3/30/13 HVAC AND INDUSTRIAL VENTILATION DUCT DESIGN

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HVAC AND INDUSTRIAL VENTILATION DUCT DESIGN©

by Bill Smith, president of Elite Software

Sizing ductwork for standard hvac applications is daunting enough, but doing

it for industrial ventilation and exhaust systems can be hair raising. At first

glance there seems to be great similarity in the design goals of these different

types of duct systems making one job as easy as the other. But ventilation and

exhaust system duct sizing has quite a few more wrinkles than standard hvac

duct design.

The main goal of designing hvac duct systems is to use the lowest cost (read

smallest) duct sizes that can be used without violating certain sizing constraints.

For example, hvac duct sections are typically sized so that air velocities don'texceed 400-800 feet per minute

and the pressure loss per 100 feet

of duct length does not exceed 0.1" wg. These constraints effectively

limit how small ducts can be sized.

If high air velocities and a large pressure drop per 100 feet of duct

are allowed, ducts can be sized relatively small. But excessive noise

and a large total pressure drop necessitating a powerful and noisy fan

are almost certain results of downsized duct system.

Still, velocity constraints can be varied for individual duct sections so that duct sizes can be selectively minimized

without adversely affecting noise considerations. Likewise, the maximum allowable pressure drop per 100 feet of

duct can sometimes be increased when it is known that the resulting greater pressure loss is still within the

capacity of the fan.

Besides meeting the desired sizing constraints, an hvac duct system should be routed so as to minimize the

individual lengths of the various duct runs. Optimally routing duct work minimizes material costs and helps to

create a naturally balanced system where the static pressure and air velocities available at each diffuser are as

similar as possible. A computer program can help to calculate pressure losses and air velocities, but intelligent

placement of the air handler and routing of the ductwork are still the art of the hvac designer.

Except for differences in desired velocities and pressure drops, all of the above would seem to apply to industrial

ventilation and exhaust duct systems. This is not so for several reasons. Industrial ventilation systems routinelyutilize components rarely seen in hvac duct systems such as hoods, dust collectors, blast gates, and other such

items.

These unique components not only require special consideration in calculating their pressure loss, they also

greatly influence the design of the duct system. For example, a hood usually has slots through which particulate or

gases are drawn through. For the hood to work properly, the connecting ductwork must allow sufficient velocity

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(typically 3,500-4,500 fpm) so that the particulate stays in suspension of the transporting air.

The dust collector of a ventilation/exhaust system not only contributes a large pressure loss, it can also vary the

density of the air stream if it is a wet collector where moisture is added. Density changes at the collector thus

affect the pressure loss calculations through all subsequent duct work.

Whereas the goal of an hvac duct system design is to use the smallest possible duct sizes that do not violate

certain constraints, the goal of a ventilation/exhaust system design is to use the largest duct sizes that will still

maintain the minimum velocity needed to keep particulates in suspension.

In ventilation/exhaust system design, maintaining the minimum velocity to keep particles in suspension is

absolutely critical. However, maintaining high air velocities requires considerable fan power and electric energy

consumption. If air velocities are maintained above what's necessary, a significant penalty is paid on operating

costs. Not to mention that an already noisy system gets even louder.

The extra material expense of large ducts is usually insignificant when compared to the operating costs saved on

fan energy. However, ducts can only be made so large before the air velocity begins to fall below the minimumrequired. Hence, good ventilation/exhaust system design strives for maximum useable duct sizes while optimal

hvac duct design aims for the smallest practical sizes.

Perhaps even more than sizing ductwork, the real problem in ventilation/exhaust system design is balancing thestatic pressure at each converging junction of ductwork. Industrial ventilation and exhaust systems consist mostlyof hoods with ductwork that converges back to a collector and fan. Weye fittings typically connect the

converging air streams from hoods.

As two air streams converge, it is important for the static pressure in each duct branch leading to the junctioncalculate to nearly equivalent values. If they do not calculate within a 5% difference, the desired cfm flow

quantities will not occur in the actual operating duct system.

Physics laws will always make the actual converging air streams have the same static pressure, but the cfm flowof air will automatically adjust to make the pressure losses balance. If the designer has not carefully calculated

and balanced the static pressure losses for his desired air flows, the actual installed duct system will simply notperform as required. Hoods will draw whatever air quantity the actual static pressure values dictate and

completely ignore the wishes of the hapless designer. The industrial ventilation/exhaust system designer has anumber of tools at hand to make static pressures of converging air streams balance for his desired flow rates.

The designer can change the sizes of the converging duct work, change a fitting on one of the duct sections,change some hood characteristics, or use a blast gate.

If the designer has followed the usual guideline of initially sizing the duct sections as large as allowed by the

minimum velocity constraint, then increasing the duct size of one branch to increase it's static pressure is not anoption. Therefore, to make the static pressures of two air streams balance, the designer usually attempts todecrease the static pressure of the branch with the highest value.

The first approach most designers try is to decrease the duct size of the branch with the highest static pressure.

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Sometimes this change alone will work, but often times it results in decreasing the static pressure of the highest

branch too much so that an imbalance still exists. At this point the designer might go back to the first computedduct size, but try using higher loss fittings or a different hood configuration on the branch with the highest static

pressure. Just changing the shape and number of slots on a hood can alter the static pressure of one branchenough to create a balance.

If all else fails in achieving calculated balanced static pressures, the designer might consider the use of a blast

gate. A blast gate allows precise and strong static pressure control, but it is considered a last resort as a blastgate is a high maintenance item with potentially dangerous side effects. Gate slides can erode or become filled

with particulate impeding their operation. A blast gate can also be improperly tampered with sometimes resultingin clogged ducts or injury to the operator.

Static pressure calculations and balancing strategy must be applied at every duct junction in a proposedventilation/exhaust system design. This kind of extensive design analysis must be done before construction so that

the actual installed system will perform as desired without expensive modifications. This is also true for an hvacduct system. It's just that hvac duct design does not require as much attention to balancing calculations as does a

ventilation/exhaust system.

Certainly, the kind of painstaking analysis described above can be long and difficult to perform. Although notimpossible to do by hand, it is much faster and easier to do with a computer program. There are many programs

available for standard hvac duct design, but most of those programs do not have provisions for the specialconsiderations of ventilation/exhaust system design. When shopping for a duct design program it is important toask whether the program is meant strictly for hvac system design or whether it can also be used for

ventilation/exhaust system design.

Whether designing standard hvac duct systems or a complex industrial ventilation/exhaust system, the more timeput in up front for analysis the less problems and headaches there will be during and after construction.

Mr. Smith welcomes your email about this article. - email

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