HVAC Lecture 2

8/10/2019 HVAC Lecture 2

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Air Conditioning Systems

(Comparison /Application /Selection)


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Air Conditioning Systems (Comparison/ Application/ Selection) 2

Contents

1. Introduction

2. System Classifications and Categories

2.1 All-Air System

2.1.1 Constant volume single zone system

2.1.2 Constant volume zoned reheat system

2.1.3 Constant volume bypass system

2.1.4 Variable volume all-air system

2.2 Air-And- Water System

2.3 All-Water System

2.3.1 Gravity convection system

2.3.2 Forced convection system (Fan coil system)

2.3.3 Ventilation systems

2.4 Unitary Refrigerant-Based System

3. Selection of Air Conditioning System

3.1 System Options Constraints

3.2 Selection Report as a Part of the Design Concept Report

3.3 Inquiries required in system selection report

References

1. Introduction

The goal of any air conditioning system is to maintain a desired

environmental condition (temperature, relative humidity, air purity, noise level,

pressurization) within a closed space. Almost in each building or application,


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there are different air conditioning system options available to the HVAC

designer to satisfy these desired environmental conditions. However,

maintaining the desired inside environmental conditions inside the space has notto be the only goal of the HVAC designer. The designer must be aware of and

account for specific goals that the owner may require other than merely

providing a desired environment. The HVAC designer is the only responsible

for considering various HVAC systems options for a certain application and

recommending the one or two systems that will perform as desired and satisfy

the specific goals of the owner and at the same time maintain the desired

environmental conditions inside the space. Then, the designer and the owner

must be collaborating to select the best design form these options. The owner

can make appropriate value judgments if the designer provides complete

information regarding the advantages and disadvantages of each option. Just as

the owner does not usually know the relative advantages and disadvantages of

different systems, the designer rarely knows all the owners financial and

functional goals. Hence, it is important to involve the owner in selecting the

system. This stage of design is called design concept stage. In this stage, the

following activities have to be carried out by the HVAC designer:

Determination of the inside design conditions suitable for the application.

Study of the architectural and the structural of the building. This stage is

important to know the available spaces and shafts and their sizes that can

be utilized by the HVAC equipment and installations.

Cooling load calculation. Rules of sump can be used in this stage. The

building cooling capacity is required to know the approximate sizing of

the HVAC equipment and installations. At the end of this stage and based

on the available building spaces for the HVAC equipment and

installations, the HVAC designer can reject many HVAC system options.


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Then each of these systems is sub classified according to the method of control

cooling in the conditioned area.

Air Conditioning Systems

All Air Air-and-Water All Water Unitary Equipment

Single zone Fan coil Fan coil units Window

Reheat Induction Unit Ventilator Through wall

Bypass Radiant Valance Unit Rooftop

Variable air vol. Two-pipe Unitary

Dual Duct Three-pipe Air Heat pumps

Multi zone Four-pipe Water Heat Pumps

2.1 AllAir System

In all-air system, complete sensible and latent cooling capacity of the

zone is removed by cold air supplied to the space. This air is centrally cooled

outside the conditioned space and supplied to the space via a duct system. No

additional cooling is required at the zone. Heating may be accomplished by the

same air stream either in the central system or at a particular zone. The heating

is accomplished in the central system if the required purpose of heating is either

winter heating of the conditioned zones or control of the RH inside the space.

The heating is accomplished at a particular zone in case of a required separate

control to the temperature and relative humidity of the space.

All-Air system can be classified as single-path systems and dual-path

systems. Single-path systemscontains the main cooling and heating coils in a

series air flow path, and use a common duct distribution system at a commonair temperature to feed all terminal air distribution devices. In a dual-path


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system, the main cooling and heating coils are in parallel flow and uses a

separate cold and warm air duct distribution systems. The use of the dual-path

system is limited due to its disadvantages of high cost of the duct system, moresizes and spaces are needed for the dual duct system and the bade energy

utilization.

Single duct system may be further classified according to the method of

control as follows:

Single Duct All-Air System

Constant Volume Variable Air Volume

Single Zone Reheat

Multiple Zoned Reheat Induction

Bypass Fan Powered

Variable Diffusers

2.1.1 Constant Volume Single zone system

This is the simplest all-air system where a supply unit is used to serve a

single temperature control zone. The zone may consist of a single room or

multiple rooms as shown in Fig. 1.


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Fig. 1 Single zone constant air volume system

Methods of Sizing

Two methods can be used to size a constant air volume system:

Zone air flow rate is calculated based on the sum of spaces airflow rates

calculated at the individual's peak load of each space (Sum of peaks).

3

AHU

R3R2R1

Space 1 Space2 Space 3

0 21


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Zone air flow rate is calculated based on the zone peak load (peak of

sums).

The second method gives smaller equipment and duct sizing for the system but

may be lead to insufficient air change per hours inside each space and high

temperature levels at the peck load time of each space.

Method of Control:

Change the supply air temperature in response to the zone load. The thermostat

is commonly located in the common return of the different spaces of the zone.

The supply air temperature is changed either by regulating the flow rate of the

cooling medium (chilled water) to the cooling unit.

Disadvantages

No separate control of the different rooms of the zone.

Additional duct clearance requirements can reduce usable floor space and

building height as a result of duct risers, fan rooms and duct work.

Longer fan operating hours are required to take care of unoccupied

periods.

Air balancing is difficult and may have to be done several times when a

common air system serves areas that are not rented simultaneously.

Accessibility to terminal devices demands close cooperation between

architectural, mechanical and structural designers.

Advantages


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Low cost of the system.

Centrally located equipment concentrate the operation and maintenance in

unoccupied areas and this permits maximum choice of filtration system,

odor and noise control level and high-quality, durable equipment.

Complete absence of conditioned area drain piping, electrical equipment

power wring and filters.

Ease of construction and operation of the system

Precise control of the temperature and humidity of the zone if the zoneconsists of single room.

The possibility of completely turning off the HVAC system of a certain

zone when required.

Adaptable to automatic seasonal changeover and to winter humidification.

Zoning of the building rooms

Two factors must take in account during grouping of multiple rooms in a single

zone:

The rooms must have similar cooling load profile along the day. This

narrows the temperature variation of each room in the zone.

The orientation and locations of the group of rooms in each zone must

lead to simple, short, ease with construction air duct routing. This reduces

the duct cost and the fan power.

Cross contaminations must be taken in account.

2.1.2 Constant Volume zoned reheat system


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High diversity of load between spaces.

The fresh air requirements are not a critical issue.

Precise control of the RH is not required.

Applications of high sensible heat ratio.

Advantages

Separate temperature control of the different rooms/zones of the building.

Smaller equipment can be used, especially when the diversity of the load

is high.

Operating cost of the variable volume systems are generally the lowest

cost as compared to the other systems. Since the volume of air is reduced

with a reduction in load, the refrigeration and fan power closely follow

the actual air-conditioning load of the building.

Efficient system for energy conservation.

No balancing is required; the system is virtually self balancing.

The system has low initial cost as compared to other systems that have

individual spaces control, because it requires only single runs of ducts and

simple control at the terminal.

Disadvantages

Dramatically Reduction of the fresh air supplied to a certain zone at part

load conditions.

Humidity control is very difficult when VAV system is used. In the

applications where the humidity level is critical, systems may have to


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limited to constant air volume. In the applications of low sensible heat

ratio, such as in conference rooms, the VAV box minimum set point is

usually limited to 50%, and reheat is added as necessary to keep humiditylevel low during part load.

Low air change per hours at part load

More complicated control system

Higher capital cost as compared to the constant air volume.

To overcome on the disadvantages of the reduction of the air change per hours atpart load, variable air volume terminal devices of different configurations are

used, including reheat, induction unit, and fan-powered systems as shown in Fig.

4.

Fig. 4 Various VAV systems

Also primary/Secondary air systems can be used to overcome on the reduction

of the percentage of the fresh air at part load. In this case the primary system

supply the conditioned outside air requirements directly to the space and the

secondary system provide additional cooling with VAV box for space load

control, see Fig. 5. Normally the second cooling coil is designed to be dry (i.e.,

sensible cooling only) to reduce the possibility of bacterial growth which can

create air quality problem.


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Fig. 5 Primary/Secondary VAV system

2.2 Air-And- Water System

In an air-and-water system, both air and water are distributed to each

space to cool the space, see Fig. 6. Both cooling and heating are carried out by

changing the air or water temperatures, or both, to control the space temperature

throughout the year. The air and water are cooled or heated in central

mechanical equipment room. The air supplied is called primary air; the water

supplied is called secondary water. Both air and waters shares the cooling load

of the building.

Fig. 6 Air-and-water system

The air side of air-and-water systems is comprised of central air-

conditioning equipment, a duct distribution system, and a room terminal unit.

The air supplied is constant volume and is called primary air, to distinguish it


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from room air, which is circulated over the room coil. It provides filtered

outdoor air for ventilation. In the cooling season, air is dehumidified in the

central conditioning unit to achieve comfort humidity conditions throughout thespace served and to avoid condensation due to normal room latent load on the

room cooling coil. The primary air normally controls space humidity. Therefore,

the moisture content of the supply air must be low enough to offset the room

latent heat gain and to maintain a room dew point sufficiently low to preclude

condensation on the secondary cooling surface. While some systems operate

successfully without a secondary coil drain system, a condensate drain is

recommended for all air-and-water system. In winter, moisture is frequently

added centrally to limit dryness. As the air is dehumidified, it is also sensible

cooled to offset a portion of the room sensible loads.

The water side consists of a pump and piping to convey water to a heat

transfer surface within each conditioned space. The heat transfer surface in the

form of a coil may be an integral part of the air terminal (as in induction units), acompletely separate component within the conditioned space (radiant panel), or

either (as can be the case in fan-coil units). The water is cooled by direct

refrigeration or by introducing chilled water from the primary chilled water

circuit; the water side is called the secondary water loop.

Method of Control

Each room temperature is controlled by varying the capacity of the coils

within the room by regulating either the water flow through it or the air flow

over it by the following techniques:

The room thermostat signal controls the chilled water control valve.

Also fan speed is used to control the room temperature. Fan speed controlmay be automatic or manual.


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Sizing Method

The quantity of primary air to each space is determined by the fresh air

requirement for this space.

The fan of the primary air AHU unit and the main duct system are sized

based on the sum of the primary air flow rates to all the spaces served by

this air handling unit.

The cooling capacity of the primary air AHU is calculated based on theair enthalpy difference across the coil. In practice to eliminate the water

condensation on the room chilled water coil, the air is strongly cooled to

make dry as possible to maintain the room dew point below the room coil

surface temperature. In this case, the primary air offset all the room latent

heat and part of the room sensible load. In this case the capacity of the

cooling coil of the terminal unit equal the room cooling load minus the

room load that was offset by the primary air system. The disadvantage of

this deep cooling of the primary air is the loss of large amount of energy

(the energy that paid in cooling the primary air) during the off position of

the room terminal unit when the room becomes empty or unused.

The chillers capacity is calculated based on the peak of sum of the entire

space load taken by the secondary units plus the load of the AHU of theprimary system.

Applications

High rise building where no space is available for large duct works.

Applications that has no enough spacing for large sizes of air handling

units if all air system is used.


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Building load rather than sum of rooms peaks determine capacity

requirements which leads to small capacity.

Low operating cost.

When all primary air is taken from outdoor, recirculation occurs only

within the room, which reduces the possibility of cross contamination.

System components are long lasting. The room's terminal units operating

dry have anticipated life of 15-25 years. The piping and ducting system

longevity should equal that of the building.

Disadvantages

The primary air supply usually is constant with no provision of shutoff.

This is disadvantages in residential applications, where tenants or hotel

room guests may prefer to turn off the air conditioning, or where

management may desire to do so to reduce operating cost.

Secondary air flow can cause the terminal units coils to become dirty

enough to affect performance. Line screen or low efficiency filters used to

protect these terminals required frequent in room maintenance and reduce

unit thermal performance.

A low chilled water temperature is needed to control space humidity

adequately and to prevent condensate on the terminal units.

Controls tends to be numerous and complex than those for many all air

systems.

The system should not be used in spaces with high exhaust requirements.

The system is inefficient if the outdoor air requirements are high.

The initial cost of the system is higher than that of the all air system.


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Humidity control is difficult.

Classifications

Air-and-water system can be classified to three types according to the terminal

unit type of the secondary water system. These types, as shown in Fig. 7, are

Air-and-water induction unit

Air-and-water fan coil unit

Air-and-water panel system

(a) Air-Water induction unit (b) Fan coil unit

Ceiling panel system

Fig. 7 Types of terminal units of air-and-water system


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2.3 All-Water System

In all water system, the room is heated or cooled by direct heat transfer

between the room circulated air and the chilled or hot water through the heat

transfer surface of the terminal unit. In this system all the room load is carried

out by the circulated water at the terminal unit. The system can be classified

according to the method of the heat transfer between the room and the circulated

water as follows,

2.3.1 Gravity Convection System

In this system hot/cold water system delivers heat/cool to a space by

water that is hotter/cooler than the air in contact with the heat transfer surface. In

this system air moves past the heat transfer surface because of the density

difference of air caused by heated or cooled surfaces. This system also transfer

large amount of heat by radiation to cold surfaces. Although this system

provides good comfort conditions, the system is suitable for heating but caution

should be exercised in their application of cooling. Examples of such systems

include the following:

Baseboard radiation

Free Standing Radiators

Wall or Floor Radiant

Bare Pipe (racked on wall)

2.3.2 Forced Convection System (Fan Coil System)


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In this system (including cooling as well as heating), the air room is

moves by fan through the room, the circulated air room is filtered and the

outside ventilation air is introduced.

Terminal units (Fan coil units) with chilled water coil, heating coil,

blowers, replaceable air filters, drain pan for condensate are designed for

this purpose.

The room fan coil units are available in many configurations to fit under

windowsills, floor mounted, ceiling mounted, and floor-to-ceiling chase

enclosed unit.

2.3.3 Ventilation Systems

Uncontrolled Local Ventilation System

In this system the outdoor air was provided to the room through the

infiltration, wall opening or window opening. In this case the amount of the

fresh air is affected by the infiltration and the stack effect. This system has many

disadvantages and does not meet the ventilation requirement of ASHRAE

Standard 62.

Central Ventilation System

In this case a central outside air pretreatment system, which maintain

fresh air at about 18-21 C, is used.

Ventilation air may then be introduced to the room either through the fan

coil unit or through a separate air outlets.

The ventilation load is carried out by this central ventilation system.


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By regulating the water flow through it by modulating the chilled water

control valve. The fan of the speed is selected high, medium or low. This

method of control leads to high RH in the space.

Two position control (on/off) of the chilled water valve with using

variable speed fan. This system gives bitter RH but it cost much.

Applications

High rise building where no space is available for large duct works

Applications that has no enough spacing for large sizes of air handling

units if all air system is used.

The system is working well in offices buildings, hotels, apartment houses

and other applications that need separate control of each space and

separate on/off control of the system for every space.

Fan coil system are used in numbers of hospitals but are less desirable

because of the low efficiency of the filtration system and difficulty of

maintaining adequate cleanliness in the space.

Applications that have great variation in the sensible heat loads.

Applications that have high diversity of loads between spaces.

Advantages

Separate temperature control of the different rooms/zones of the building

with the possibility of adjusting each thermostat for a different

temperature at relatively low cost.


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Separate heating and cooling sources in the primary air and secondary

water gives the occupant in each space the possibility of choice of heating

or cooling.

Less space is required for the distribution system.

The sizes of the central air handling units are smaller compared with those

of other systems.

The ability to shut off local terminals in unused areas.

No cross contamination from circulated air between rooms.

The system can be use for existing building retrofitting.

Ventilation air supply is positive and may accommodate recommended

outside air quantity.

Spaces can be heated without operating the ventilation air system via the

secondary water system.

Building peak load rather than sum of rooms peaks determine capacity

requirements which leads to small capacity.

Low operating cost.

Disadvantages

All-Water-System requires much more maintenance then central all air

system and this work must be done in occupied area.

Condensate pan and drain system is required and this must be cleaned and

flushed periodically, condensate disposal can be difficult and costly.

Filters are small, low in efficiency, and required frequent changing to

maintain air volume.


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Summer room humidity level tends to be relatively high

Low filtration efficiency leads to unclean coil and low performance of it.

It is also difficult to clean the coil if necessary.

The system is inefficient if the outdoor air requirements are high.

The initial cost of the system is higher than that of the all air system.

2.4 Unitary Refrigerant-Based System

Unitary air conditioning equipment is an assembly of factory matched

refrigerant cycle components for inclusion in filed designed air-conditioning

systems. Some of the many types of unitary air conditioners available include

the following characteristics:

Arrangement: single or split

Heat rejection: air cooled, evaporative condenser, water cooled.

Unit exterior: decorative for in space applications, functional for

equipment room and ducts, weather proofed for outdoors.

Placement: Floor standing, wall mounted, ceiling suspended, roof

mounted.

Indoor air: vertical up flow, counterflow, horizontal, 90 and 180 turns

or for use with forced air furnaces.

Locations:

Indoor- exposed with plenums or furred in ductwork concealed in closest,

attic, crawl spaces, basements garages, utility rooms, or equipment room,

wall built in, window.

Outdoor- roof top, wall mounted, or on ground.


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Unitary air conditioner, in contrast to room air-conditioner, includes fans

capable for operating with ductwork, although some unit may be applied with

plenum.

Central unitary air conditioner tends to serve zoned systems, with each

zone served by its own unit. For large single spaces where central systems are at

their best advantages, multiple units are often advantageous because as load

sources move within the large space, the many smaller interlocked and

independent systems have more flexibility than one central system.

Applications

Building that need air conditioning of only small numbers of rooms that

are far away from each other.

Applications that need metering and accounting of energy to each tenant.

Applications of small cooling capacities

Applications of intermitted use of the rooms.

Air conditioning of existing building.

Apartment and dormitories

Control

The control of the unitary units is on-off control to the refrigeration machine via

a thermostat in the zone or in the return air duct.


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Advantages of multiple units systems.

Simple and inexpensive individual room control.

Individual air distribution for each room, usually with convenient and

simple adjustment by the occupant.

Heating and cooling capability at all times, independent of the mode of

operation of other spaces in the building.

Individual ventilation air, normally operating whenever the conditioner

operates.

Consistent performance assured by manufacture-matched components.

Usually some space saving.

Usually lower initial cost

Only one terminal zone is affected in the even of equipment malfunction.

Usually quick availability and installation are possible.

Equipment serving spaces that become vacant can be turned off locally or

from a central point, without affecting occupied spaces.

High quality and reliability because manufacture assembles components.

System operation is simple.

Energy can be metered directly to each tenant.

Equipment locations allows for shorter duct runs, reduced duct space

requirements, and ease of service access.

Disadvantages

Operating sound level can be high


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How much area and volume from the building can be taken for the HVAC

equipment.

How the HVAC service is sale or rented to the tenant of the spaces of the

project.

What are the parameters that required to be controlled and what are

precision and accuracy of the control system.

What are the special equipments of the HVAC system, ACH, clean spaces

or not, sound level, fresh air requirements, etc.

Since these factors are interrelated, the owner and the designer must consider

how each affects the other. The relative importance of these factors differs with

different owners and often changes from one project to another.

3.1 System Option Constraints

The first step in selecting a system or rejecting other systems is to

determine and documents constraints dictated by performance, capacity,

available space and other factors. These constraints narrow the choice to systems

that can fit the applications. Examples of these constraints are:

Cooling load

Known the cooling load often narrows the choice of the system that can fit

within the available space and are compatible with the building architectural.

Zoning Requirements

The required zoning of the building, the degree of control required in each zone

and the equipment space required for individual zones also narrows the system

choice. The factors that must be taken in account in zoning a building are:


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The required special support forms the stricture for the major components.

The heavy of the major components and the stricture can hold it or not.

Space available to house the equipments and its location relative to the

occupied space.

The acceptability of the components obtruding into the occupied space,

both physically and visually.

The space available in the false ceiling to pass the duct and piping system

or inserting the terminal devices.

First and operating cost

The first and the operating cost of the HVAC system can narrow the selection of

the system. These cots must be within the budget limit of the project. The major

factors that affect the cost of each of the HVAC system are:

The operating energy source of the system; electricity or natural gas and

the price of each source, and the possibility and cost of extending these

sources to the building.

The control system, accuracy and the number of variables required to be

controlled.

Centralize the system or using individual components. The cost of a

central unit is smaller than the cost if individual units are used.

Maintenance cost of the HVAC system.

Cost of the spare parts of the system.

3.2 Selection Report as a Part of the Design Concept Report

The last step of the system selection is preparing a report, by HVAC

designer, containing a summary of the design and selection criteria. This report


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2. How much will the system cost to own compared to others considered?

What are the recovery time of the initial investment, and the future cost of

replacement equipment?

3. Will the system deliver the desired uniform temperature under varying

whether and solar conditions?

4. What are the operating costs-energy costs, maintenance, operating labor

and supplies of this system compared to the others?

5. What reliability can the owner expect compared to other systems.

6. If the system flexible enough to meet changes in the owner's needs? What

is required to add a new zone? Can it meet the increased capacity

requirements of space when new equipment (load) is added?

Finally the system selection report should conclude with a recommended

system choice along with reasons for the choice.

References

1. ASHRAE HANDBOOK, Fundamentals

2. ASHRAE HANDBOOK, HVAC Systems and Equipment

3. Harry J. Sauer and Ronald H. Howell, Principles of heating ventilation and

air conditioning, a text book supplemented to the 1989 ASHRAE Handbook-

Fundamental.

Documents

HVAC Lecture 2