Variable+Flow+Hydronic+Systems

7/29/2019 Variable+Flow+Hydronic+Systems

http://slidepdf.com/reader/full/variableflowhydronicsystems 1/7

ble Flow Hydronic Systems http://www.kele.com/Tech/HVAC/Hydr

7 2/20/2006

Designing & Commissioning Variable Flow Hydronic Systems

article by Gil Avery, PE

This article reprinted with permission from ASHRAE JOURNAL July 1993, used with permission

Direct return Variable Flow HydronicSystems (VFHS) must be designed tobe self-balancing. However, inpractice many design engineers willuse customary pipe sizing routines,piping detail drawings, andspecifications that apply to morefamiliar constant flow systems.Substituting a 2-way valve in place of a 3-way valve and bypass pipe, while

retaining the balancing valve andbalancing specification ultimatelycreates control problems and energywaste. The use of balancing valves onVFHS is detrimental to theperformance of the system because itreduces the authority of the controlvalve and adds a permanent restrictionin every branch (References 1 & 2). This restriction increases the pumpingcosts for the life of the building.

Specifications for VFHS must stressthe importance of the valve actuatorand the need for high quality valvebodies to withstand the additionaldynamic forces and static pressuresthat are present in these systems. Theworking pressures for VFHS are always higher than for equivalent constant flow hydronic systems (CFHS).On many VFHS the use of conventional HVAC control valves may not be suitable. This article addressesthese problems and the important role the balancing contractor has in testing/commissioning these systems.

In order for a VFHS to be self-balancing, the sensor controlling the coil valve must always be in control of the flow. This is generally the condition when the supply air thermostat controls the coil valve on a variableair volume air handling unit.

If the valve controller is aroom thermostat, then itshould be locked at designtemperature. If thesetpoint can be adjustedby the public so that thecoil valve stays open, then




7 2/20/2006

the system is no longer self balancing. The VFHS shown in Fig. 1& 2 are identical exceptfor the chiller roomlocations. In Fig. 1, thechiller room is located onthe bottom floor

(equipment elevation 0 ft)and in Fig. 2 it is locatedon the top floor(equipment elevation 450ft). Fig. 3 shows thepressure drops across thecomponents in a typicalbranch at design flowconditions.

Unless otherwise noted, allheads shown in Fig. 1, 2,& 3 are dynamic frictionlosses (drop) when all coilvalves are handling designflows. The followingparameters also apply tothese figures:

The coil valves are

sized for a 15 ftdrop. The branch drop is 30 ft when the valves are wide open. The differential pressure controller "DP" will maintain 30 ft across the ends of the supply and returnmains by operating the variable speed drive on pump P-5. The supply and return mains are sized for an 8 ft drop between adjacent branches. The chiller loop (all equipment above cross-over points "A" & "B" in Fig. 1) supplies chilled water tothe secondary loop (all equipment below cross-over points "A" & "B"). The design head for pump P-5 is 110 ft at maximum speed. The air handling units are located at equipment elevations 90 ft apart. The static hydronic head is 450 ft.

The drop across branch "F" in Fig. 1 and in Fig. 2 is 30 ft and the drop across the coil valves handling designgpm is 15 ft. The valves will be 100% open and the valve plugs will lift 100% when handling design flow.

The drop across Branch "B" through "E" will vary between 46 and 94 ft depending on their distance frompump P-5. The maximum plug lifts will be greater than 80%, but less than 100% when handling designflows.

The drop across Branch "A" in Fig. 1 and Fig. 2 is110 ft and the drop across the coil valves handlingdesign gpm is 95 ft. The valve plug lifts




7 2/20/2006

approximately 80%, and will not lift more than thisexcept on start up or at other times when the coil-loadexceeds design.On start-up, all control valves will be wide open andpump P-5 will be handling more than design flow. Acheck valve in the cross-over between points "A" &"B" will prevent mixing return water with supplywater, since coil pump P-5 may be handling more

water on start-up than the total design flows of thechiller loop pumps. In this mode, the coil pump P-5will be in series with the chiller loop pumps.

The cool-down time will be reduced because the chillers and coils will operate at slightly higher than designflows; therefore, the building will actually cool down faster than one with a constant flow system or avariable flow system that has been manually balanced. The cross-over pipe and check valve can be sized tohandle the flow of the largest chiller, since flow sensor FS-1 will determine the number of chiller pumps tooperate. At no time would less pumps operate than the secondary system demands. For this reason, thecross-over does not have to be sized for total system flow.

In buildings with multiple risers, it may be necessary to have a differential pressure control for each pair of risers so as to adjust the pump speed to satisfy the branch with the greatest load.

Buildings with coil valves controlled by a DDC system can further reduce the water transport energy byresetting the differential pressure sensor from the valve operating with the greatest plug lift.

VALVE SIZING The pressure difference across the 2-waymodulating control valve on VFHS can varyfrom the differential pressure when thefully-opened valve is handling design flow, up

to the highest differential pressure when thevalve is closed and the pump is operating atmaximum speed. Because of the wide range inpressure differentials, valve sizing on VFHSis extremely important. The valve should beselected so that the pressure drop across thevalve is at least half the drop in the coilbranch as shown in Fig. 3. The valve will nothave enough authority to modulate properly if less drop is taken across the control valve thanacross the other components in the branch. Valve drops greater than this are a plus for controlability since the

valve has more authority, but a minus as far as pump energy is concerned. A valve sized for half the drop of the branch is a conservative compromise.

VALVE ACTUATOR SIZING The wide variable pressure differentials across the branches dictate the use of large valve actuators that canclose tightly and can precisely position the valve plugs. Actuators sized to close against at least 1 1/2 timesthe pump head will insure good valve plug positioning and will also minimize any spring range shift causedby high differential pressures across the valve. The spring range shift on pneumatic valves with smallactuators can be so severe that both the heating and cooling valves can be partially open. For example, manyfour-pipe chilled-hot water systems have heating valve operators with 3 to 8 lb springs and cooling valveoperators with 8 to 13 lb springs. If, because of high valve pressure differentials and undersized actuators, the




7 2/20/2006

spring range is shifted to 5 to 10 lb on the heating valve, then simultaneous heating and cooling can occur.

The control valves in Figs. 1 & 2 should have actuators sized to close against 165 ft (pumphead x 1.5). Valvemanufacturers often question why a valve that is selected for a drop of 15 ft needs an actuator that will closeagainst 165 ft.

VALVE BODY STATIC RATINGCommercial HVAC valve bodies are generally rated @ 250 psig, which is satisfactory for most installations.

The valve bodies on VFHS must withstand not only the static hydronic head, plus the imposed expansiontank pressure, but they must also withstand these pressures plus the full pump head when the valve is closed.All valves, pipes, and fittings on VFHS should be selected for operation with the pump at full speed,so as towithstand the maximum pump head when the pump drive or differential pressure sensor fails, and the pumpis operated on the backup full speed starter. The maximum head when the pump is operating in the backupmode may be 10 to 15% higher than the design head. The head will increase as the flow decreases and thepump head rides up the pump curve.

The valve body design pressure for VFHS is equal to the reserve compression tank pressure, plus static, pluspump head at cut off.

The valve body design pressure for CFHS is equal to the reserve compression tank pressure, plus static, pluspump head at duty point, minus friction drop.

In Fig. 1, the piping below coil B must be rated for design pressures over 250 psi. These design pressureswould be required whether the system was designed for variable flow with 2-way valves or for a manuallybalanced CFHS with 3-way valves.

In Fig. 2, piping designed for pressures greater than 250 psi is required for piping below coil "E" in theVFHS, but piping with working pressure of less than 250 psi could be used throughout on the equivalentCFHS.

VALVE BODY DYNAMIC RATING The dynamic body rating is defined as "the maximum flow differential pressure (psid) that the wetted partsare designed for." This is a "gray" area with the valve manufacturers and there do not appear to be anystandards for the HVAC industry to follow. The typical valve selection charts may be confusing since theylist close-off ratings much higher than dynamic ratings. For example, one manufacturer shows a 2" valvewith a close off rating of 55 psid but the footnotes limit the dynamic rating to 25 psid for "modulatingapplications." Another manufacturer shows a dynamic rating of 35 psid in the "wide open" position and yetlists a close off rating of 160 psid with a 50 square inch pneumatic actuator. The use of valve bodies withdynamic ratings too low for the application can be a real disaster because the valve seats deteriorate.

The dynamic body differential pressure rating for any valve on a VFHS should be at least 1.5 times the

design pump head. On large installations, it may be necessary to use industrial valves to meet this standard. The dynamic valve rating for those in Figs. 1 & 2 should not be less than 165 ft (71.4 psid). The 50% safetyfactor should provide enough reserve rating to handle the higher pressure differentials when the pump isoperated in the full speed mode.

VALVE FLOW CHARACTERISTICSValves for VFHS must have ports and plugs that are characterized so that the coil output (heating or cooling)is approximately a linear function of the valve stroke. Valves with equal percentage ports most nearly meetthis requirement and are a standard product with most manufacturers. They should be used exclusively onVFHS.




7 2/20/2006

PRESSURE RISE RATIOVFHS are designed to be self-balancing, and the balancing is done by the last segment of the plug lift. Thissegment is not usable to modulate the flow of water to the coil under design operating conditions. It istherefore desirable to minimize this loss in lift as much as possible so as to utilize more of the valve lift forflow modulation. This can be accomplished by minimizing the drop in the mains and all components in thebranches except the control valve.

The vertical axis in Fig. 4 is "percent plug lift" and the horizontal axis is the "pressure rise ratio." The

pressure rise ratio (PRR) is the ratio of duty point pump head to valve pressure drop at design flow, with thevalve wide open. The ratio for the system in Fig. 1 & 2 is 110 ft (pump head) divided by 15 ft (specifiedvalve drop), or a PRR of 7.3. The curve in Fig. 4 shows that the maximum lift is 80% when the branchdifferential pressure is 110 ft with design flow through the valve. The last 20% of the plug lift is used tobalance the system.

The curve can be used to determine the maximum lift for any equal percentage 2-way valve that has a designflow drop equal to one half of the total branch drop.

Fig. 4 shows that the equal percentage valve is a forgiving valve in that increasing the pressure rise ratio hasa relatively small effect on the maximum plug lift. Valves are only available in a limited number of sizes, and

therefore most valves selected by the control contractor are oversized. This increases the installed PRR onmost branches. Designing the VFHS so that this ratio is less than 10 will generally insure that the decrease ineffective plug lift does not increase the gain in the control circuit so much that the control loop is unstable.

On large systems with long mains, it may be necessary to add tertiary pumping zones in order to avoid thehigh pump heads and thus high pressure rise ratios that would be required of a single pump that had to handlethe pressure drop of the long main plus the drop in the risers and branches.

TESTING/COMMISSIONING THE VFHSWhen constant flow hydronic systems are balanced, the balancing contractor generally starts with all branchcontrol and balancing valves wide open. When testing the VFHS, the reverse procedure should be used.

Close coil valves except the one on the critical branch that appears to have the highest pressuredrop. This valve is left wide open.Adjust the differential pressure controller so the pump delivers design flow through this branch. Then close the coil valve on this branch and run the pump at full speed to be sure that there is noflow in the branch and that the actuator closes the valve tightly against the maximum pumphead.Check every other branch in the same manner to ensure that the drop on the remainingbranches is less than the apparent critical branch and that all valves close tightly.If another branch has a higher pressure drop, then reset the differential pressure controllerupwards to satisfy this branch.

When testing the drop in the branches, all readings can be taken at the remote coil branch where thedifferential pressure sensor is located. There is no flow and therefore no drop in the mains between the sensorand the branch under test. It is not necessary to move the differential pressure test gage to the branch beingtested since the differential pressure will read the same across the branch as it reads across the sensor, as longas all other branch valves are closed.

If the building has an operational DDC system, all branch flows and all valves can be tested at the DDCconsole if the testing procedure outlined above is followed. The branch differential pressures can all be readacross sensor "DP" and the branch flows can all be read through flow sensor "FS-1" in Fig.1. This procedurecan also be done annually, as part of the maintenance program, to detect any valves that have deteriorated




7 2/20/2006

and may not be closing tight.

The flow sensor should be of industrial quality, preferably one with no moving parts and with an accuracy of at least 1% of flow. When using the flow sensor for testing a valve for leakage it must be capable of readingvery low flows as well as accurately sense total system design flow.

The testing/commissioning procedure is critical and should be a part of the check-out of every VFHS. Acommon procedure is to neglect checking the drop through each branch. The technician may just lower the

differential pressure setting until one branch is starved. This method is not recommended because there is noway of detecting a blockage or if a component in one of the branches has an unusually high or low pressuredrop.

Very often, valves are furnished with the same pipe connections but different internal port sizes (differentCv). Testing each branch of the VFHS will reveal if all valves are installed in the right branches. If a branchis found with an excessively high or low pressure drop, it may be an indication that the wrong valve wasused. For example, in Fig. 1 all branches are shown with a 30 ft pressure drop. If there were a restriction inBranch "D" so that the pressure drop across this branch was actually 60 ft instead of 30 ft, the differentialpressure controller would have to be set for 60 ft just to satisfy this branch. The water transport energy woulddouble for THE LIFE OF THE BUILDING if this problem were not corrected. The pressure drop through

every branch on a VFHS should be checked by the balancing contractor and, if any branch is found to havean unusually high or low drop, it should be documented and corrected so that the VFHS can perform at peakefficiency.

SUMMARY

Use valves with equal percentage ports.Size all branches for approximately the same pressure drop.Size all valves so that the pressure drop through the open valve at design flow is equal to orgreater than the drop in the rest of the branch.Select all valve actuators to close off against a differential pressure at least 1 1/2 times the design

pump head.Select valve bodies that have a dynamic differential pressure rating at least 1 1/2 times the designpump head.Minimize the pressure drop in the mains and branch piping. Take as much drop across thecontrol valve as practical.Keep the pressure rise ratio below 10.Select valve bodies with static ratings greater than the static hydronic head, plus the compressiontank reserve pressure, plus the pump cut off head, at maximum pump speed.

References:

1) Hansen, E.G. 1985. Hydronic System Design and Operation. New York: McGraw-Hill.

2) Avery, G. 1990. "Balancing a variable flow water system will ruin the control system."ASHRAE Journal.Vol. 32, No. 10, October, p. 30.

3) Honeywell, Inc. 1992. Tradeline Catalog. Minneapolis, Minnesota: Honeywell, Inc.

4) Barber Colman Co. 1992. Controline Catalog. 6th ed. Rockford, Illinois: Barber Colman Co.




Documents

Variable+Flow+Hydronic+Systems