Specifications

ManualsBrandsZephyr ManualsHousehold AppliancesS)

Engineering Notes

Zephyr Fans

PENNBARRY

The following section includes engineering and technical

data, guidelines and system explanations related to air mov-

ing and control devices. Fan laws and system descriptions

are consistent with industry standards, definitions and

accepted practices. They are provided to assist system

designers in sizing, selecting and defining their air moving

and control systems as well as explaining variables inherent

in system design.

Flow And Static Pressure

For any change in static pressure (SP), a squared relationship

is applied to the flow ratio. This is expressed by the formula:

CFM

Where P

is the original static pressure,

is the desired static pressure,

CFM

is the original flow rate in cu. ft. per minute, and

CFM

is the desired flow rate in cu. ft. per minute.

This formula is based upon performance of a fan at one point

on a system. This data can be used to calculate a fan per-

formance curve indicative of all five points from 0" SP (max-

imum flow) to maximum SP (0 flow).

CFM

RPM

CFM

RPM

CFM

RPM

CFM

RPM

NOTE: Direct drive fans can change speed only through the use of a speed

controller and cannot increase speed beyond catalog maximum. Fans must

operate on single phase in order to change speed.

Air Systems

An air system may consist simply of fan with ducting con-

nected to either the inlet or discharge or to both. A more com-

plicated system may include a fan, ductwork, air control

dampers, cooling coils, heating coils, filters, diffusers, noise

attenuators, turning vanes, etc. The fan is the component in

the system which provides energy to the airstream to over-

come the resistance to flow of the other components.

Component Losses

Every system has a combined resistance to flow which is

usually different from every other system and is dependent

upon the individual components in the system.

The determination of the “pressure loss” or “resistance to

flow”, for the individual components can be obtained from the

component manufacturers. The determination of pressure

losses for ductwork and branch piping design is well docu-

mented in standard handbooks such as the ASHRAE

Handbook of Fundamentals and SMACNA Duct Design

Manual.

The System Curve

At a fixed volume flow rate (CFM) through a given air system

a corresponding pressure loss, or resistance to this flow, will

exist. If the flow rate is changed, the resulting pressure loss,

or resistance to flow, will also change. The relationship gov-

erning this change for most systems is:

PRESSURE

/ PRESSURE = (Q

/ Q)

Q = CFM

= change

Interaction Of System And Fan Performance Curve

If the system characteristic curve, composed of the resis-

tance to flow of the system and the appropriate System

Effect Factors have been accurately determined, then the fan

selected will develop the equivalent and necessary pressure

to meet the system requirements; i.e. the fan will deliver the

designated flow rate when installed in the system.

The point of intersection of the system curve and the fan per-

formance curve determines the actual flow volume. If the

system resistance has been accurately determined and the

fan properly selected, their performance curves will intersect

at the design flow rate. Refer to Figure 1. The normalized

Duct System A from Figure 1 has been plotted with a nor-

malized fan performance curve.

The volume flow

rate through the

system in a given

installation may be

varied by changing

the system resis-

tance. This is usual-

ly accomplished by

using fan dampers,

duct dampers, mix-

ing boxes, terminal

units, etc. Figure 1 shows the volume flow rate may be var-

ied from 100% design Q (Point 1, Duct System A), to approx-

imately 80% of the design q by increasing the resistance to

flow, thus changing the system curve characteristics to Duct

System B. This results in fan operation at Point 2 (the inter-

section of the fan curve and the new Duct System B).

Similarly, the volume flow rate can be increased to approxi-

mately 120% of the design Q by decreasing the resistance to

flow, thus changing the system curve characteristic to Duct

System C. This results in fan operation at Point 3 (the inter-

section of the fan curve and the new Duct System C).

()

Figure 1