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As discussed in Choosing a Balancing Design Strategy( Mechanical Business, January/February 2008), there are manymethods that can be used to balance the fluid flow in an HVACsystem to ensure every terminal unit has at least design flowavailable.

For the purpose of this article, we will assumethat the building we are looking at is using acombination of pipe stretching techniques,with provision for manual circuit balancing.It is possible that any combination of othertechniques, such as reverse return piping,automatic flow limiting, or one-pipe loadpump balancing, may also be used. Shouldthat be the case, many of the sameprinciples apply, and only the method of actual flow adjustment to design flow differs.

As variable volume hydronic fluid flowdesigns become more prolificfor overall energy savingsopportunities, the most common terminal unit control and regulationcircuits used in newconstruction are basedon two-way controlvalves. However,circuits based on three-way control valves arestill very common inconstant volume systems,or at the end of parallel two-way circuits to avoid completely shutting thesystem down, should all or a significant portion of control valves happen to close at the same time, which couldhappen during night setback activation.

For a constant volume scheme, the system will now run at optimum efficiency, with design flow delivered to each terminalunit. The control valve will dictate whether this flow runsthrough the terminal unit, or through the bypass back to thepump. While these systems can operate quite efficiently,

ASHRAE 90.1 recommends limits on the amountof flow that is bypassed in a system, hence

favour is now generally given tovariable-volume systems.

When two-way control valves are used with avariable speed pump in a proportionallybalanced HVAC system, as demand is met andcontrol valves close, the system resistanceincreases.As this resistance increases, thepump is slowed to maintain design flowthrough the demand loads that remain open.

The pump speed is controlled basedon inputs from discrete pressure

sensors, or via sensorlessvariable speed pumptechnology. This schemeassures consistent controlvalve authority despitedynamic diversity, andhelps reduce controlovershoot and hunting

conditions, ultimatelyavoiding excess wear and

premature failure of thecontrol valves.

In the next issue, well look at various balancingvalve technologies and how they perform in the

HVAC system.

HYDRONICS by Rod Brandon

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COMMISSIONING

Regardless of which method or combination of methods are employed, responsible project management dictates that, uponcompletion, the HVAC system be thoroughly tested by a third party to ensure the system engineers design intent is met anda formal report be produced that details the actual performance realized and any non-conformance issues found. Mostcommissioning authorities recommend that design flows throughout the system be tested and verified accurate to within five percent (plus or minus) of design flow.

COMPLETE SYSTEM BALANCINGProportional Balancing Method

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Lets take a closer look at the stepsinvolved to proportionally balance a

multi-storey building.A typical schoolmay have two floors with 100 terminalunits each, whereas a typicalmetropolitan building may have 25floors with 20 terminal units per floor.

For illustration purposes, well look at only two floors with three terminalseach. All of these principles will apply,regardless of the number of floors andnumber of terminal units per floor.

1Ensure the system is running wide

open. Set all control valvescompletely open, turn all balancing

valves completely open, and ensure the pump is running at design speed.

2 Locate the most disadvantaged terminal unit.This isusually the one on the highest floor and furthest from thesupply risers. (Figure 2, CBV2-3) This is the startingreference flow. This valve is always left wide open. Note that at this point, the flow through this terminal is likely well belowdesign flow. If not, there is a good chance the pump is severelyover-sized.

3 Adjust the balancing valves for all other terminal units onthe floor (CBV2-2 & CBV2-1) to match the flow that willprevail through the most disadvantaged unit. Note that there is interactionbetween valves, andas any one isthrottled, the flowthrough the othersincreases.Experiencedbalancers can oftenpredict the amount of compensationrequired for thisinteraction and canadjust all valves toachieve a commonflow through all in

just one pass.

4 Repeat step 1 to 3 for the next most disadvantaged floor. When complete, each floor will be balanced individuallyand separate from each other.

5To balance the floors together,leave the riser balancing valve of the most disadvantaged floor wide

open (Figure 3, CBVR-2). Adjust theriser valves of other floors, until all floorsare at an equal flowrate.

6 Throttle the main systembalancing valve (CBV Main) toachieve 100% design flow. Theentire system is now proportionallybalanced, with design flow available toall circuits. However, having the mainCBV throttled is like driving a car withone foot to the floor on the gas pedaland the other on the brakes.Though thesystem is balanced, optimum efficiency isnot realized.

7 With the system running wide open, measure the pressuredrop across CBV main both as throttled and completelyopen. This pressure difference is the amount of excesspump head generated.

8 Shut down the system and have the pump impellertrimmed toeliminate theexcess head andbring the pumpinto the most efficient zone of the performancecurve.

9 Reinstall theimpeller,open CBVmain, and verifythat design flow isrealized. Thesystem is nowproportionallybalanced with design flow available to all terminal units, andoperating at optimum efficiency.

10 Release all control valves for normal operation.Rod Brandon is a technical marketing specialist with S.A. Armstrong Limited, a global supplier of HVAC and fluid flow equipment and

solutions for residential, commercial and industrial applications. Hecan be reached at info@armlink.com.

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