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.
Combined System is not
recommended. The Separate
system as shown on the
previous page is now used.
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.
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, selfsiphonage 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
1-21/2o, 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 Schemes
Medical 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.
Downpipe
and gully
CAR
PARK
SINK
WASHING
MACHINE
WC
WHB
SHOWER
SINK
DISHWASHER
WC
WHB
BATH
WHB WC
BIDET
SHOWER
SHOWER
WC
WHB
BIDET
BATH
WHB WC
SHOWER
SHOWER
BUILDING No.2
BUILDING No.1
GROUND
FLOOR
CAR
PARK
Entrance
25Omm dia. Foul water
300mm dia. Surface
water
100mm dia. Vent to rise
vertically outside building to
high level and terminate
above openable window
Manhole
with No.1
bird guard.
Downpipe
and gully
SINK
WC
100mm
WASHING
MACHINE
100mm
WHB
SHOWER
Manhole
No.2
100mm
BATH
WHB WC
BIDET
SHOWER
SHOWER
Manhole
No.6
100mm
Manhole
No.5
100mm
SINK
DISHWASHER
WC
WHB
Manhole
No.4
WC
WHB
BIDET
BATH
WHB WC
SHOWER
SHOWER
100mm
CAR
PARK
100mm dia. Vent to
rise vertically outside
building to high level
and terminate above
openable window with
bird guard.
BUILDING
No.2
BUILDING
No.1
GROUND
FLOOR
Manhole
No.7
Manhole
No.3
CAR
PARK
Manhole
No.8
Entrance
NOTES:
1. All underground foul water pipes
from sanitary appliances to
manholes are to be 100mm
diameter uPVC to aid cleaning.
2. All other pipes between manholes
to be sized.
25Omm dia.
Foul water
300mm dia.
Surface water
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.
50mm to 100mm drain
connector
External wall
100mm underground
foul water to
manhole
50mm common
horizontal drain above
ground
Cleaning eye
40mm sanitary appliance outlets
above floor level.
Typical Connection of Two Sanitary Appliances
Outside
2.
Join some sanitary appliances drainage pipework above ground
inside as shown below.
This method has the advantage that footpaths outside are not
obstructed.
External wall
100mm
underground foul
water to manhole
50mm to 100mm drain
connector
40mm sanitary appliance
outlets from traps above
floor level.
50mm common
horizontal drain
above ground
Cleaning eye
Typical Connection of Two Sanitary
Appliances
Inside
Sizing Main Foul Water Sewer
Appliance
No. off
Basin
Bath
Sink (large)
WC (9.0 litre)
Bidet
Washing Machine
Shower
Dishwasher
5
2
2
5
2
1
5
1
TOTAL = 177
From graph 150 mm pipe is suitable.
Discharge
Units each
Type of
application
Public
3
12
8
10
8
8
8
8
Total
Discharge
units
15
24
16
50
16
8
40
8
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.
Distance
Pipe
Flow direction
Fall
FALL IN DRAINAGE PIPE
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
Gradient
=
1 in 80
=
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
Fall
=
=
0.0125 x 50
0.625 metres or 625mm.
The previous diagram may be completed by adding a pipe gradient.
Distance
Pipe
Flow direction
Gradient
1 in 80
Fall
FALL & GRADIENT IN
DRAINAGE PIPE
Invert Levels
The Invert Level of a pipe is the level taken from the bottom of the inside of
the pipe as shown below.
Crown of pipe
Section through
pipe
Water level
Invert level
INVERT LEVEL OF PIPE
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.
Manholes
A 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.
Cover and
frame
Brick wall
Pipe channel
for access to
system
Sloping
concrete/mortar
bed or haunching
Concrete base
BRICK BUILT MANHOLE
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.
Sloping
surface
Underground
sewer
Access cap
Excessive
gradient
SEWER ON A SLOPING
SITE
Back Drop
manhole
Sloping
surface
Back Drop
manhole
Access cap
Underground
sewer
Normal pipe
gradient
Vertical
section of
pipe
USE OF BACK DROP
MANHOLES
An easier way to construct a back drop manhole is to use an internal vertical
section of pipe as shown below.
Sloping
surface
Underground Foul
Water pipe
Cast Iron screw
down lid
Access cap
Step Irons
Drop
distance
Outlet section
of pipe
Sloping surfaces
Back Drop
manhole
BACK DROP MANHOLE WITH
INTERNAL OUTLET PIPE
Drainage Pipe Sizing
Foul Water Pipe Sizing
The following method is one way of sizing pipework.
1. Choose a minimum gradient for all pipes, say 1:80
2. Use the table below to calculate the total number of discharge units in
pipe.
No.
Appliance
No. of
units
WC
basin
bath
shower
sink
washing machine
dish washer
14
3
7
4
6
4
4
Total units
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
2
1
1
1
0
0
WC
basin
bath
shower
sink
washing machine
dish washer
14
3
7
4
6
4
4
28
6
7
4
6
0
0
51
Total
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 1:80
2. Use the table below to calculate the flow rate in each section.
SURFACE TYPE
AREA
IMPERMEABILITY
FACTOR (f)
TOTAL
(A) m2
(A x f)
Road or
pavement
Roof
0.90
Path
0.75
Garden
0.25
Access road,
parking
0.90
0.95
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
Q
=
(A x f) 50
50
x
f
((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
IMPERMEABILITY
FACTOR (f)
(A) m2
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
=
factor
area drained x rainfall intensity x impermeability
Q
=
A
Q
=
(A x f)
Q
=
103.50 x 50 =
Q
=
1.438 litres/second per house X 12 houses.
Q
=
17.25 litres/second.
x
50
x
f
50
5175 litres/hour
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.
Manhole Lid
Vent and rodding access.
Sometimes rodding access only.
Vent and
rodding
access
Ground Level
Effluent from
dwelling
Sludge
Compartment wall
Septic Tank
Water to
soakaway
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)
Capacity (m3)
=
=
(4 x 0.14) + 1.8
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.
Top Water Level
(TWL)
Water to soakaway
Ground Level
Inlet
Water Level
50 to 100mm
Sludge
Septic Tank Levels
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.
Pipe filled
with water
Manhole
1.5 metre
head of
water
End stopped
Section of pipe
under test
Drainage System Water Test
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 SYSTEMS
DOMESTIC 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 SYSTEMS
Non-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 Control
Constant 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.
Os
ADP W
s
Ss
R
Ms
Constant Dew
Point line
Mw
ow
FIG. 8.3.2 PSYCHROMETRIC CHART SHOWING SUMMER & WINTER CYCLES
A typical air-conditioning system is shown below.
Cooling Coil
Supply fan
Preheater
Damper Motor
T1
Fresh
T3
T2
M1
+
air
Anti-Frost
heater
Exhaust
air
Reheater Battery
M3
V3
-
+
V1
T4
Supply
air
+
V2
Humidifier
V4
V5
Recirculated
air
M2
Room
H1 T5
Return
air
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 Control
Air-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
Refrigerant flow
& return
Refrigerant to air
heat exchanger
Air ON
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 airconditioning.
Controls for Lighting Systems
Lighting 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
Length of
Size the pipework for the heating system
shown below.
Section
Flow & Return
(metres)
Also determine the pressure drops.
The total lengths of the section are:
1
16
2
6
3
6
Angle
Valves
Panel Radiators
1.7 kW
Pump
Oil/Gas
Boiler
Cast Iron
Gate
valve
Elbow
2.3kW
1.7 kW
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.
Angle
Valves
Panel Radiators
1.7 kW
Tee
2.3kW
1.7 kW
Tee
Section
2
Pump
Section
3
Section
1
Tee
Oil/Gas
Boiler
Cast Iron
Gate
valve
Elbow
Tee
Pipe Sizing Table
Secti
on
Ref.
1
2
3
Heat
Outp
ut
Pip
e
Hea
t
loss
.
Total
Heat
in
secti
on
kW
Col.1
+2
4
6
7
Wat Pip Leng
er
e th of
Flow Siz pipe
e
F&R
Rate
m
Total
Equivalent
length of
Fittings
m
m
dia
.
m
kW
15%
of
1.
5
kg/s
8
5.7
0.8
55
6.56
0.1
6
22
10
Total Pressu TOTAL
Pipe
re
PRESSU
Leng
RE
th
drop
DROP
Col.
per
6+7 metr Col. 8
e
x9
m
Pa
Pa/
m
kW
1
9
16
Boiler = 2.5
22.48
3 Gate Valves
= 0.4 x 3 = 1.2
Note: the equivalent length le comes from the
4 Elbows =
CIBSE guide pipe sizing table.
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
150
3372
2
4.0
0.60
4.60
0.11
22
6
2 Tees = 0.2 +
Factor for
reduction
6.88
80
550
13.25
130
1723
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
0.25
5
1.96
0.05
15
6
2 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
Total
The total pressure drop in the 3 sections is 5645 Pascals (Pa).
5645