heat emitters and controls

7/28/2019 Heat Emitters and Controls

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LOW PRESSURE HOT WATER

HEAT EMITTERS & CONTROLS

JOSEPH GATT

WEDNESDAY, 14 MARCH 2012


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BTEC HIGHER NATIONAL DIPLOMA: BUILDING SERVICES ENGINEERING 1

CONTENTS

1.0 TYPES OF HEAT EMITTERS ..................................................................................................................... 21.1 RADIATORS ...................................................................................................................................................................................... 21.1.1 CAST IRON ..................................................................................................................................................................................... 21.1.2 ALUMINIUM SECTIONAL ............................................................................................................................................................... 21.1.3 LOW SURFACE TEMPERATURE (LST) ................... ...................... ...................... ...................... ...................... ..................... ......... 21.2 WARM AIR HEATERS ..................... ..................... ...................... ...................... ...................... ...................... ...................... ............... 31.2.1 NATURAL CONVECTORS .............................................................................................................................................................. 31.2.2 FAN CONVECTORS ....................................................................................................................................................................... 31.2.3 UNIT HEATERS .............................................................................................................................................................................. 31.3 RADIANT HEATERS .......................................................................................................................................................................... 31.3.1 METAL RADIANT PANELS ............................................................................................................................................................. 31.3.2 METAL RADIANT STRIPS ................... ..................... ...................... ...................... ...................... ...................... ..................... ......... 31.3.3 METAL RADIANT CEILINGS ................... ..................... ...................... ...................... ...................... ...................... ..................... ..... 32.0 LPHW CLOSED SYSTEM .......................................................................................................................... 52.1 A TYPICAL SCHEMATIC DIAGRAM FOR LPHW CLOSED SYSTEM ...................... ...................... ..................... ...................... ......... 52.1.1 MAIN FUNCTION OF THREE CONTROL FEATURES ................... ...................... ...................... ...................... ..................... ......... 52.1.1.1 ACTUATOR ..................... ..................... ...................... ...................... ...................... ...................... ...................... ..................... ..... 52.1.1.2 FLAME FAILURE VALVE ...................... ..................... ...................... ...................... ...................... ..................... ...................... ..... 52.1.1.3 PRESSURE RELIEF VALVE ..................... ..................... ...................... ..................... ...................... ..................... ...................... .. 52.2 PLANT ROOM LAYOUT ..................................................................................................................................................................... 62.3 DATA SHEETS OF THE SELECTED BOILER ..................... ...................... ..................... ...................... ..................... ...................... .. 63.0 CALCULATION ........................................................................................................................................... 73.1 THE NUMBER AND REQUIRED OUTPUT OF HEAT EMITTERS ................ ...................... ...................... ...................... ................... 73.2 FLOOR PLAN LAYOUTS ..................... ..................... ...................... ...................... ..................... ..................... ...................... ............. 93.3 PIPE SIZING .................................................................................................................................................................................... 103.4 PUMP SIZING .................................................................................................................................................................................. 113.5 SCHEMATIC DIAGRAM OF THE SYSTEM ...................... ...................... ...................... ...................... ...................... ..................... ... 133.6 COMPARISON OF ONE-PIPE AND TWO-PIPE LPHW SYSTEM ..................... ...................... ...................... ...................... ............. 133.6.1 ONE-PIPE SYSTEM ...................................................................................................................................................................... 133.6.2 TWO-PIPE SYSTEM ..................................................................................................................................................................... 144.0 COMMISSIONING PROCEDURE OF LPHW SYSTEM ........................................................................... 154.1 SETTINGS PUMPS TO WORK ................... ..................... ...................... ...................... ...................... ...................... ..................... ... 154.1.1 PRESSURISATION SETS ............................................................................................................................................................. 154.1.2 CHECKS BEFORE PUMP STARTS ................... ...................... ...................... ...................... ...................... ..................... .............. 154.1.3 INITIAL RUN ...................... ..................... ...................... ...................... ...................... ...................... ...................... ..................... ... 154.1.3.1 CHECKS ON ACTIVATING THE MOTOR STARTER ................... ...................... ...................... ...................... ..................... ....... 154.1.3.2 PRELIMINARY CHECK OF PUMPS .................... ...................... ...................... ..................... ...................... ..................... ........... 154.1.3.3 RUNNING-IN PERIOD ...................... ..................... ...................... ...................... ...................... ..................... ...................... ....... 164.1.3.4 STANDBY PUMP ....................................................................................................................................................................... 164.1.3.5 SECONDARY PUMP ...................... ..................... ...................... ...................... ..................... ...................... ...................... .......... 164.1.4 FURTHER VENTING ..................... ..................... ...................... ...................... ...................... ...................... ...................... ............. 164.1.5 COMPLETION CERTIFICATION ..................... ..................... ...................... ...................... ...................... ..................... .................. 164.2 BALANCING AND REGULATING WATER FLOW RATES ..................... ...................... ...................... ...................... ..................... ... 164.2.1 FLOW RATE MEASUREMENT TOLERANCES .................................. ...................... ...................... ..................... ...................... ... 164.2.2 BASIS OF PROPORTIONAL BALANCING .................... ...................... ...................... ...................... ...................... ..................... ... 164.2.3 PUMP SHUT-OFF HEAD TEST ................................ ...................... ...................... ...................... ...................... ..................... ....... 174.2.4 PRELIMINARY FLOW RATE CHECK ........................... ...................... ...................... ...................... ...................... ..................... ... 174.2.5 BALANCING BY COMPENSATED METHOD .................... ...................... ...................... ..................... ..................... ...................... 174.2.5.1 BALANCING A BRANCH ................... ..................... ...................... ...................... ...................... ...................... ..................... ....... 174.2.5.2 BALANCING THE REMAINDER OF THE SYSTEM ................................ ...................... ...................... ..................... .................. 184.2.6 REGULATION BY TEMPERATURE BALANCE .................... ...................... ...................... ...................... ..................... .................. 184.2.6.1 PROCEDURE ...................... ..................... ...................... ...................... ..................... ...................... ...................... ..................... 18APPENDICES ................................................................................................................................................. 19 APPENDIX A: RECOMMENDED COMFORT CRITERIA FOR SPECIFIC APPLICATIONS ..................... ...................... ..................... ... 19APPENDIX B: PROPERTIES OF WATER AT SATURATION ...................... ...................... ...................... ...................... ..................... ... 20APPENDIX C: PIPE SIZING SPREADSHEET .................... ...................... ...................... ...................... ...................... ..................... ....... 21APPENDIX D: VELOCITY PRESSURE LOSS FACTORS FOR PIPE FITTINGS ..................................... ...................... ..................... ... 22REFERENCES ................................................................................................................................................ 23


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1.0 TYPES OF HEAT EMITTERS

According to (Stephen Frazer), there are many types of heat emitters to be used for space heatingrequirements. Most common ones in use are listed herein.

Heating system Emitter type

Radiators Steel panel; Cast iron; Aluminium; Bathroom

Warm airheaters

Natural convectors; Fan convectors; Industrial warm air heaters; Unit heaters; Skirtingheaters; Trench heating

Radiant heaters Metal radiant panels; Metal radiant strips; Metal radiant ceilings; Gas radiant heaters

Underfloorheaters

Pipe underfloor heating; Electrical underfloor heating

Electric heaters Electrical tubular heating; Storage heaters; High temperature heaters

1.1 RADIATORS

Strictly speaking, radiators do not radiate all their heat into the space, but up to 80% may be convected,typically for a double panel radiator about 30% of total heat output is radiated and 70% is emitted byconvection.

Radiators are widely used in buildings to provide central heating. These emitters are usually positioned at low level, typically under windows. Heat outputs vary up to around 3 kW.

A typical radiator height for a house is 600 mm, but other sizes are used depending on the location. Radiators can be described by various means, but the type of material used in the manufacture is the

main method of distinction.

1.1.1 CAST IRON

Cast iron sections are bolted together. They are very robust, heavy radiators. Good heat transfer but expensive.

1.1.2 ALUMINIUM SECTIONAL

Aluminium radiators are more expensive than steel panels but they are lightweight with high heat output. The material used and production techniques ensure a clean smooth finish but one of the problems with

using aluminium is corrosion of the metal in contact with hot water which may have a small quantity of airabsorbed in it.

An inhibitor can be provided as a capsule inserted in the radiator or special additives can be added tothe water during commissioning of the system to overcome this problem.

1.1.3 LOW SURFACE TEMPERATURE (LST)

Some manufacturers make LST radiators for the use in hospital, old peoples homes, nursery schools,etc.

One method of limiting the surface temperature of a radiator to about 45 C is to cover the hot metalparts with an outer casing.


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1.2 WARM AIR HEATERS

1.2.1 NATURAL CONVECTORS

Convectors are more used to heat up the space quicker than radiators. Typical locations are entrance hall, foyer, kitchen, bathroom, small hall or auditoria and small workshop. A natural convector has no fan but has more output than most radiators.

1.2.2 FAN CONVECTORS

A fan-assisted convector has even more output and is more common in use, but in some applicationsfan noise can be a nuisance.

Convectors operate by heating air, which passes over the finned pipe through which hot water passes. Convectors can be recessed into walls so that they appear to be part of the fabric of a building and may

have a decorative panel on the front to add to their appearance. The fins are mechanically fixed to the tubes and extend the heating surface so that all the heat output is

purely convective. The heating tubes are enclosed in a cabinet with louvers at the bottom to allow coolerair to enter and louvers at the top to emit warm air into the space.

The convector may also have some form of control such as damper to alter the flow of air.

1.2.3 UNIT HEATERS

Unit heaters are very like fan convectors in operation in that they blow out warm air from a heatexchanger. The heat exchanger uses steam or hot water to heat the air, which is forced over the tubesand fins by a powerful fan.

Since the steam or hot water may be higher than 100 C and the fan may cater high volume of air, theoutput range between 10-300 kW.

Unit heaters may be used in factories, workshops and warehouses.

1.3 RADIANT HEATERS

1.3.1 METAL RADIANT PANELS

Radiant panels are a good way to heat up large spaces such as factory, workshop and warehousesbecause the air is not heated directly but the surfaces below the panels are heated.

This is less expensive way to heat large volumes. High temperature water or steam is passed through a series of pipes that are connected to steel panels.

The panels heat up to about 100-150 C and radiate heat downwards into the occupied space. Panel sizes are usually several metres long by about 1 metre wide. They can be suspended from the roof or from a wall at high level, either vertically or at an angle to direct

radiant energy into the space below.

1.3.2 METAL RADIANT STRIPS

For workshops, strip heaters can be used when supplied with high or medium temperature hot water orsteam.

The more radiant strips the more is the heat output. Strips may be wide up to 1 metre but usually are continuous throughout the factory or workshop. They are also suitable for mounting between stacking rows in a warehouse. Some high outputs as 3 kW per metre at 100 C can be achieved with radiant strips.

1.3.3 METAL RADIANT CEILINGS

This type of radiant heating system incorporates the whole ceiling. One type uses a ceiling made from metal plates above which are clipped pipes containing hot water. The pipes heat the metal ceiling below which in turn heats the room below by radiation.

Typical heat output is about 160 W/m

2

of floor area. To make the system more efficient, some insulation is added on top of the pipes. Radiant ceilings can be used in a wide variety of buildings such as schools and office blocks.


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The system is very silent in operation but requires temperature control to ensure a comfortableenvironment.


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2.0 LPHW CLOSED SYSTEM

2.1 A TYPICAL SCHEMATIC DIAGRAM FOR LPHW CLOSED SYSTEM

2.1.1 MAIN FUNCTION OF THREE CONTROL FEATURES

2.1.1.1 ACTUATOR

An actuator is a device that may receive electrical energy and converts it to some kind of motion. Forinstance, the temperature sensor sends a signal to the actuator that it shall open or close the three-portmixing valve. Hence the hot water return is then recirculated depending on the requirements. This allows theboiler to work less if the return water temperature will be sufficient to recirculate in the flow instead ofentering the boiler and reheated again.

2.1.1.2 FLAME FAILURE VALVE

(Hall & Greeno, 2009) explain that a thermoelectric valve has an ancillary thermocouple sensing elementwhich consists of two dissimilar metals joined together at each end to form an electrical circuit. When thethermocouple is heated by the gas pilot flame, a small electric current is generated. This energises an

electromagnet in the gas valve which is retained permanently in the open position, allowing the gas to passthe relay valve. If the pilot flame is extinguished, the thermocouple cools and the electric current is no longerproduced to energise the solenoid. In the absence of a magnetic force, a spring closes the gas valve.

2.1.1.3 PRESSURE RELIEF VALVE

According to (Smart Products, 2011), a pressure relief valve performs the important role of limiting theexcessive pressure in a system. It can also be designed to open at a fixed pressure. Relief valves protectyour system from over or under pressurization. When the pressure of a system is exceeded, the relief valveopens and the unwanted pressure is thrown away, normally to drain. Once the pressure reaches safe levels,the relief valve will close allowing normal operation of the system to continue. Pressure relief valves are alsoknown as vacuum relief valves, blow-off valves, pop-off valves, pressure regulating valves, safety valves,

and purge valves.


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2.2 PLANT ROOM LAYOUT

2.3 DATA SHEETS OF THE SELECTED BOILER

Source: Viessmann


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3.0 CALCULATION

3.1 THE NUMBER AND REQUIRED OUTPUT OF HEAT EMITTERS

Step 1: From the design data, the design heat loss from the first and ground floor is 60 and 45 kWrespectively.

Step 2: Calculate the mean water temperature between the flow and return. According to (BSRIA, 2007), the

flow and return temperatures for LPHW systems are typically 82 C and 71 C respectively.

Step 3: Calculate the system temperature difference between the mean water temperature and roomtemperature. According to (CIBSE, 2008), the recommended comfort temperature for open-plan offices inwinter is 21-23 C (Appendix A). It is assumed that the design room temperature is to be 22 C.

Step 4: Find the output correction factor by interpolating between the figures on the table below. As thesystem temperature difference in this case is equal to 90% above 50 C, then the required factor will be thatof the 50 C factor plus 90% of the difference between the 50 C and 55 C factors.

Source: BSRIA

Correction factor for 55 C is 0.898 Correction factor for 50 C is 0.789 Correction factor required for 54.5 C Interpolate between 0.898 and 0.798

Therefore:

90 % of 0.100 needs to be added to the correction factor for 50 C, hence the correction factor for 54.5 C is:

Step 5: To obtain the corrected required output figure for first and ground levels:


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Step 6: The type of heat emitter from the radiator selection chart below is to be type 3, 56 sections, with an

output power of 3.918 kW. The size of the radiator is 2240 mm x 600 mm.

Source: BSRIA

Step 7: Calculate the number of radiators required at first and ground floor levels.

Step 8: Recalculate the boiler size. (CIBSE, 2006), suggests to apply a pre-heat factor of 1.2 to the radiatorheating load to allow for admittance of the structure. This gives a required heating plant capacity of:


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Source: Viessmann

3.2 FLOOR PLAN LAYOUTS


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3.3 PIPE SIZING

Calculate the mass flow rate at each panel. (BSRIA, 2007) suggests that 6% needs to be added to theradiator power output to compensate to the heat loss across the pipe. According to (CIBSE, 2008), thespecific heat capacity of water at 77 C (mean water temperature) is 4.1955 kJ/kgK (Appendix B). Accordingto (BSRIA, 2007), an appropriate temperature drop across the system for LPHW is 10-20 K.

Radiator power output is 3.918 x 1.06 = 4.153 kW Temperature drop across system is 82-71=11 C

Mass flow rate alone is not enough to determine the pipe size. One need to limiting maximum pipe pressureloss per metre run in Pa/m. (CIBSE, 2007) suggests a suitable starting point of 250 Pa/m.

The tables below illustrate the pipe sizing across the system. Pipe sizes can be based on the mass flow rateat a fixed pressure drop of 260 Pa/m. This approach can only be used to determine the pipe size. The exactpressure drop and velocity of a particular pipe is explained in section 3.4. According to (CIBSE, 2008), heavygrade steel containing water at 75 C is as follows:

For instance; a mass flow rate of 0.090 kg/s at 260 Pa/m is 20 mm diameter with a velocity of 0.62 m/s(Appendix C).


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(CIBSE, 2008) recommends water velocities as follows:

Steel pipe up to 50 mm diameter: 0.75-1.5 m/s Steel pipe over 50 mm diameter: 1.25-3 m/s

As seen in the tables above, the water velocities at each section are acceptable. According to (BSRIA,2007), there are limits to acceptable fluid velocity. High velocities lead to noise and erosion while low velocitycan give problems with air-locking. While pipe capital costs are obviously related to diameter, the runningcosts for pumped systems are proportional to pressure loss. Therefore, pipe sizing involves valueengineering.

3.4 PUMP SIZING

Having obtained the mass flow rate across the system is not enough to determine the size of the pump. Thedynamic losses need to be calculated. The dynamic losses occur due to the fluid friction in the pipes.

In order to determine the dynamic losses, one needs to find out the index run of the system. (BSRIA, 2007)describes that the index run within a system is that circuit that has the highest resistance to the flow of waterand supplies the index heat emitters. This is the worst case possible when considering pressure losseswithin a system. It is usually, but not always, the longest circuit in the system. Sometimes, a shorter run witha greater number of fittings or items of equipment can be the index run.

The index pressure drop is required in order to successfully size the pump for the system. If the pump canwork to the pressure demands of the index run then all other circuits will work.

Once the index run is established, one needs to sub-divide the circuit into several sections in order todetermine the pressure loss across each section. The tables below illustrate a typical exercise for the givenheating system.

From (CIBSE, 2008) pipe sizing spreadsheets, the exact velocity and pressure drop of a particular pipe canbe determined. For instance; section F in the table below requires a 50 mm pipe with a mass flow rate of1.62 kg/s, as in the table above. Unlike the table above, the pressure drop and velocity of the same pipe are140 Pa/m and 0.83 m/s, respectively (Appendix D).

The pipe velocity is needed to calculate the velocity pressure in the pipe. This can be achieved by thefollowing equation:


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Where the density of water shall be that of the mean water temperature, in this case 973.6 kg/m3

at 77 C(Appendix A), and K factors for various fittings are found on (CIBSE, 2007) (Appendix D).


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3.5 SCHEMATIC DIAGRAM OF THE SYSTEM

3.6 COMPARISON OF ONE-PIPE AND TWO-PIPE LPHW SYSTEM

3.6.1 ONE-PIPE SYSTEM

According to (Oughton & Hodkinson, 2008), these types of circuit are theoretically the simplest possible andhere hot water from the boiler is fed to each radiator in turn with the cooler water from each radiator beingfed back to the same pipe. Natural convection causes the hot water to rise into the radiator displacing coolerwater back into the pipe. As a result the temperature of the water is gradually reduced as it enters eachsuccessive radiator. This makes the control of the distribution of heat difficult. In order to compensate for thisdecay in temperature, the size of successive radiators is usually increased and care is taken to select asuitable temperature drop across both of them and the piping circuit. For example, consider a system having10 radiators, each of which is required to provide the same output. The temperature drop across the circuitmight be chosen to be 10 K and that across each individual radiator, to be 15 K. The water temperature inthe single pipe would fall by an average of 1 K after each radiator and thus the mean temperature of the firstand the last would be 72.5 C and 63.5 C, respectively.

One could argue that this type of system is the easiest and cheapest to install, but control of individualradiators is virtually ineffective as the first radiator gets hotter than the second, etc. The emitters must besized to offer negligible resistance and therefore have large waterways making them bulky and costly. Thesesystems were popular 3040 years ago and many of them are still in operation. They have been superseded


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by two-pipe circuits as these are generally simple to design and install, requiring smaller pipework and allcommercially available heat emitters are now designed for integration into two-pipe systems.

The calculations for one-pipe systems are tricky and tedious and it could be argued that one of thesereasons they went out of favour is because of the failure in the past by engineers and plumbers to usesimple arithmetic calculations for heating systems designs. Examples of these systems are shown in thefigure below.

3.6.2 TWO-PIPE SYSTEM

This arrangement has, in most respects, come to supersede the single-pipe configuration and has aninherent logic as far as parallel circuits are concerned. Flow and return mains originate from the boiler plantand each main or sub-circuit consists of branches from them. Each branch conveys an appropriate quantityof water to and from whatever heat emitting terminals are connected to it. In an ideal world, all circulatingpipework would be insulated perfectly, and the water inlet temperature at each terminal would be exactly thesame as that leaving the boiler. Similarly, the temperature of water returned to the boiler would be exactlythe same as that leaving each terminal. In practice, heat losses from the pipework will reduce thetemperature at the various inlets to a level which will vary, roughly, according to how distant each is from theboiler. Likewise, the water returned to the boiler will be at a lower temperature than that at which it leaves theoutlets of the various terminals.

In terms of hydraulic balance, systems may suffer from a number of difficulties. It will be obvious that themost distant heat emitter is disadvantaged, by comparison with that nearest to the boiler, in this respect. Theproblem will be reduced if either the heat emitters or the final sub-circuit pipework to them offer a highresistance to water flow, relative to that of the remainder of the system.

In terms of physical arrangements, two-pipe circuits are extremely flexible and may be set out to suit thebuilding facilities available. They may be applied to all types of hot water systems whatever the watertemperature and to supply all types of heat emitting equipment.

Source: Faber & Kells


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4.0 COMMISSIONING PROCEDURE OF LPHW SYSTEM

4.1 SETTINGS PUMPS TO WORK

4.1.1 PRESSURISATION SETS

If the system is designed to be pressurised by a pressurisation set then this will need to be commissionedand the system brought up to normal operating pressure and vented before circulating pumps arecommissioned. The manufacturers detailed guidance for commissioning the pressurisation set and settingpressure limit switches shall be followed.

The water treatment specialist should fill the pressurisation set reservoir with water containing therecommended concentration of water treatment chemicals.

4.1.2 CHECKS BEFORE PUMP STARTS

With the system filled check that:

All normally open valves are fully open. All bypass and normally closed valves are fully closed. All thermostatically controlled valves are fully open and will not be affected either by ambient air or water

temperature during balancing procedures. A method of operating automatic control valves is available and that these are motored to normal

operating mode. The pump suction and return valves are fully open on the selected pump. The delivery valve is closed on any standby pump unless non-return valves are fitted; the suction valve

should be left open (it is not good practice to leave any vessel full of water and completely isolated, as atemperature rise could cause an excessive build-up of pressure).

The pump casing is vented of air. The selected pump discharge or flow valve is partially closed to limit the initial start current.

4.1.3 INITIAL RUN

4.1.3.1 CHECKS ON ACTIVATING THE MOTOR STARTER

Where appropriate check that:

The direction and speed of rotation of the motor shaft are correct. The motor, pump and drive are free from vibration and undue noise. For star-delta starters the starter sequence timing has been adjusted as necessary in the light of motor

starting current. The motor running current is balanced between phases and does not exceed the motor nameplate

stated rating. There is no sparking at the commutator or slip rings where fitted. There is no overheating of the motor. There is no seepage of lubricant from the housing. There is no overheating of the bearings. The water flow to water-cooled bearings is sufficient. On multi-speed motors, motor running currents are correct. The ventilation systems of air-cooled motors are operating correctly.

4.1.3.2 PRELIMINARY CHECK OF PUMPS

Avoid starting the motor repeatedly. This prevents overstressing the fuses, switchgear and motor. Open the discharge valve gradually until the motor current reaches either the design value or the motor

full-load current, whichever is the lower.

Compare the pump pressure developed against the system design circulating pressure using the pumpdifferential pressure gauge. Where excessive circulating pressure is developed the cause should beinvestigated.


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4.1.3.3 RUNNING-IN PERIOD

The pumps must be run in in accordance with the manufacturers instructions. Do not leave pumpsunattended or allow them to run outside working hours.

Check that:

All bearing and motor temperatures remain within manufacturers specifications. Pump gland nuts are adjusted to give correct drip rates (this does not apply to mechanical seals). After four hours running all strainers are inspected and cleaned; if they are excessively dirty then

continue frequent inspection and cleaning.

4.1.3.4 STANDBY PUMP

The standby pump should be commissioned as detailed in sections 4.1.1 and 4.1.2.

4.1.3.5 SECONDARY PUMP

After checking the primary strainers, isolate the primary system. The secondary system valves can be

opened and the secondary pumps started. Re-open the primary circuit valves.

4.1.4 FURTHER VENTING

After initial running of the pumps (primary/secondary as applicable) the pumps should be stopped and thesystem re-vented. It may be necessary to repeat this a number of times. After applying heat to a heatingsystem, further venting may also be required.

4.1.5 COMPLETION CERTIFICATION

On the satisfactory compliance with the provisions of pre-commission and section 4.1, the installationcontractor should complete a certificate to that effect. The designer or his/her representative should

countersign the certificate. Certification should include the documents pertaining to chemical cleaning andflushing. Copies of these certificates should be passed to the commissioning specialist before the system isbalanced.

4.2 BALANCING AND REGULATING WATER FLOW RATES

4.2.1 FLOW RATE MEASUREMENT TOLERANCES

It is the responsibility of the system designer to state acceptable tolerances for the regulation of flow rates forvarious sections of a particular system, taking reasonable account of the impact of tolerances on systemperformance and the practicality of measuring and adjusting very low flow rates. In deciding on theappropriate tolerances, the designer should bear in mind that the cost of regulation can increase significantly

where close tolerances are specified.

Where the commissioning specialist considers that the tolerances required by the system designer areimpractical (e.g. due to inadequate provisions in the system design or very low flow rates) he/she shouldformally advise the designer of this, clearly stating and explaining the reasons.

4.2.2 BASIS OF PROPORTIONAL BALANCING

Prior to balancing and regulating the flow rates in a system the commissioning specialist should produce awritten method statement detailing the intended balancing procedures. This method statement should beformally agreed with the principal contractor and system designer and form part of the overall commissioningplan.

Proportional balancing is used for constant volume systems and for variable volume systems at maximumflow. However, heat balance methods are sometimes necessary in final branch or terminal distribution. Thisinvolves temperature measurements.


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Safety warning: balancing a system requires the coupling of manometer probes into pressure test points.The water temperature must be below 55 C when this is done. An MTHW or HTHW system must not, underany circumstances, be commissioned at operating temperature as serious injury may result.

In MTHW and HTHW systems, self-sealing pressure or temperature measuring points on valves or orificesmust be preceded by a manually operated isolating device.

4.2.3 PUMP SHUT-OFF HEAD TEST

To verify the operational performance of a pump it is necessary to check the measured performance againsttest data provided by the manufacturer.

The performance test should be carried out as follows.

Connect a suitable differential pressure gauge across the suction and discharge pressure test points ofthe pump.

With the pump running, slowly close the discharge valve. Do not run in this condition for longer than 15minutes (check manufacturers guidance) or the pump will overheat and may be damaged.

Determine the shut-off pressure differential,check against the manufacturers data for zero flow then

slowly re-open the discharge valve. Where the test result coincides with the manufacturers test data proceed to the next step. Otherwise

draw a curve parallel to that shown on the published data, starting at the shut-off head pressure. Record the total pressure with the differential pressure gauge at full flow rate and read the actual flow

from the manufacturers data, or from the corrected graph curve as appropriate. If the performance is inadequate refer back to the installer and designer.

4.2.4 PRELIMINARY FLOW RATE CHECK

With all valves fully open, measure and record the total actual flow rate and compare this with the totalsystem design flow rate. Where necessary, close the main regulating valve to provide a flow ofapproximately 110 % design flow rate.

Where the initial measured flow rate is less than 100 % of the design flow rate with the system fully open,then a value of less than 100 % will result at the conclusion of balancing.

Regulation of a system at less than 100 % of design flow rate should not be attempted without formallyadvising the installing contractor and system designer.

The reason for the low flow rate condition should be investigated and, where possible, corrective measuresimplemented prior to balancing the system.

4.2.5 BALANCING BY COMPENSATED METHOD

4.2.5.1 BALANCING A BRANCH

A manometer is connected to the reference valve. The design flow in the reference valve is regulated by thepartner valve. A second manometer should be used to set and adjust the terminal balancing valves startingwith the terminal next to the reference terminal and proceeding upstream.

Adjust each terminal commissioning valve to give the design flow in that terminal but maintain observation ofthe reference valve manometer. If this changes more than 10 % during the balancing process then bring thepressure differential back to the original by operating the partner valve only.

Continue with all other commissioning valves in the branch, working upstream, observing the referencepressure drop and adjusting it with the partner valve. In this way all terminals on that branch will be inbalance at design flow.


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If, when adjusting a terminal valve, the design flow cannot be obtained when fully open, this terminal istermed the index unit. In this case, measure the actual maximum flow obtained in this terminal anddetermine the %DFR. Re-adjust the reference valve to give the same %DFR, then lock in position.

The design flow in the reference valve is obtained by reopening the partner valve and the balancingprocedure repeated using the new setting on the reference valve.

4.2.5.2 BALANCING THE REMAINDER OF THE SYSTEM

By using the same methodology as in section 4.2.5.1, each branch, riser and header can be progressivelybalanced.

4.2.6 REGULATION BY TEMPERATURE BALANCE

4.2.6.1 PROCEDURE

Operate the system at a moderate flow temperature. This will be governed to some extent by theambient temperature but a minimum of 55 C is suggested.

If the system is controlled by an outdoor compensator this should be adjusted to ensure that a constant

flow temperature is maintained during the balancing. Throttle each terminal unit regulating valve until all returns register an equal temperature measured by a

contact thermometer consistently applied.


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APPENDICES

APPENDIX A: RECOMMENDED COMFORT CRITERIA FOR SPECIFIC APPLICATIONS

Source: CIBSE


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APPENDIX B: PROPERTIES OF WATER AT SATURATION

Source: CIBSE


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APPENDIX C: PIPE SIZING SPREADSHEET

Source: CIBSE


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APPENDIX D: VELOCITY PRESSURE LOSS FACTORS FOR PIPE FITTINGS

Source: CIBSE


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REFERENCES

1. BSRIA. (2007). BSRIA Guide 30/2007: A Guide to HVAC Building Services Calculations, Second Edition. Image Data Group.2. CIBSE. (2003). Commissioning Code W: water distribution systems.3. CIBSE. (2008). Consice Handbook. Norfolk: Page Bros (Norwich) Ltd.4. CIBSE. (2006). Guide A: Environmental Design.5. CIBSE. (2007). Guide C: Reference Data. Norfolk: Page Bros. (Norwich) Ltd.6. David Bownass. (2001). Building Services Design Methodology: a practical guide. London: Spon Press.

7. Hall & Greeno. (2009). Building Services Handbook.8. Oughton & Hodkinson. (2008). Faber & Kell's Heating & Air Conditioning of Buildings, 10th Edition. Elsevier Ltd.9. Smart Products. (2011, March 10). Retrieved from http://www.smartproducts.com/index.php10. Stephen Frazer. (n.d.). Retrieved March 10, 2012, from Students notes for building services engineering: http://www.bsenotes.com/

heat emitters and controls

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