testing, adjusting & balancing - ashrae

8/10/2019 Testing, Adjusting & Balancing - ASHRAE

1/7

May 2 00 1 AS HR AE J our na l 1 9

ASHRAE Journal TAB

W

A Primer on Testing,

Adjusting and Balancing

About the Author

By Andrew P. Nolfo, P.E.Member ASHRAE

The TAB firm is part of the construction

delivery team along with the design engi-neer, mechanical contractor, and controls

contractor. They all have the same goal:

deliver a project that satisfies the design in-

tent. If an adversarial relationship develops

among team members, the TAB firm oftenis perceived as the watchdog. Sometimes

this watchdog role identifies design and/or

installation errors. However, this watchdog

role is ancillary to the TAB firms main func-

tion: helping the system to work properly

by balancing the fluid flows to their correct

proportion.

This article discusses some common

problems that can complicate the TAB

firms work. The article addresses air and

hydronic systems. It also addresses how

these mistakes can be avoided at the de-

sign stage or fixed in the field. Addition-ally, the article discusses the application

of fan and pump curves to TAB work. Fi-

nally, the article discusses how TAB firms

use other diagnostic tools and data to ob-

tain unknown data.

Traverse LocationMeasuring airflow in ducts is important.

Duct traverses are used for everything from

measuring total airflow to determining cor-

rections factors for direct reading hoods. Thelack of a suitable traverse location is prob-

ably the single greatest issue in TAB work.

A suitable location is one where there is fully

developed airflow, i.e., one where the ve-locity profile is reasonably uniform across

the plane of the traverse location. While the

industry debates the accuracy of various

duct traverse protocols, the larger issue is

determining and using a suitable traverseplane to obtain accurate, repeatable read-

ings.

Many of todays TAB specifications re-

fer to ANSI/ASHRAE Standard 111-1988,

Practices for Measurement, Testing, Adjust-

ing, and Balancing of Building Heating,

Ventilation, Air-Conditioning, and Refrig-

eration Systems. Standard 111 is being re-

viewed with a new edition expected in 2001.

The current version lists several character-

istics of an ideal traverse plane:

a. A uniform velocity distribution

means that 80%90% of the velocity pres-sure measurements are greater than 10%

of the maximum velocity pressure.

b. Airflow should be at right angles to

the traverse plane.

c. The cross section of the traverse planeshould not be an irregular shape, and the

shape area should be uniform in the vicin-

ity of the traverse plane.

d. The traverse plane should be located

to minimize the effects of leaks.

Appendix D of the standard offers addi-tional guidance in locating a suitable

traverse plane. It suggests an effective

length of 2 duct diameters downstream

of a centrifugal or axial fan outlet. This

length is based on a velocity of 2,500 fpm

(12.5 m/s) or less. For velocities greaterthan 2500 fpm (12.5 m/s), add one more

duct diameter for each 1000 fpm (5 m/s)

in excess of 2500 fpm (12.5 m/s). For rect-

angular duct, Equation 1 provides an

equivalent diameter:(1)

where, a and b are the rectangular

duct dimensions.

As an example, a 10,000 cfm (5000 L/s)

system with a 30 in. 20 in. (76 cm 51

cm) discharge duct (2,400 fpm [12 m/s])

would require about 5.8 ft (1.7 m) of unob-structed, straight duct upstream of an ideal

traverse location. Another 1 diameter, or 2.7

ft (0.8 m) of downstream duct should also

exist before a fitting, takeoff or other ob-

struction is encountered. This would requirethat the mechanical equipment room be de-

signed to accommodate an air-handling unit

with a straight, horizontal discharge duct 8.5

ft (2.6 m) long.

As an example, Figure 1shows this air-

handling unit (10,000 cfm, 30 in. 20 in.

[4719 L/s, 76 cm 51cm] duct) with a

traverse plane 3 ft (0.9 m) from the dis-

charge elbow. This location is 1.1 diam-

eters from the disturbance. A traverse taken

there would not be accurate. The elbow

would also create a substantial pressure

drop due to turbulence associated with sys-tem effect. This discharge is in a broken-

back condition.

The NEBB Procedural Standard for the

Testing, Adjusting, and Balancing of Envi-

ronmental Systems says the accuracy of a

Andrew P. Nolfo, P.E.,is the techni-cal director for the National Environ-mental Balancing Bureau (NEBB). Heis a corresponding member of TC 9.7,TAB, and a member of TC 9.9, Build-

ing Systems Commissioning.

( ) 214 abEL =


2/7

2 0 A SH RA E Jou rn a l ww w.as h rae jou rna l .o r g M ay 20 0 1

ASHRAE Journal

pitot tube traverse is determined by the availability

of a location to perform the traverse. The standard con-tinues to identify a location that has six to 10 diam-

eters of straight duct upstream of the test location. The

standard also has a statement regarding the practicality

of finding such an ideal location: this condition willnot be found very often in the field, therefore, use the

best location available. The procedural standard also

discusses ways to correlate total airflow when a less than

ideal location exists.

If the traverse location cannot avoid elbows, off-

sets, transition, branch take-offs or other items that

would cause turbulence, one solution is to obtain more

readings closer together for a better average reading.If a total supply cannot be directly read, it might be

possible to measure several main branches and add the resulting

airflows together. When all else fails, the TAB technician must

use the best available location and correlate the airflow reading

to other gathered data, such as brake horsepower calculated fromfan pressure drop and electrical performance.

Determining Outside Air QuantityIf supply and return airflow are measured accurately, outside

airflow could easily be determined from Equation 2:

outsidereturnsupply QQQ +=(2)

When accurate direct measurements are not available, anothermethod to determine the outdoor air uses static pressure. If accu-

rate supply airflow can be determined, the outdoor air can be de-

termined by measuring static pressure in the return duct. The out-

door air must be completely sealed off for this test. As an example,

consider a 10,000 cfm (5000 L/s) supply fan with a minimum out-door air requirement of 10%. With the fan delivering the required10,000 cfm (5000 L/s) and the outside air intake sealed, measure

the static pressure in the return duct just before the mixing box.

Since the outdoor air is completely sealed, the return air must equal

the supply air. Suppose the measured return static pressure is 1.0

in. w.c. (250 Pa). Since flow varies with the square root of the

pressure, the TAB technician can then open the outdoor air damper

until the static pressure in return duct equals 0.81 in. w.c. (202 Pa),

as illustrated by Equation 3:

( )0.52121 PPQQ = (3)

Where,

Q1 = Return airflow with minimum outdoor air(9,000 cfm [4500 L/s])

Q2 = Return airflow with zero outdoor air

(10,000 cfm [5000 L/s])

P1 = Return air static pressure at minimum outdoor air

P2 = Return air static pressure at zero outdoor air

(1.0 in. w.c. [250 Pa])

Substituting into Equation 3 would give:

( )

( ) 5.01

5.0

1

250000,5500,4

0.1000,10000,9

P

P

=

=

Solving forP1:= 0.81 in. w.c. (202 Pa)

Many TAB technicians and engineers are familiar with using

temperatures to estimate outside air quantity. This technique uses

Equation 4 in conjunction with Equation 2:

outsidereturnsupply QOATQRATQMAT += (4)

Where:MAT = Mixed air temperature

RAT = Return air temperature

OAT = Outside air temperature

The accuracy of the temperature measurements introduces

another level of potential error. Unless the air in the return air

duct is thoroughly mixed, the measured temperatures could beoff as much as 1F to 4F (0.5C to 2C). The outdoor air mea-

surement will probably experience an error. The mixed air tem-

perature is the most suspect and could be off as much as 5F to

20F (2C to 10C) depending on the temperature.

Many air-handling unit mixing chambers do not really mix theairstreams well. Some use opposed blade dampers that provide

excellent throttling capabilities but poor mixing. Parallel blade

dampers provide better mixing but poor control. As a result, per-

fect mixing is rarely achieved. The errors are magnified when

the supply, return, and outdoor air are in the range of 45F to

70F (7C to 21C) because the differences are small. In colder

climates, errors in this method of reading the outdoor air quantity

diminishes when the temperature is in the range of 0F to 35F (

18C to 2C).

As an example, an air-handling unit is to provide 10,000 cfm

(5000 L/s), with a minimum of 1500 cfm (750 L/s) of outside air.

The actual field conditions have made it impossible to measure

the supply and outdoor airflows accurately. The return air, out-door air, and mixed air temperatures are respectively measured

as: 74F, 45F, and 70F (23.3C, 7.2C, and 21.1C). The return

airflow is accurately measured at 8,900 cfm (4450 L/s). Substi-

tuting these values into Equations 2 and 4 and solving would

yield:Qsupply= 10,324 cfm (5154 L/s)

Qoutside = 1,424 cfm (704 L/s)

The supply and outdoor airflows appear to be within 5% of the

design requirements.

Suppose measurement errors and incomplete mixing meant

the actual temperatures were: 75F, 44F and 72F (23.9C, 6.7C,and 22.2C). Substituting these values into Equations 2 and 4

and solving would yield:


3/7


TAB

Qsupply= 9,853 cfm (4938 L/s)

Qoutside = 953 cfm (488 L/s)The supply airflow is still within 5% of the design require-

ment, but the minimum outdoor air is 36% below the design re-

quirement.

Although this method of can provide accurate data, the best ap-plication is when it is used by trained technicians to provide an

acceptable check to a suspect measurement.

Duct LeakageA good duct traverse can also be used to determine duct leak-

age. Suppose a VAV system needs to supply 10,000 cfm

(5000 L/s). The design diversity is 10%, so the sum of all the

VAV terminals is 11,000 cfm (5500 L/s). With terminals equal to1000 cfm (500 L/s) closed (zero airflow), the TAB technician

has tested the remaining terminals. The sum of the outlets is 8,500

cfm (4250 L/s). A supply duct traverse, as shown in Figure 1,

measures supply airflow of 9,500 cfm (4750 L/s). The sum of all

the air outlets is measured as 8,500 cfm (4250 L/s). Which one, ifeither, is correct? If the supply air duct traverse was reliable, thetechnician could be justified in concluding that the problem is

duct leakage.

Determining Pump FlowLike measuring airflow on a fan, pump total flow measure-

ments can sometimes be suspect. Since water is non-compress-

ible, the problems are not as severe as for fans. Most TAB techni-cians will determine flow by measuring the differential pressure

between the pump discharge and the pump suction. Pump manu-

facturers provide pressure taps machined into the body of the

pump or on the suction and discharge flanges of larger pumps.By closing the discharge valve, the differential pressure at no-

flow conditions can also be measured. The discharge valve is

then opened to its original condition. By using the manufacturers

pump curve and these two pressure measurements, the techni-

cian can estimate total flow. The differential pressure at the no-

flow conditions is used to verify which impeller is installed in the

pump. The differential pressure at the operating condition will

determine the actual flow condition.Using the pump curve to determine pump flow can be inaccu-

rate when the pump has a flat curve. As an example, Figure 2

is the manufacturers pump curve for a pump selected to provide

800 gpm (50.5 L/s) at 68 ft (204 kPa) head. This design point is

identified as Point 1. It shows that a 9 in. (229 mm) impeller willsatisfy these design conditions and that a 20 hp (15 kW) motorshould be provided. The technician tested the pump and mea-

sured a shut-off differential pressure of 75 ft (225 kPa) and an

operating differential pressure of 71 ft (213 kPa). These points

are identified as Points 2 and 3 respectively.

Based on the technicians findings, it appears that the pump

Advertisement for the print edition

formerly in this space.


4/7


5/7


( ) 5.02121 PPQQ = (3)

Where:

Q1= Actual Chilled Water Flow (gpm or L/s)

Q2= Design Chilled Water Flow (gpm or L/s)

P1= Actual Measured Pressure Differential

(ft or kPa)

P2= Manufacturers Design Pressure differential (ft or kPa).

As an example, a chiller has a design flow of 1,000 gpm (63.1

L/s) with a stated differential pressure of 20 ft (60 kPa) head. The

TAB technician measured differential pressure across the dedi-

cated pump and estimated the flow at 850 gpm (53.6 L/s) from a

flat pump curve. The technician wants to check the reading so itmeasures differential pressure across the chiller at 19.2 ft (57.6kPa) of head.

Substituting into Equation 3:

( )

( ) 5.0

5.0

0.606.571.63

0.202.19000,1

=

=

X

X

Solving forX:= 979 gpm (61.8 L/s)

The technician concludes that the flow through the chiller is 979

gpm (61.7 L/s). There is a caveat to this scenario and to all other

data associated with equipment. The manufacturers rating is based

on laboratory conditions. In this case, long, straight runs of pipe

are at the chiller. Additionally, the pressure drop is measured im-mediately at the equipment connection. The measured pressure

drop as shown in Figure 4must be corrected for the pressure drop

associated with the additional pipe, valves and fittings at the chiller.

One method is to calculate the pressure drop associated with the

additional pipe, valves, and fittings. This pressure drop must be

subtracted from the measured pressure drop and the corrected pres-sure drop be used to verify chiller flow.

Sizing Balancing ValvesThis final common mistake is easy to correct at the design

stage but costly to correct in the field. A balancing device is

similar to a control device and should be sized accordingly.




6/7


7/7


key data cannot be directly measured?As previously discussed, airflow measurement of mixed air-

streams can be determined by the temperature measurement. This

can also be applied to coils. If the entering and leaving air tempera-

tures can be adequately determined, airflow can be estimated by

applying the conservation of energy equations. As an example, ahot water heating coil has the following conditions:

Entering air temperature 55F (13C)

Leaving air temperature 90F (32C)

Hot water flow 50 gpm (3.16 L/s)

Entering water temperature 180F (82C)

Leaving water temperature 150F (66C)

Since the energy from the hot water must be transferred fromthe water to the airstream, the energy balance can be written as

Equation 5:

( ) ( )

( ) ( )13321.232668242003.16L/s(air)

55901.1015018050050cfm

=

=

(5)

Substituting and re-arranging gives:

airwater

airwater

1.232L/s4200L/s

1.10cfm500gpm

=

=

cfm = 19,480 cfm (9071 L/s)

ConclusionMost TAB firms feel their objective is to orchestrate all pieces

of the mechanical system to a workable, operable system. Theworld is not an exact science. Their ability to perform these ser-

vices depends on taking accurate, repeatable measurements. The

design professionals and installing contractors can greatly enhance

the TAB firms work by understanding the difficulties that mostof todays projects present to the TAB work.

BibliographyNational Environmental Balancing Bureau (NEBB) Procedural Stan-

dard for Testing, Adjusting, Balancing of Environmental Systems, 1998/

Sixth Edition.

ANSI/ASHRAE Standard 111-1988,Practices for Measurement, Test-

ing, Adjusting, and Balancing of Building Heating, Ventilation, Air-Con-

ditioning, and Refrigeration Systems.

Air Movement & Control Association InternationalAMCA 203-90.

Rishel, J.B. 2001. Applying affinity laws for centrifugal pumps.

HPAC Engineering,February.



testing, adjusting & balancing - ashrae

Documents