

From BS 6297 Small domestic sewage treatment works and cesspools the Septic tank capacity is;

Capacity (m3) = Number of residents x 0.14 + 1.8

For a house with four occupants the capacity is;

Capacity (m3) = (4 x 0.14) + 1.8

Capacity (m3) = 2.36 m3.

Positioning

Septic tanks must be sited at least 7m from the habitable part of the building (preferably downslope), within 30m of a suitable tanker access.

The drainage field or mound serving the septic tank must be at least 15m from any building, 10m from any watercourse, permeable drain or soakaway, etc and not be covered by drives, roads or paved areas.

Steep sloping sites should be avoided. Sites should be remote from ditches, streams and wells.

Compartments

Septic tanks are normally divided internally into compartments.This allows the new effluent to settle and be digested before it is passed into the outlet. Also, it means that the route from inlet to outlet is not direct, thus ensuring that liquid circulates before reaching the outlet, giving more time for digestion.

If constructed in block or brick, mortar is left out of the vertical joints between the masonry units at about half-liquid depth to make the slotted wall.

Levels

The level of the invert of the outlet pipe fixes the TWL (top water level) of the tank. When the water reaches that level, the tank is full to capacity, and it will overflow by discharge through the outlet.

In order that the inlet pipe does not become full, the inlet should be slightly higher than the outlet (say 50 - 100mm). This means that there will be a slight cascade into the tank.

To ensure that the scum on top of the liquid neither impedes influent nor escapes as effluent, both inlet and outlet pipes should be fitted with a tee as shown above.

Cess Pits

A cess pit is a sealed storage tank into which sewage is drained until it can be removed for disposal. The sewage is not treated in the tank just stored.

In some areas a septic tank is not suitable, there may be no suitable drainage in the subsoil, and a cess pool is the only answer.

Older cess pits are usually cylindrical pits lined with either brick or concrete. Modern cess pits are made from fibre glass, steel or polyethylene. Current building regulations require cess pits to be able to hold at least 18,000 litres of sewage. It is estimated that each person produces 115 litres of sewage a day. For a family of four this means that the tank will need emptying about once a month.

Seepage Pits

Other sewage systems that have been used in the past are seepage pits or large soakaways. These systems typically involve discharging septic tank treated sewage into a deep, cylindrical pit that is open on the sides and bottom. Sometimes these pits can be constructed using honeycombed brickwork, or concrete manhole sections with perforations in the walls. The holes are frequently filled with large stones or gravel and a cover (probably in concrete ) placed over the hole. If the ground strata for the whole depth is good and will absorb the effluent these can be satisfactory, but if not then these can cause problems as the end result will be a large hole filled with septic tank effluent.

Testing

Drains must be tested before and after backfilling trenches.

Water Test

BS 8005 gives details of Water tests.

This is suitable for sewers up to 750mm diameter.

The section of pipework to be tested is blocked at the lower end with a test pipe upstand at the higher end. This test pipe is often located in an inspection chamber or manhole.

The test pipe has a 1.2 to 1.5 m head of water in it to produce a meaningful test with adequate pressure.

This should stand for 2 hours and if necessary topped up to allow for limited porosity (clay pipes). For the next 30 minutes, maximum leakage for 100 mm and 150 mm pipes is 0.05 and 0.08 litres per metre run respectively.

BS 8005 requires maximum leakage of 1 litre per hour per metre diameter per metre length of pipe.

Air Test

BS 8005 gives details of Air tests.

The drain is sealed between access chambers and pressure tested with hand bellows and a 'U' gauge (manometer).

Build up air pressure initially to 100mm water gauge.

After 5 minutes adjust the air pressure to 100mm water gauge.

The pressure must not fall below 75 mm during the first 5 minutes, that is, a drop in pressure of 25mm over 5 minutes.

Smoke Test

The length of drain to be tested is sealed and smoke pumped into the pipes from the lower end. The pipes should then be inspected for any trace of smoke. Smoke pellets may be used in the smoke machine or with clay and concrete pipes they may be applied directly to the pipe line.

TYPICAL CONTROL SYSTEMSDOMESTIC HEATING SYSTEMS

The drawing below shows one method of controlling a domestic heating system. More details are given in the section – Domestic Heating Controls.

The system shown uses minimal electrical on/off control but utilises thermostatic valves to give mechanical control.

One of the disadvantages of the system is that during the summer the radiators would have to be turned off manually if hot water was required from the hot water cylinder.

The system shown below was for a large house with adjoining flat.The system is divided into 3 circuits with a possible fourth circuit for domestic hot water heating.Three pumps were used but another approach may be to use one larger pump and 3 zone valves for the circuits.Individual room control is achieved by installing thermostatic radiator valves (TRV) on most rads.

INDUSTRIAL HEATING SYSTEMS

STEAM SYSTEMSNon-storage Calorifiers using steam as the primary heating medium normally use a 2-port on/off or modulating control valve to maintain the desired temperature in the Secondary

Flow. An immersion thermostat measures the Secondary Flow temperature and regulates the flow of steam accordingly as shown below.A direct acting control valve may suffice where simple constant temperature is required. This uses a wax filled thermostat which can send a signal directly to the control valve.Always install a strainer before control valves in steam systems so that the valve seat does not become damaged by high velocity particles.

Controls for Air-Conditioning

Control systems for air-conditioning installations are complex and varied. A lot can depend on the control system sold by the control equipment manufacturer and also on how sophisticated the engineer wants the controls to be. Dew point control is one basic method for air conditioning systems and an understanding of dew point is necessary.

Constant Dew Point ControlConstant dew point control for air-conditioning systems means that the supply air is maintained at a constant dew point. The supply air condition will therefore be at the same moisture content for both summer and winter conditions as shown on the psychrometric chart below.

O

MRWADP S

FIG. 8.3.2 PSYCHROMETRIC CHART SHOWING SUMMER & WINTER CYCLES

o

M

w

s

s

ss

w

Constant Dew Point line

A typical air-conditioning system is shown below.

Control Sequence

1. In summer it is best to admit minimum outside air. Damper motors M1, M2 and M3 are adjusted by

temperature sensor T1 which measures outside air temperature.

Minimum fresh air is usually about 20% of the total supply air quantity. The actual minimum fresh air quantity can be calculated for high occupancy buildings based on statutory requirements and recommended levels.

2. In summer dew point is controlled by thermostat T2

operating the cooling coil control valve V2 which will alter the amount of chilled water through the coil. Thermostat T2 has a set point say of 10oC and if there is a rise in outdoor temperature or return air temperature then T2 will sense this temperature rise and open the cooling coil valve V2 to admit more chilled water through the coil and maintain the thermostat set point of 10oC in this example.If the outside air cools a little in summer, then thermostat T2 will eventually sense an air temperature lower than its

set point and will reduce the amount of cooling by diverting more chilled water away from the cooling coil.

3. In mid-season, when the outdoor air temperature is cool enough ‘free cooling’ may be used. This is probably when outside air temperature is lower than space temperature and is measured by sensor T1. If the outdoor temperature falls to say 21oC then sensor T1

will operate the damper motors to increase the amount of fresh air admitted to the system. The proportion of fresh air can be increased if the outdoor temperature falls to some outdoor temperature where 100% fresh air is an advantage for ‘free cooling’.

4. In mid-season dew point will continue to be controlled by thermostat T2 operating the control valve V2 as for summer.

5. In winter, with outside conditions well below that required in the conditioned space, valve V4 will maintain dew point control by thermostat T2. Valve V4 operates the heater battery.The anti-frost preheater in the outside air supply will have thermostat T3 operating valve V3 to maintain a temperature of say 4oC. If the anti-frost preheater is electric then switches are used to control electric elements.

6. If the amount of latent heat changes within the conditioned space then humidistat H1 can be arranged to reset thermostat T2 and to control valve V5 to introduce humidity to the air stream.The change in latent heat is a variation in dew point.The humidistat H1 can be placed either in the space or the

return air duct.Dry-Bulb

7. The final dry-bulb temperature leaving the plant will be controlled by thermostat T4 operating valve V4 to admit heat to the heater battery.

8. To provide for variation in sensible load in the conditioned space, and thus for adjustment of dry-bulb temperature leaving the plant, thermostat T5 placed either in the space or the return air duct may be arranged to reset thermostat T4 .

Controls for Refrigeration Systems

Air-Cooled Condenser ControlAir-cooled condensers are increasing in popularity because of the absence of water piping, also water does not come in contact with air as in cooling towers and their simple operation.Their use in air-conditioning is usually limited to about 70kW total plant output.

Air OFF

Air ON

Refrigerant flow & return

Refrigerant to air heat exchanger

FIG. 9.4.1 AIR-COOLED CONDENSER

The demand for cooling varies widely throughout the year and even varies widely from morning to afternoon to evening in warm summer days. This variation in demand can be met by good control of each item of refrigeration and air-conditioning plant.

It is relatively easy to control fan speeds or more often switch on and off fans in systems with several roof-mounted air-cooled condensers in accordance with the cooling requirement.

It is always desirable to keep a stable condensing pressure with reciprocating compressors, otherwise in mild periods when the refrigeration plant is on, the condensing pressure will be too low to meet a reduced demand for air-conditioning.

Controls for Lighting SystemsLighting systems may be automatically controlled resulting in annual electrical energy savings.

In modern offices for example the lighting level may be about 25 Watts per metre square floor area. If this can be reduced even by a small percentage significant savings can still result.

One typical method of control is to switch off the row of lights near windows on the external walls.This is achieved with the use of a photo-cell so that when the required lighting level can be maintained by daylight ( about 400 - 700 lux) a row of light fittings may be automatically switched off.

One of the problems associated with switching off lights is that even light distribution is no longer achieved and although the average lighting level may be adequate in a room there may be a noticeable difference in lighting levels in some areas.

Another system of lighting control measures room occupancy and lights are switches on only when the room is occupied. Movement sensors or a door switch can be used to indicate if the room is occupied or not. A typical installation would be a toilet with a door switch so that when the door opens the lights are automatically switched on (and if necessary the ventilation also). A time delay built into the system will avoid frequent switching.PIR’s (Passive Infra-red Sensors) act as movement and occupancy sensors and are useful in offices, classrooms where occupancy is variable.

Example 6

Size the pipework for the heating system shown below.

Also determine the pressure drops.

Section

Length of

Flow & Return

(metres)

1 16

2 6

3 6

The total lengths of the section are:

the pipe circuits may be divided into sections as shown below.

Different pipe sections carry different flow rates of water.

There are 3 sections in the system below.

Sections include both flow and return pipes.

Pipe Sizing Table

Section

Ref.

1

Heat Outp

ut

in section

kW

2

Pipe

Heat

loss.

15% of 1.

kW

3

Total Heat

Col.1+2

kW

4

Water

Flow

Rate

kg/s

5

Pipe

Size

mm

dia.

6

Length of pipe F&R

m

7

Total Equivalent length of Fittings

m

8

Total Pipe Leng

thCol. 6+7

m

9

Pressure

drop per

metre

Pa/m

10

TOTALPRESSU

RE DROP

Col. 8 x 9

Pa

1 5.70.855 6.56 0.1

622 16

Boiler = 2.5

3 Gate Valves = 0.4 x 3 = 1.2

4 Elbows = 1.0 x 4 = 4.0

2 Tees = 0.2 x 2 = 0.4 (Straight thro tees)

TOTAL

TEL = x le le = 0.8

TEL = 8.1 x 0.8 = 6.48 m

22.48 150 3372

2 4.0 0.60 4.60 0.11 22 62 Tees = 0.2 + Factor for 6.88 80 550

Note: the equivalent length le comes from the CIBSE guide pipe sizing table.

reduction

Reduction: A2/A1 = 0.0001767 /

0.000380 = 0.465

for Tee

TOTAL for 2 Tees = 1.1

TEL = x le le = 0.8

TEL = 1.1 x 0.8 = 0.88 m

3 1.7 0.255

1.96 0.05 15 62 Elbows = 1.0 x 2 = 2.0

2 Angle Valves = 5.0 x 2 =10.0

1 Panel Radiator = 2.5

TOTAL = 14.5

TEL = x le le = 0.5

TEL = 14.5 x 0.5 = 7.25 m

13.25 130 1723

Total 5645

The total pressure drop in the 3 sections is 5645 Pascals (Pa).