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MODULE 71

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A water system in the air conditioning industry may be defined as the heat transfer medium whereby the fluid from one source, e.g. a boiler or chiller is circulated using a pump through pipe work to a series of terminal devices or units and then recirculated back to its source.

Balancing is measuring and adjusting the system to get the design water flow. Unless specifications require otherwise, it’s normally considered there’s an adequate balance when the water quantities measured are within ±10 per cent of design quantities.

DIVERSITYIn any variable flow system the balancing procedure should be discussed with the designer and clearly understood.

In some installations, particularly large high-rise systems with the four facades of the building exposed, the designer may have purposely built in diversity i.e. the pumps will not have been selected to be able to provide design flow to all sections of the building simultaneously. Typically this could be of the order of 80 per cent of the sum of the terminal flows, i.e. a diversity factor of 0.8.

In commissioning this type of system it may be necessary to shut off several risers to reduce the flow to match the pump capacity whilst the remaining risers and their terminal units are balanced. In turn, several of the balanced risers may then need to be turned off to enable the remainder of the system to be balanced.

When the designer has taken account of diversity and selected pumps and thermal plant for the diversified load only, special balancing techniques are necessary to ensure that terminals can receive design flow rate when needed.

These techniques will include the restriction (stage by stage) of flow to risers and branches so that all terminals can be balanced for their individual design flows.

REDUNDANCYDesigners quite often install chiller capacity in excess of the design full load requirement to provide a measure of standby or redundancy. It is therefore necessary for the commissioning person to ascertain from the designer the number of pumps and chillers/boilers operating and hence the system design flow rate. It is possible that this flow rate may not equate to the sum of the individual terminal unit’s flow rates.

PROPORTIONAL METHODThis method is based on the principle that variations in the differential pressures across a circuit will change the flow to the terminals in the circuit.

Simple system (terminal balancing only)Figure 1 is a diagrammatic representation of a simple system comprising four air handling units in a plant room and two fan coil units on a branch line. The balancing procedure for such a system is as follows:

a) With all control valves locked open and the balancing valves fully open, adjust the main throttling valve to achieve a pump flow of approximately 100 to 110 per cent of the system design flow rate.

b) Measure the flow rate at all terminals.

c) Calculate the percentage flow rate ratio for each terminal i.e. the terminal having the lowest ratio being known as the “index terminal” and will normally be located towards the most distant terminal from the pump.

d) If the index is not at the end of the system, adjust the percentage flow rate of the valve at the end of the system to 2–3 per cent above the index and then use this end terminal as the index terminal for the remainder of the balancing procedure.

e) Now proceed to the next terminal towards the pump, adjusting its flow to approximately 2–3 per cent above the new (end unit) index percentage flow rate.

f) Now proceed to subsequent terminals.

g) As terminals are set, recheck the index terminal percentage flow rate. If it has increased, recalculate the target flow percentage for the next terminal.

h) Throughout the balancing process do not change the setting of the index terminal or any of the terminals that have been previously set.

i) It is beneficial to have a second flow instrument connected to the index terminal to monitor any flow variation.

j) Recheck and adjust the main throttling valve to achieve the system design flow rate. Revisit all terminals and check/record their final flow rate.

When a branch serving several terminals is encountered, that branch is balanced in its entirety commencing with the most remote terminal and using the index established in points e, f, g and h.

www.hvacrnation.com.au | HVAC&R Nation | May 2014

WATER SYSTEMBALANCINGThis month’s Skills Workshop looks at balancing water systems within air conditioning installations.

PART ONE

MAIN THROTTLING VALVE

REFERENCE VALVE

TERMINAL VALVE(TYPICAL)

PUMPFig. 1: Simple system – valve locations.

HVAC&R SKILLS WORKSHOP ▲ Module 71

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More complex systemsIn more complex systems, there will be groups of terminals fed from branches or risers etc. Risers can often have multiple branches.

In principle, the branches with the highest flow are balanced first to divert flow to less favoured branches. However, the order in which branches and risers are balanced may need to be rearranged because of the layout and flow on individual projects.

Branch balancingThis part of the balancing procedure is conducted after all the terminals have been balanced.

Fig. 2 is a diagrammatic representation of a system with several terminals, branches and risers.

The balancing procedure is as follows.

a) Ensure all control valves remain locked open and the branch balancing valves are fully opened.

b) Check and adjust the main throttling valve to ensure a pump flow of approximately 100–110 per cent of the system design flow rate.

c) Measure the flow rate in all branches.

d) Calculate the percentage flow rate ratio for each branch i.e.,

the branch having the lowest ratio being known as the “index branch”.

e) Adjust the flow rate of the branch with the highest ratio to its correct value. When this has been achieved, move to the next highest ratio branch and adjust its flow rate and so on until all branches are balanced.

f) Revisit all branches and check/record the flow rates. This may indicate that further trimming is necessary in which case repeat the procedure detailed in the last point.

g) Revisit all branches and check/record their final flow rate.

Riser balancingThis part of the balancing procedure is conducted after all the terminals and branches have been balanced.

The balancing procedure is as follows:

a) Ensure all control valves remain locked open and the riser valves are fully opened.

Fig. 2: Valve locations – branch and riser balancing.

BRANCH BALANCING VALVE

TERMINAL VALVE (TYPICAL)

PUMP

MAIN THROTTLING VALVE

RISER BALANCING VALVE BRANCH BALANCING VALVE

BRANCH BALANCING VALVE

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Next month: Water system balancing – part two

b) Check and adjust the main throttling valve to ensure a pump flow of approximately 100–110 per cent of the system design flow rate.

c) Measure the flow rate in all risers.

d) Calculate the percentage flow rate ratio for each of the risers i.e.

the riser having the lowest ratio being known as the “index riser”.

e) Adjust the flow rate of the riser with the highest ratio to its correct value. When this has been achieved move to the next highest ratio riser and adjust its flow rate and so on until all risers are balanced.

f) Revisit all risers and check/record the flow rates.

If design flow is not achievable on some risers a compromise solution must be agreed with the designer.

Mains balancingThis part of the balancing procedure, if required, is conducted after all the terminals, branches and risers have been balanced.

If a system has a number of mains serving numerous buildings then the riser balancing procedure would be repeated for the main pipes.

Final stepRecheck and adjust the main throttling valve at the pump to achieve the system design flow rate. Revisit all risers and check/record their final flow rate.

COMPENSATED METHODThis further development of the proportional method gives two specific advantages.

• Reducing balancing labour by making only one measurement at each balancing valve.

• Reducing pumping costs by selecting the lowest admissible pressure drop in the balancing valve.

The compensated method makes it possible to achieve correct flows everywhere with one adjustment at each valve, regardless of the size of the plant. To ensure that the differential pressure does not change in parts of the plant that have already been balanced, the pressure drop across a particular valve (referred to as the reference valve) is monitored.

When balancing the branches, the branch valves are the reference valves and the riser valves are the partner valves.

When balancing the risers, the riser valves are the reference valves and the main throttling valve is the partner valve.

As other valves are adjusted, the differential pressure changes and this is detected as a change in the pressure drop across the reference valve.

On no account must the hand wheel on the reference valve be moved to keep the pressure drop constant since this would change the differential pressure over other valves that have already been adjusted.

To achieve constant pressure drop over the reference valve, the partner valves are, in sequence, adjusted a small amount until the design pressure drop on the meter across the reference valve is reinstated.

In this manner, as each new valve is adjusted, compensation is made for the new distribution so that design flows are maintained in all the valves already adjusted.

Presetting the reference valveTo determine the correct preset value for the reference valve, select an appropriate design pressure drop in the reference valve according to one of the following criteria.

• At least 3kPa.

• At least the pressure drop for the valve when it is fully open at design flow.

Terminals having the same pressure dropa) Commence the balance on the terminal on the branch furthest

from the pump.

b) Set the reference valve at the correct preset value and lock the hand wheel memory on the correct setting.

c) Connect the pressure drop meter across the reference valve with indication transmitted to the partner valve.

d) Turn the hand wheel of the partner valve until the desired design pressure drop in the reference valve is read. It may be necessary to close one or more valves upstream to achieve adjustment.

e) Adjust all valves using a second pressure drop meter achieving the design water flow in each branch or riser starting at the valve next to the reference valve and proceeding upstream. For each adjusted valve, compensate for the change in differential pressure by adjusting the partner valve.

f) When the valve closest to the partner valve has been adjusted, all flows in the terminals on this branch or riser are considered correct.

Other procedures are available to deal with design pressure drops that are known but different, also when the design pressure drops are different and unknown. ▲

MORE INFORMATIONThe information in this month’s Skills Workshop was taken

from AIRAH’s design application manual DA24 Water System Balancing. This manual, and others in the DA suite, is available

for purchase on AIRAH’s online store at www.airah.org.au

www.hvacrnation.com.au | HVAC&R Nation | May 2014

APPLICATION MANUAL

DA24

THE AUSTRALIAN INSTITUTE OF REFRIGERATION, AIR CONDITIONING AND HEATING

WATER SYSTEM BALANCING