Refr Sys Design

7/29/2019 Refr Sys Design

1/13

Air Conditioning System Design

For buildings air conditioning, small distributed air-con. units can be installed in the desired locations

especially in the apartment buildings. For smaller ton capacities below 5 ton, window units can be used,

and box units can be used for larger capacities. However, this system suffers from the high operation

cost with lower efficiency.Central air conditioning or better known as HVAC systems (Heating, ventilating and air conditioning)

provides much higher efficiency for large scale air conditioning.

Central HVAC Systems

HVAC central systems can be divided into three main types

All Air systems: These systems use air as the cooling/heating medium and it consists of AirHandling Units (AHU) with air ducting. These systems can be divided into single-zone ormulti-

zone systems. based on ducting design, the system can be divided into: single-ducting or multi-

ducting.

Single Ducting: in this design, the conditioning of air (i.e. hot and cold air mixing) is prepared inside

the AHU and then delivered to the building through a single duct with single return duct.

Double Ducting: in this design, two air ducts are coming out of the AHU, one is for cold air and the

other for warm air. The preparation of the conditioned of air occurs inside the space in a

mixing/control box, then returns to the AHU in a single duct.

All Water systems or Hydronic system: This system has the advantage of lower initial cost andlower space requirement since no air ducts are used. water can be cooled using water chillers or

cooling towers and for water heating, steam boilers are commonly used. hot or cold water is

distributed through the building for heating/cooling and thermal power is transferred by Fan-coil

units inside the different zones. These systems suffers from the lack in Humidity Control unless

separate air dehumidifiers are used such as desiccant system.

Air-Water systems: this design has the advantage of both systems with AHU and air ducts for somezones and water/steam piping with fan-coil units for the other zones.

When Air is used as the cooling/heating medium, two main designs can be used:

I. Constant air volume (CAV) systems: is a lower cost system with higher reliability and lowermaintenance requirement. It accomplish cooling and heating by varying air temperature while

keeping the airvolume as constant.

II. Variable air volume (VAV) system: this system provides better temperature and humidity controlwith higher energy saving especially at part loads. For different cooling/heating loads, air flow rate

is changes while keeping temperatureasconstant. Variable Air Volume (VAV) Systems. Air

flow is controlled by varying the fan speed and also by a terminal control box (VAV box) at the

different zones. VAV box dampers opens and closes via a thermostat control inside the space.

Central HVAC Air-System Components

Min components of air HVAC systems are the AHU's and air ducting.

First: Air Handling Units (AHU)

AHU is a device used to prepare and circulate the conditioned air to the building.

The common sections inside AHU are:


2/13

1. Fan section:

Air handlers typically employ a large squirrel cage blower driven by an AC electric motor. The blower

may operate at a single speed in (CAV) systems or driven by a Variable Frequency Drive to change the

blower speed in (VAV) systems. For large blowers, motors are commonly connected with the blower

through belting since large blowers operates usually below 1500rpm especially in centrifugal blowers. Inlarge AHU's, multiple blowers are used, typically placed at the front of the AHU known as supply fans,

and augmented by fans in the return air duct known as return fans ,pushing the air into the mixing

chamber.

Fan or blowers can be divided into two main types:

(1) axial fans: air flow passes axially (parallel to the fan axis) and it can be calcified into three main

types:

Propeller fan: the impeller with a multi-blade is not contained in a tube (or casing). Propeller fans arelow-pressure, high-capacity units.

Tube-axial fan: the impeller is placed within a cylinder or tube. Tube-axial fans can be used on low-and medium-pressure, small scale systems.

Vane-axial fans are similar to the tube-axial fans but with guide vanes that straighten out the axialspiral airflow for higher pressure outlet.

(2) Centrifugal fans: air flow is directed outward (radially) from the fan wheel. These fans are able to

move more air at higher pressures and with less noise than axial fans and are most commonly used in air

handling units.

The three principal types of blades are the forward-curved blade, the radial blade, and the backward

inclined blade.

backward-inclined blade fan wheel, the tip of the blade is inclined backwards away from the rotationdirection. This lets the fan to move air at higher pressures compared to forward-curved fan at higher

efficient (less horsepower) for most air volume and pressure ranges. They are commonly used for

medium- and high-pressure systems.

Forward-curved blade fans are used primarily for the low-pressure HVAC applications.Radial blade fan wheel, the tip of the blade projects straight out from the fan shaft. The radial blade fan

can work at a higher pressure than either the forward-curved or the backward-inclined blade fans.

However, to move the same amount of air as the other two types, the radial fan wheel requires morehorsepower. Due to its low efficiency, it not commonly used in HVAC applications.


3/13

Fan power can be calculated using the following equation:

Air flow rate for commercial fans is measured in cubic foot per minute (1CFM = 1.699m3/h).

2. Coils & Humidifier section.Based on the required application, AHU may need to provide heating/cooling and humidification to

change the supply air condition. Commercial AHU contain coils that circulate hot water or steam for

heating , and chilled water of Freon for cooling and dehumidification. However, for humidification,

additional units are required using steam or water injection. AHU's can be designed for all-year-around

system with both summer and winter air conditioning equipment.

3. Mixing chamberIn order to maintain indoor air quality, air handlers commonly have provisions to allow the introduction

of outside air into, and the exhausting of air from the building. Mixing the right amount of cooler outside

air with warmer return air can be used to approach the desired supply air temperature and increase

system efficiency by reducing power consumption of air heaters. The mixing box (sometimes known as

mixing Plenum Box) has dampers to control the fresh, circulated and exhausted air percentage and the

dampers can be automatically controlled based on temperature, humidity and CO2% (in some modern

systems).

In some designs, return air is not to be mixed with fresh air due to contamination issues, thus, A heat

recovery heat exchanger, may be fitted in the AHU instead of the mixing box.

4. Filter section.Air filtration is always employed with AHU's in order to provide clean dust-free air to the building

occupants. Based on shape, filters can be: a frame-type that can range from low efficiency pre-filters toHigh Efficiency Particulate Air (HEPA) and a Bag ofPocket- filters. In normal HVAC applications,

allowed particulates sizes are in the range 5-0.5 microns in diameter, and filters below 95% efficiency

are sufficient. However, special care has to be paid for some applications such as pharmaceutical or high

sensitive electronic industries and intensive care health centers, Thus, HEPA filters with 99.9% and

0.1m can be implemented but with higher pressure drop through the filters.

The induced fresh air into AHU or inlet air duct should pass through a coarse air pre-filter of low

efficiency (below 50%) to prevent large particles from interring the system.

Electrostatic precipitators are also used for air filtration by ionizing suspended particles in the air. Very

low pressure drop occurs through the filter but with certain electrical consumption.


4/13

Pressure Drop in the Ducting System:

Total pressure drop through the ducts is calculated by adding the total pressure drop calculated by

Bernoulli equation between sections (1-2) and the pressure drop through thermal gravity effect (Pse)

pressure drop from Bernoulli equation is presented by:

Static pressure (Ps) and it can be measured experimentally by U-tube manometer. Dynamic or velocity pressure ( ). Pressure head ( ), resulted from potential energy difference. It is neglected for gases.

pressure drop/gain by thermal gravity effect

is calculated from

The draft (pressure difference) is caused by

the difference in air density. For hot flow,

air is drifted upwards, thus, if the air stream

is upwards, then the draft will be added as

pressure gain (not drop). For cold coils,

natural draft is pushed downwards.

The figure below illustrates pressure drop/rise inside ducts. If pressure gauge of U-tube manometer was

used to measure air pressure inside duct, the reading will indicate the static pressure only, while dynamic

pressure can be determine from the known air velocity. Reducing the flow cross-section area will drop

the static pressure but dynamic pressure will increase due to velocity elevation and vice-versa for flow

area increment.


5/13

The pressure drop due to friction losses is calculated from the following equation:

The following chart is used to determine the regtangular duct size from calculated cicular duct diameter.

DUCT SIZE DESIGN METHODS

In this course, four duct design methods will be discussed. Constant pressure and velocity methods are

simple and adequate for many application, but on the other hand are non-balancing methods, thus,

require balancing dampers through the system. Other methods such as static pressure regain and T-

method are self-balancing and optimistic methods and do not require control dampers (theoriticallyspeaking). The methods are:

A. Constant Pressure Loss or Equal Friction Method.B. Constant velocity Method .C. Static Pressure Regain or Recovery Method.D. T-method.

First: Constant friction loss (Pressure) method

The equal friction method, as its name implies, is based on maintaining the same pressure drop per unit

of duct length (or friction rate) throughout the system.The duct size is based on the flow rate through a

particular section of duct, and design value for the friction rate. The shaded area of the friction chart

below is the suggested range of friction rate (pa/m) and air velocity. When energy cost is high and

installed ductwork cost is low, a low friction- rate design is more economical. For low energy cost and


6/13

high duct cost, a higher friction rate is more economical. Each section is sized using the fixed (designed)

friction rate, and the total pressure drop for each run is simply the sum of the pressure drop of each

individual section, that includes straight duct friction loss, pressure losses through fittings such as

elbows, takeoffs, etc. The unit fan speed is selected to provide the design (CFM) and produce enough

pressure difference to overcome pressure losses in the supply and return branches having the greatest

pressure drop. Note that duct systems designed using the equal friction method is not self-balancing.

Balancing dampers must be installed in lower pressure loss branches to balance the system.

Principle design variables are the design velocity (chosen for noise control) or the design friction loss

(pa/m). To avoid disturbing noise levels, maximum velocities should be kept within experienced limits:

Comfort systems - air velocity 800 to 1400 fpm (4-7m/s). Industrial systems - air velocity 1500 to 2400 fpm (8-12m/s). High speed systems - air velocity 2000 to 3600 fpm (10-18m/s).This is one of the simplist design methods, however, it may increase the number of reductions compared

to other methods, and often requires adjusting dampers resulting in more system cost compared to other

methods. This method is well suited for CAV systems, but in VAV systems, dampers have to be

rebalanced every time the system goes in part load.

Second: Constant Velocity Method

The constant velocity method of calculation is most suited for dust and fume extraction ruturn (exhaust)

ducting systems where the object is to entrain dust particles in the air stream. The velocity must be

maintained at a sufficiently high level to carry the particles to a collector receptacle. The velocity is then

reduced to a level which will no longer support or suspend the particles and they then simply drop out of

the air stream for collection.Proper air flow velocities for the application considering the environment are selected. Sizes of duct

(cross section area) is then given by the continuity equation:


7/13

A proper velocity will depend on the application and the environment. The table below

indicate commonly used velocity limits.

Third: Static Regain Method

This method refers to increase or regain of static pressure in the ductwork when the air velocity

decreases. The Static Regain method of duct sizing is based on Bernoulli's equation, which states thatwhen a reduction of velocities takes place, a conversion of dynamic pressure into static pressure occurs.

With this method, the air speed in the duct is reduced near each branch or diffuser so that the dynamic

pressure conversion obtained exactly balances the pressure drop of the air in the trunk of the next duct.

This means there is the same static pressure near all the branches and all the diffusers, thereby obtaining

an intrinsically balanced air distribution system without having to use throttling devices. Compared to

the two previous methods, this method usually involves a larger surface area of the panels, but lower

electric fan power and easier balancing of the plant. For complex plants, it may be advisable to apply

two methods simultaneously; the constant pressure loss method for sizing the main trunk, with insertion

of adjustment air locks on the branches; the static pressure recovery method for sizing the branches fitted

with terminals to obtain the same operating pressure in the latter.

For static pressure regain calculations, pressure drop equation is rearranged by setting ps,2 equal to

ps,1 (neglecting thermal gravity effect term). This means that the change in static pressure from one

section to another is zero, which is satisfied when the change in total pressure is equal to the change in

velocity pressure. Thus,

and

where pt,1-2 is total pressure loss from upstream of junction 1 to upstream of junction 2. For each

main section, the straight-through and branch sections immediately downstream of the main duct section

are determined by iteration of that sections size until new pressure drop equation is satisfied. However,

there could be cases when the straight or branch sections need to be larger than the upstream section to

satisfy the equation. However, making downstream sections larger than upstream sections is not

practical. The largest straight-through or branch size should be limited to that of the upstream section.The imbalance that occurs is resolved during total pressure balancing of the system.

To start system design, a maximum velocity is selected for the root section (duct section downstream of

a fan). The shaded area on the friction chart (shown earlier) is the suggested range of air velocity. When


8/13

energy cost is high and installed ductwork cost is low, a lower initial velocity is more economical and

Vice versa. Because terminal sections often require additional static pressure to operate VAV terminal

boxes properly, that static pressure requirement is added into the section after it is sized using static

regain to avoid the downstream section being larger than the upstream section. For calculating duct

sizes, the pressure losses of CAV terminal boxes, grilles, diffusers, etc. should be included in the sizing

iterations. After completing duct sizing by the static regain method, any residual unbalance can be

reduced or eliminated by calculating the systems total pressure (pressure required in the critical paths)

and changing duct sizes to increase the paths total pressure to approximate what is needed in the critical

paths.

Fourth: T-Method

T-method optimization (Tsal et al. 1988) is a dynamic programming procedurebased on Bellmans

(1957) tee-staging idea.

T-method simulation determines the flow in each duct section of an existing system with a known

operating fan performance curve. The simulation version of the T-method converges very efficiently

with only few iterations that are sufficient to obtain a solution with a high degree of accuracy. Ductwork

sizes are determined by minimizing the objective function:E = Ep (PWEF) + Es

where

E = present-worth owning and operating cost

Ep = first-year energy cost

Es = initial cost

PWEF = present worth escalation dimensionless factor (Smith 1968),

The objective function includes both initial system cost and present worth of energy. Hours of operation,

annual escalation and interest rates, and amortization period are also required for optimization. The

following constraints are necessary for duct optimization: Continuity. For each node, flow in equals flow out.

Pressure balancing. Total pressure loss in each path must equal fan total pressure; or, in effect, at any

junction, total pressure loss for all paths is the same.

Nominal duct size. Ducts are constructed in discrete, nominal sizes. Each diameter of a round duct or

height and width of a rectangular duct is rounded to the nearest increment, usually 25 or 50 mm, or

according to ISO standards where applicable. If a lower nominal size is selected, initial cost decreases,

but pressure loss increases and may exceed the fan pressure. If a higher nominal size is selected, the

opposite is true: initial cost increases, but section pressure loss decreases. However, this lower pressure

at one section may allow smaller ducts to be selected for sections that follow. Therefore, optimization

must consider size rounding.

Air velocity restriction. Maximum allowable velocity limitation (ductwork regenerated noise).

Construction restriction. Architectural limits may restrict duct sizes. If air velocity or construction

constraints are violated during an iteration, a duct size must be calculated.

HVAC Duct Design Procedures

The general procedure for HVAC system duct design is as follows:

1. Study the building plans, and arrange supply and return outlets to provide proper distribution of air ineach space. Adjust calculated air quantities for duct heat gains or losses and duct leakage . Also,

adjust supply, return, and/or exhaust air quantities to meet space pressurization requirements.


9/13

2. Select outlet sizes from manufacturers data.3. Sketch the duct system, connecting supply outlets and return intakes with the air-handling units/air

conditioners.

4. Divide the system into sections and number each section. A duct system should be divided at allpoints where flow, size, or shape changes. Assign fittings to the section toward the supply and return

(or exhaust) terminals.

5. Size ducts by the selected design method. Calculate system total pressure loss; then select the fan(refer to Chapter 20 of the 2008 ASHRAE HandbookHVAC Systems and Equipment).

6. Lay out the system in detail. If duct routing and fittings vary significantly from the original design,recalculate pressure losses. Reselect the fan if necessary.

7. Resize duct sections to approximately balance pressures at each junction.8. Analyze the design for objectionable noise levels, and specify lined duct, double-wall duct, and

sound attenuators as necessary. Refer to the section on System and Duct Noise.

Air Duct Configuration

There are several basic types of supply and return duct systems. Any one of the system types, or a

combination of different types, can be utilized to fit the needs of a particular structure. Some of the

general duct configurations are listed below:

A. Radial systemB. Extended plenum systemC. Reducing plenum systemD. Reducing trunk duct systemE. Perimeter loop system.

First: Radial Duct System:

The radial duct system in its simplest form consists of a central supply plenum that feeds a number of

individual branch ducts arranged in a generally radial pattern. It also can be designed and sized so that

each individual duct leaving the plenum can divide to two or more supply outlets. This is frequently the

case because of the number of supply outlets required to condition the structure. The direct ducting

maximize air flow (with lower pressure drop),and this system is most adaptable to single-story homes.

Traditionally, this system is associated with an air handler that is centrally located so that ducts are

arranged in a radial pattern. However, symmetry is not mandatory, and designs using parallel run outs

can be designed so that duct runs remain in the conditioned space.


10/13

Second: Extended Plenum System:

The extended plenum duct system generally consists of one or two boxlike pieces of ductwork extending

from the main plenum at the indoor unit. This extended plenum has the same dimensions (height and

width) from the starting collar to the end of the run. Branch runs to feed the supply outlets are tapped

into the extended plenums. The best results are achieved when the maximum length of the extended

plenum is not greater than 24 ft from the AHU. If two plenums are used, this total length can be

extended to 48 ft. If longer distances are required based on the physical layout of the structure,consideration should be given to using other designs such as the reducing plenum or the reducing trunk

duct system. Another downside of the system is lower flow rates at the first branches (the branches

closest to the indoor blower) due to the higher velocities in the plenum.

Third: Reducing Plenum System:

The reducing plenum duct system can be used when the physical size or layout of the structure requires

greater distances than the length constraints imposed on the extended plenum (24 ft). Simply, when the

air velocity lost to the branch runs reaches approximately 50% (known as 50% rule), the plenum size is

reduced to regain the velocity in the remaining portion of the plenum. This reduction also improves the

air flow characteristics at the branch ducts that are closest to the air-handling unit. This system is

relatively easy to fabricate and install. Additional sheet metal sometimes is required to build the system,

but if done correctly it can deliver good results. It may be necessary to balance the system branch

dampers properly.


11/13

Forth: Reducing Trunk System

It is very similar to the reducing plenum system, with the exception that the trunk run is reduced in

cross-sectional area after each branch takeoff. These multiple reductions make it possible to maintain

more uniform pressure and/or air velocity in the trunk, which improves air flow in branches for constant

velocity or pressure in the trunk. This type of system generally takes more sheet metal to build and

requires more labor to fabricate and install. Another major concern is that there are more joints to seal

(to prevent air leakage). The reducing trunk system also can be applied using lengths of round duct andmanufactured fittings. Round duct systems can significantly reduce the cost of labor for fabrication and

installation. The reduced trunk system can be used very successfully in a structure that spreads out in

two or more directions. A properly designed reducing trunk system represents the ultimate in an

engineered duct system with each portion of trunk duct specially sized so that the trunk is

proportionately reduced after each branch takeoff. The system is well-balanced since each branch is

specifically engineered but is more complex and costly than other standard duct systems because of

custom nature of design, ductwork and installation.

Fifth: Perimeter loop system

The perimeter loop duct system is well-suited for buildings that are constructed using concrete-slab-on-

grade with no space between the ground and the structure. This type of construction is most often seen in

warmer climates, where ground freezing and thawing is less of a concern and where there is no need for

heat ducting underneath the floor. It generally performs better than the radial system in such

applications. However, the perimeter loop system does have the disadvantage of being a little more

difficult to design and more expensive to install. It is basically laid out around the perimeter of the

structure next to the edge of the slab. The entire perimeter loop is the same size duct. The loop is fed by

four or more ducts radiating out from the central plenum. They are usually the same size as the loop

duct. The boot boxes are sized to deliver the proper flow rate to each room of the structure.


12/13

Heat Exchanger Design Methods

Many type of heat exchangers are used in air conditioning systems, the main heat exchanger design

methods are:

First method: , C*, NTU:

NTU is the number of transfer units. The heat capacity rate for tube side Ct and shell side Cs both can be

calculated using the general heat capacity equation:

C (W/K) =.

m . Cp

The smaller heat capacity rate for either shell or tube sides is known as: Cmin ; and the larger as Cmax.

The heat capacity ratio C* can be calculated using equation:

max

min

C

C*C

The maximum temperature difference (Tmax) can be calculated using equation:

T max = T tube,inTshell,in

Since all thermal power lost from one side is gained by the other side, thus:

ttss TCTCQ

In order to measure the thermal performance of the heat exchanger, effectiveness () is used. It is defined

as the ratio of the actual heat transfer rate through the heat exchanger to the maximum possible heat

transfer rate. It is calculated using equation:

maxminmaxminmaxTC

TC

TC

TC

Q

Q ttss

After that, the number of transferred units is determined charts. And with known overall heat transfer

coefficient (U), the heat exchanger surface area (A) is calculated using equation:

U

NTUCA

.min

Second Method: P, R, NTU:

Fluid 1 is usually taken for shell side and fluid 2 is tube, the temperature effectiveness for tube (P1) can

be calculated for tube side using equation:

max

,,

1T

TTP

isos

The heat capacity rate ratio (R1) can be calculated using equation:

t

s

C

CR

1


13/13

After that, the number of transferred units for fluid 1 (NTU1) is determined from P1, R1 charts. And

with known overall heat transfer coefficient (U), the heat exchanger surface area (A) is calculated using

equation:

U

NTUCA

1.1

Third Method: MTD

This is the mean temperature difference method. The log-mean temperature difference (LMTD=TIm). It

is calculated using the following equation for the cross flow arrangement:

II

I

III

TT

TTT

lnIm

TI and TII are the temperature difference at each end. If we assume counter flow arrangement, then

the equations are:

TI = T t,in -T s,out

TII = Tt,out -T s,in

The surface area (A) of the heat exchanger is calculated using:

Im.. TFUQ

A

Where (F) is the mean temperature difference correction factor.

Refr Sys Design

Documents

Transcript of Refr Sys Design