Authentic Ductwork Calculation

7/29/2019 Authentic Ductwork Calculation

1/10

Kirkuk Technical College

Refrigeration and Air conditioning Department

4Th

Year

Air Conditioning Systems

Compiled by: Aumid Abdulrahim, Assistant Lecturer

Reference: Chapter 8 of the recommended book

Air-conditioning Principles and Systems, Fourth edition by Edward G Pitta

HVACLectures

October 2

2012Air duct system, design Week one
http://www.google.com/imgres?q=Air+duct+system,+design&num=10&hl=sv&biw=1280&bih=598&tbm=isch&tbnid=DXQsP8rSaFyCxM:&imgrefurl=http://www.zoningnews.net/?p=48&docid=BEXrFlZ1t7cPkM&imgurl=http://www.zoningnews.net/wp-content/uploads/2010/05/New-Picture-3.jpg&w=789&h=398&ei=9SEZUPPAMs7P4QSln4HwBA&zoom=1&iact=hc&vpx=79&vpy=33&dur=3850&hovh=159&hovw=316&tx=160&ty=70&sig=106376247813177082638&page=1&tbnh=79&tbnw=157&start=0&ndsp=20&ved=1t:429,r:14,s:0,i:116


2/10

2

Introduction:In planning or servicing an HVAC system, it is often necessary to determine pump and fan pressurerequirements and piping or duct pressure losses. These and related problems can be solved by anapplication of some principles of fluid flow which apply to the flow of wate, and air in air conditioningSystems.

Objectives

After studying this chapter, you will be able to:1 .Use the continuity equation to find flow rate.2. Use the energy equation to find pump and fan pressures.3. Find velocity from total and static pressure.4. Determine pipe and duct sizes.

The continuity equation:

The flow of water through piping and air through ducts in HVAC systems is usually under conditionscalled steady flow. Steady flow means that the flow rate of fluid at any point in a section of pipe or ductis equal to that at any other point in the same pipe or duct, regardless of the pipe or duct's shape or

cross section. That is, the same quantity of fluid is passing through every section at a given moment.

For example, in Figure above , suppose the flow rate of water past section 1 were 10 GPM (gallons perminute). If there is steady flow, there must also be 10 GPM flowing past section 2. To see this more

clearly, if less than 10 GPM were flowing past section 2, say 4 GPM, ask, what happened to theremaining 6 GPM that left section 1? It cannot disappear or be lost (unless there is a hole in thepipe!).Similarly, there cannot be more flow at section 2 than at section 1 because there was only 10GPM available initially. In HVAC systems, the density of the air or waterflowing generally does not

change significantly. When the density remains constant, the flow is called incompressible*. Steadyflow is a special case of a general principle called either the conservation of mass principle or thecontinuity principle.The continuity principle can be expressed as an equation. which is called the continuity equation.For incompressible steadyflow, the continuity equation is.VFR = constant = A1 X V1 = A2 X V2 (1.a)

Where:VFR = volume flow rate of fluidA I, A2 = cross-sectional area of pipe or duct at anypoints I and 2V1 V2 = velocity of fluid at any points I and 2

The continuity equation can also be expressed using mass flow rate (MFR) rather than VFR. FromEquation 2. 1, mass equals density times volume.Then, with constant density (d),

MFR = constantwhered x VFR = d x A1 X V1 (1.b)d x A2 x V2

MFR = mass flow rated = density of fluid* Most flows in HVAC systems are incompressible flows.


3/10

3

4. The Flow Energy Equation

When the energy balance principle is applied to flow in a pipe or duct, it may be statedas follows: between any two points 1 and 2

or

where:E1, E2 = energy stored in fluid at points 1 and 2

E add = energy added to fluid between points 1 and 2E lost = energy lost from fluid between points 1and 2The energy of the fluid at any point consists of . pressure, velocity (kinetic energy), and elevation(potential energy). The energy added may be that of a pump or fan. The energy lost is due tofriction. There may be other energy changes (e.g., a temperature change), but they are usuallysmall and may be neglected.

The flow energy equation applied to flow in a duct or pipe

(2a)

where


4/10


5/10

5

Thus the total pressure energy that a fluid has at any point can be considered to consist of twoparts, its static pressure energy and its velocity pressure energy.The velocity pressure concept isuseful in measuring velocities and flow rates in piping and ducts. If the velocity pressure can bemeasured, the velocity can be found by solving Equation 4 for V:

When measuring airflow, using in. w. as the unit of pressure and velocity in ft/min, if theappropriaten conversion units are substituted in Equations 4 and 5:

Many testing and balancing instruments for measuring flow utilize the relationships among total,static, and velocity pressure. Figure below shows an example. In Figure (a), a manometer isconnected to the duct to read static pressure. In Figure(b), a manometer reads total pressurebecause in addition to being exposed to the static pressure, the impact tube at the end of themanometer faces the

oncoming airstream, and therefore receives the velocity pressure energy as well. By connecting

the two manometers as shown in Figure (c), the difference between total and static pressure-the

velocity pressure-is read directly. The pitot tube (Figure below) is another air flow measuring

device that works in the same manner. The probe that is inserted in the duct has two concentric

tubes. The opening facing the airstream measures the total pressure and the concentric holes

are exposed to static pressure, so the velocitypressure is read directly. A number of readings areusually taken across the duct to get an average velocity.

4

5

6

7


6/10

6

6. PRESSURE LOSS FROM FRICTION IN PIPING AND DUCTS

We have seen from the flow energy equation that one of the effects the pump or fan mustovercome is the pressure loss due to friction. Friction is a resistance to flow resulting fromfluid viscosity and from the walls of the pipe or duct. In previous examples, we haveassumed values of friction pressure loss. Actually we must be able to calculate it. For the type offlow usually existing in HVAC systems, called turbulent flow, the pressure loss or drop due tofriction can be found from the following equation (called the Darcy-Weisbach relation):

Where:

Hf = pressure loss (drop) from friction in straight pipe or duct

f = a friction factor

L = length of pipe or duct

D = diameter of pipe or duct

V = velocity of fluid

The friction factor (f) depends on the roughness of the pipe or duct wall. Rougher surfaces willcause increased frictional resistance. This means that by using and maintaining smoothsurfaces, friction decreases and less energy is used. The other terms in the equation alsoindicate useful information. Lower velocities and larger diameters reduce Hfand therefore result in lower energy consumption. although the pipe or duct cost then increases.Although Hfcould be calculated each time from Equation 8, charts that are much easier touse and show the same information have been developed for water flow and air flow.

7. FRICTIO N LOSS FROM AIR FLOW IN DUCTSPressure loss from friction for air flow in straight round ducts is shown in Figure 8.21.It shows

this information.This chart is suitable forclean galvanized steel round ducts with about 40

joints per 100 ft, and with air at standard conditions. It can beused for the general range ofHVAC temperatures and for altitudes up to 2000 ft.

8


7/10

7


8/10

8

To find the friction loss in rectangular section ducts, Figure below must first be used. This chartshows equivalent round duct sizes. The equivalent round duct is defined as the round duct thatwould have the same friction loss as a rectangular duct found in the chart

8. ASPECT RATIOAt first consideration, it might seem that the equivalent round duct would have the same cross

sectional area as a rectangular duct for the same friction loss. This is not quite true. Arectangular duct with the same friction loss will have a greater area than a round duct. This isbecause the rectangular shape, with a greater ratio of surface to cross section, causes morefriction. This problem becomes worse as the aspect ratio increases. The aspectratio is the ratio of the dimensions of the two adjacent sides of a rectangular duct.As a general rule, the aspect ratios of rectangular ducts should be as low as possible to keepfriction losses reasonably low and thereby avoid excess energy consumption. A high. aspectratio will also mean more sheet metal and therefore a more expensive system. Unfortunately, theheight available for horizontal ducts is often limited by the clearance above hung ceilings,resulting in high aspect ratios.


9/10

9

9. PRESSURE LOSS IN DUCT FITTINGSIn addition to the pressure loss in straight lengths of duct, there is a pressure loss when the airflows through duct fittings (elbows, tees, transitions). These pressure losses, called dynamiclosses are due to the turbulence and change in direction. They can be expressed in either of twoways. One is the equivalent length method, explained later. Another procedure is called theloss coefficient method. With this method, the pressure loss through a duct (or pipe) fitting isexpressed as follows:

(9)

Below


10/10

01

10. PRESSURE LOSS AT FAN INLET AND OUTLETThere will also be a pressure loss at the fan inlet and outlet, the value of which depends on theshape of the fan-duct connection. This is called the system effect. Some values of the resultingloss coefficient C are shown in Tables. An inspection of the types of connections in Table 8.9will show the importance of considering the system effect and of installing fans with goodconnections. A list of system effects can be found in the Air Moving and ConditioningAssociation(AMCA) Manuals.

Sketch for (a) No straight inlet duct. (b) Long straight inlet duct.

Next lecture will be to design a system and calculate pressure losses and required fan.

Authentic Ductwork Calculation

Documents

Transcript of Authentic Ductwork Calculation