Hot and Cold Water Design

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Contents

• Water Consumption and Demands• Pipe Sizing and Water Storage• Pumping Systems and Performance• Other Design Considerations• Exercise

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Water Consumption & Demands

• Common water supply systems• Cold water system

• Potable/fresh water• Flushing (salt water in HK)• Cleansing water• Swimming pool filtration• Irrigation (e.g. for landscape)• Fountain circulation• Make-up water of cooling tower, etc.

• Hot water system (e.g. in hotels & hospitals)

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Water Consumption & Demands

• Major tasks of water systems design:• Assessment & estimation of demands• Supply scheme & schematic• Water storage requirements• Piping layout• Pipe sizing• Pump capacity

• Designers require a wide range of info.• Water usage, patterns of use, flow loads

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Water Consumption & Demands

• Practical info on water usage• Very few experimental studies on this!

• Theoretical framework• Fit the data & provide a design method• Based on statistics & probability, e.g. binomial

distribution

mnmm PP

mnmnP −−×

−

= )1()!(!

!

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Water Consumption & Demands

mnmm PP

mnmnP −−×

−

= )1()!(!

!

Pm = Probability of occurrence

And n is the total number of fittings having the same probability and m is number of fitting in use at any one time.

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Water Consumption & Demands

• Designers have to make do with very limited practical info. & make up by engineering judgement• Many design guides are from western countries

• Need to understand the context/circumstance• Is it similar to average/typical?• Any foreseeable special requirements?

Estimatedtotal wateruse in England& Wales

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Water Consumption & Demands

• Simultaneous demands• Most fittings are used only at irregular intervals• No need to size pipework on continuous max.

• Evaluate the ‘probable maximum’ using a ‘loading unit’ rating

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Water Consumption & Demands

• Apply probability theory• Assume random usage with fittings (true?)• Determine max. frequencies of use• Estimate average water usage rates & time

• The theory is valid with large nos. of fittings• Often expect to be exceeded at 1% time only

• Reliability and risk management

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Water Consumption & Demands

• Loading Units• Definition – A factor or number given to an appliance

relating the flow rate at its terminal fitting to • the length of time in use• the frequency of use for a particular type and • use of building.

• WC flushing cistern = ? L.U.• The answer may be 1 or 2 or 5 L.U.• Shower head = ? L.U.• The answer may be 2 or 3 or 6 L.U.

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Water Consumption & Demands

• Why there are 3 values on loading units??• The answer is the assumption of use, that is, low, medium or

high rate of use.• Low use assumes 20 min interval between use• Med use assumes 10 min interval between use• High use assumes 5 min interval between use

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Water Consumption & Demands

• Design flow considerations• A small increase in demand over design level will

cause a slight reduction in pressure/flow (unlikely to be noticed by users)

• Exceptional cases, such as:• Cleaners’ sinks• Urinal flushing cisterns (constant small flow)• Team changing rooms at sport clubs• Special events

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Pipe Sizing & Water Storage

• Pipe sizing procedure• Assume a pipe diameter• Determine the flow rate• Determine the effective pipe length• Calculate the permissible loss of head• Determine the pipe diameter

• Usually, flow velocities shall be < 3 m/s• The higher the temperature of the water, the

lower would be the limit of flow velocity

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Pipe Sizing & Water Storage

•Avoid oversizing & undersizingOversizing• High cost – extra but unnecessary

• Delay in getting at outlets

• Increase heat loss from distributing piping

• Undersizing• Slow or even no water during peak demand

• Variation in temperature & pressure at outlet (obvious in mixer for shower)

• High noise level

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1. Pipe reference -Mark pipe reference on the schematic drawing and enter the pipe reference on the table

2. Loading units- Determine the loading units according to the

outlet fittings

3. Flow rate (l/s) - Convert loading units to flow rate

4. Pipe size (mm diameter) - Make assumption to the pipe size

5. Loss of head (m/m run) - Find friction resistance per metre

Pipe Sizing & Water Storage

Pipe Sizing – parameters to be listed out (page 1 of 3)

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6. Flow velocity (m/s)- Find the flow velocity

7. Measured pipe run (m)- Measure the straight pipe length

8. Equivalent pipe length (m) - Find the frictional resistance in fittings (as

equivalent length)

9. Effective pipe length (m)- Total length (= straight + equivalent length)

10. Head consumed - total length x metre/ metre loss

Pipe Sizing & Water Storage

Pipe Sizing – parameters to be listed out (page 2 of 3)

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11. Progressive head (m) - Add head consumed to progressive head

12. Available head (m) - Check available head available at point of delivery

13. Final pipe size (mm) - Compare progress head with available head to

confirm any correction of pipe diameter.

Pipe Sizing & Water Storage

Pipe Sizing – parameters to be listed out (page 3 of 3)

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Pipe Sizing & Water Storage

Pipe Fitting Loss :

1) K value

2) Equivalent length

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Pipe Sizing & Water Storage

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Pipe Sizing & Water Storage

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Pipe Sizing & Water Storage

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•Useful formulae for pipes:•Relative discharging power

•Thomas Box formula

5

=

dDN

5

5

1025 ×××

=L

Hdq

Pipe Sizing & Water Storage

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Pipe Sizing & Water Storage

• Water storage allowance depends on:• Type and use of buildings• Number of occupants• Type and number of fittings• Frequency and pattern of use• Likelihood and frequency of breakdown of supply

• Often design for 24-hour reserve capacity

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Pipe Sizing & Water Storage

• Recovery rate and hot water storage• Recovery period = time to heat up the stored water• Too high a storage volume: unnecessary costs• Inadequate storage: loads not met• Need to consider these factors:

• Pattern of use• Rate of heat input to the stored water• Recovery period for the hot water storage vessel

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• Need to consider these factors:•Any stratification of the stored water

Pipe Sizing & Water Storage

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Water heater – good mixing of water

Pipe Sizing & Water Storage

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Pipe Sizing & Water Storage

• Formula to calculate recovery period• M = V T / (14.3 P)

• M = time to heat the water (min.)• V = volume of water heated (litres)• T = temperature rise (oC)• P = rate of heat input to the water (kW)

• It can be applied to any pattern of use• It ignores heat losses from storage vessel

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Pump Systems & Performance

• Centrifugal pumps are commonly used• Two types of systems:

• Closed systems• Recirculation

• Open systems• Open to atmosphere

Pump pressure effects in an open system

= h x density x 9.81= atmospheric= 101,325 Pa

Thus, h = 10.33 m

Neutral point

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Pump Systems & Performance

• Pump considerations• Practical suction lift is 5 m maximum• Also known as net positive suction head (NPSH)

• Pump location is important for both closed and open systems• Open system: not excessive to avoid cavitation

• Power• Close system: Influence water level of open vent pipe &

the magnitude of antiflash margin (temp. difference between water & its saturation temp.)

• ‘Self-priming’ to evacuate air from suction line

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Pump Systems & Performance

• Pump characteristics• Characteristics curves (e.g. from catalogue):

• Total head• efficiency

• No-flow conditions (flow = zero)• Close valve pressure• Need to prevent over-heat

Pump characteristics curves (centrifugal)

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Pump Systems & Performance

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Pump Systems & Performance

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Pump Systems & Performance

Q = Flow, p = Pressure, P = Power

Centrifugal pumps Positive Displacement pumps(very few using in plumbing system)

- Capacity varies with head- Capacity proportional to pump speed- Head proportional to the square of pump speed- Non self-priming- Suitable for low-viscosity liquid

- Capacity substantially independent of head- Capacity proportional to speed- Self-priming- Suitable for various liquids (reduced speeds usually necessary for high viscosity

Main characteristics of centrifugal & positive displacement pumps

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Pump Systems & Performance

• Pump characteristics (cont’d)• Pumps connected in series:

• Double the pressure

• Pumps connected in parallel:• Double the flow

• Dissimilar pumps may not be in parallel

Pumps in series

Pumps in Parallel

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Pump Systems & Performance

• Pump characteristics (cont’d)• Pumps with steep characteristics

• Change in pressure -> small change in flow rate• Useful where pipes tend to scale up

• Pumps with flat characteristics• Change in flow -> small change in pressure• Useful where extensive hydraulic balancing is needed

• For closed systems, pressure at zero flow shall be greater than static height of the system to ensure initiation of flow

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Pump Systems & Performance

• Pump characteristics (cont’d)• Pumps with constant speed cannot respond to

changes in load• Require a bypass to ensure constant flow

• Variable speed pumps• Provides for savings in pumping costs during partial

load

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Pump Systems & Performance

Types of centrifugal pumps:-

1) Vertical and horizontal

2) Single and multiple stages

Pump materials to suit the environment, e.g. stainless steel pumps for salt water system

Vertical, multiple stages pump

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Horizontal centrifugal pump

Pump impeller

Pump Systems & Performance

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Other Design Considerations

• Noise & vibration• Pipe noise

• Pipe should not be fixed rigidly to lightweight panels

• Flow noise• Keep velocities under control

• Pump noise• Use rubber hose isolators, resilient inserts, acoustic

filters

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Other Design Considerations

• Water hammer• Such as when a valve is closed rapidly• Pulsating type of noise by shock waves• Preventive measures:

• Prevent sudden closing of the valve• Absorb pressure peaks (e.g. by pneumatic vessels)• Increase the attenuation of pressure waves when

transmitted through the pipework• Design the pipework to avoid long straight pipe runs• Restrict water velocities (e.g. to a maximum of 3 m/s)

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Other Design Considerations

• Back siphonage• Occur when water mains pressure reduce greatly• Contamination of water may happen• Contamination might also occur due to gravity &

backpressure backflow• Anti-siphonage device and design precautions

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Other Design Considerations

• Water economy & energy conservation• Economy of water

• A key factor in the design (to conserve water)• Measures:

• Detect water leakage• New & innovative flushing arrangements (e.g. low-water and

pressure flushing cisterns)• Water plugs, self-closing taps, spray taps, aerators, etc.

• Energy conservation• Insulation of hot water pipe, fittings & vessels• Use of fresh water for cooling tower make-up

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Further Reading

• Garrett, R. H., 2000. Hot and Cold Water Supply, 2nd ed., Blackwell Science, Malden, MA

• Hall, F., 1994. Building Services and Equipment, 3rd ed., Vol. 2

• Moss, K. J., 1996. Heating and Water Services Design in Buildings

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Web Links

• Plumbing design example for a house (layout and schematic diagram)• http://arch.hku.hk/teaching/project/f-pl.htm

• Apartment Building : Plumbing Schematic• http://arch.hku.hk/teaching/envctrl/KPC_pro/pro1

54.html

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Design of Cold & Hot Water System– Exercise One

Assume that a cold water storage cistern of 9000 litres capacity installed at the roof of the building is to be refilled very two hours.

The vertical height and the horizontal run from the water main to the ball valve is 26 m and 9 m respectively. The friction loss of pipe and fitting may be taken as 10.5 m.

It is given that the pressure on the main supplying cistern is 300 kPa at the time of refill.

Calculate the diameter of this supply main by making use of Thomas Box formulae.

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Design of Cold & Hot Water System– Exercise Two

Water supply from a cistern is shown on the right diagram.

Fill in the blanks of the below table.

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Design of Cold & Hot Water System– Exercise Three

A storage type water heater has the following characteristics: -

Heat input to water is 3 kW.

Hot water sufficient for 2 baths (each bath draws off 60 litres at 60oC and 40 litres supply main water to give a mixed water temperature of 40oC) & one kitchen sink (10 litres) followed by one further bath (also draw-off 60 litres at 60oC and 40 litres supply main water which is at 10oC to give a mixed water temperature of 40oC) after 30 minutes.

Determine the minimum capacities of a storage type water heater of a residential flat assuming that there is good mixing of the stored water.