HEATING, VENTILATION

AND AIR CONDITIONING

SYSTEMS

"Heating, Ventilating and Air Conditioning - Analysis and Design," 6th Edition, F.C. McQuiston, J. D. Parker and J.D. Spitler, John Wiley & Sons, 2000.

This material is reproduced with permission of John Wiley & Sons, Inc.

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Dr. Suraj JOSHI

Chapter 1: Introduction

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Chapter 2: Air-conditioning Systems

Chapter 4: Comfort and Health - Indoor Air

Quality (Homework reading)

Chapter 1: Introduction

• Year round control of indoor environment required

• Temperature, humidity and air quality control

• Examples:

• Manufacturing or printing plant

• Electronics laboratory

• Food processing factory

• Large office complex

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What is air conditioning?

Definition by Wills Carrier

Air conditioning refers to the control of temperature,

moisture content, cleanliness, air quality and air circulation

as required by occupants, a process, or a product in the

space.

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Abbreviations

• HVAC: Heating, Ventilation and Air Conditioning

• ASHRAE: American Society of Heating, Refrigerating and

Air Conditioning Engineers, Incorporated

• ESCO: Energy Services Company

• BAS: Building Automation Systems

• WACS: Web-Accessible Control Systems

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ASHRAE Handbooks

• Because of the wide scope and diverse nature of HVAC, literally thousands of engineers have developed the industry.

• Their accomplishments have led to selection of material for the ASHRAE Handbooks, consisting of four volumes

(i) HVAC Systems and Equipment,

(ii) Fundamentals,

(iii) Refrigeration, and

(iv) HVAC Applications

• Research, manufacturing practice, and changes in design and installation methods lead to updating of handbook materials on a four-year cycle.

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Common HVAC Units and Dimensions

• gpm (gallons per minute) for liquid volume flow rates

• cfm (cubic feet per minute) for air volume flow rates

• in.wg (inches water gauge) for pressure measurement in

air-flow systems

• ton (12,0000 Btu per hour) for the description of cooling

capacity or rate

• ton-hour (12,000 Btu) for cooling energy

• J (Joule equivalent) = 778.28 ft-lbf/Btu

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Energy vs. Power

• Power is the rate at which energy is produced or consumed

• Electrical power (KW) depends on the size

• Size is also called capacity, or load, or demand

• Large users of electricity are almost always charged during certain months for the maximum rate at which energy is used (maximum power) during defined critical periods of time

• This is in addition to the charge for the amount of energy used

• Demand charge is the charge for maximum power or rate of use

• Peak demand period is the critical period when demand charges are the highest

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• Fig. 1.1

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Fundamental physical concepts

• First law of Thermodynamics leads to energy balance

• In some cases, balance is on closed system or fixed mass

• Closed systems are able to exchange energy (heat and work) but not

matter with their environment.

• Often this involves a control volume, with the balance on mass

flowing in and out considered along with energy flow

A system is separated from its

surroundings by a boundary which may

be notional or real, and delimits a finite

volume

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Heating

• Heating is performed to

(a) bring a space up to a higher temperature than existed

previously; or

(b) replace the energy being lost to colder surroundings to

maintain a desired temperature range

Heat transfer that is manifested solely in raising or

maintaining the temperature of the air is called sensible

heat transfer.

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Furnace, air handler, zone

• In a furnace, air is heated directly by hot combustion

gases obtained from the burning of some hydrocarbon

fuel (like natural gas or fuel oil)

• An air handler typically contains heating and/or cooling

coils, fans for moving the air, and filters

• A zone is a region of a building controlled by an individual

thermostat

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In a blow-through type, the fan pushes the air through the coil or coils

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In a draw-through type, the fan is downstream of the coil and is pulling the air

through the coil.

• These typical air-handlers might furnish air to several zones, the regions of the building that are each controlled by an individual thermostat.

• One or more air handlers might furnish all of the air needed for space conditioning on one floor, or for several adjacent floors in a multistory building.

• Heating water might be piped from boilers located in the basement to mechanical rooms containing air handlers located on conveniently spaced floors of a high-rise building.

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Governing equation

Specific heat of air cp = 0.24 Btu/(lbm-F) = 1.0035 J/g-K

Specific heat of water cp = 1 Btu/(lbm-F) = 4.18 J/(g-K)

Cooling

• Cooling is the transfer of energy from a space, or from air

supplied to a space, to make up for the energy being

gained by that space.

• Energy gain comes from warmer surroundings, or

sunlight, or from internal sources

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Heat removal problem

• Air at 1 atm and 760F is flowing at the rate of 5000 cfm. At

what rate must energy be removed, in Btu/hr, to change

the temperature to 580F, assuming that no

dehumidification occurs?

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A damper is a valve or plate that stops or regulates the flow of air inside a duct or other air handling equipment.

A thermostat is a device that automatically regulates temperature, or that activates a device when the

temperature reaches a certain point.

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•Both the cooling and the heating coils might be installed in a typical air handler.

•Placed in series in the airstream, as the coils could provide either heating or

cooling but not both at the same time.

•Placed in parallel, the coils would be capable of furnishing heating for one or

more zones while furnishing cooling for other zones.

Humidification and Dehumidification

• Humidification is increasing the amount of water vapor in the airstream for maintaining desired humidity levels in a conditioned space

• Dehumidification is reducing the amount of water vapor in the airstream for maintaining desired humidity levels in a conditioned space

• Latent heating is the energy involved in moisture addition (humidification) only.

• Latent cooling is the energy involved in moisture removal (dehumidification) only.

• Total heating/cooling provided by a coil

= Sensible heating/cooling + Latent heating/cooling

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Latent energy in humidifying/ dehumidifying process

For water, enthalpy of vaporization = 1061 Btu/lbm = 40.65 kJ/mol = 2257 kJ/kg

The enthalpy of condensation (or heat of condensation) is by definition equal to the enthalpy of vaporization with the opposite sign

Enthalpy changes of vaporization are always positive (heat is absorbed by the substance)

Enthalpy changes of condensation are always negative (heat is released by the substance)

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Chapter 2: Air-conditioning Systems

THE COMPLETE SYSTEM

• In all-air heating and cooling systems, both energy and ventilating air are carried by ductwork between the furnace or air handler and the conditioned space.

• The all-air system may be adapted to all types of air-conditioning systems for comfort or process work.

• It is applied in buildings requiring individual control of conditions and having a multiplicity of zones, such as office buildings, schools and universities, laboratories, hospitals stores, hotels, and ships.

• All-air systems are also used for any special applications where a need exists for close control of temperature and humidity, including clean rooms, computer rooms, hospital operating rooms, and factories.

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Schematic showing the major elements bringing energy to or removing energy from the

airstreams passing through air handlers, typical of the central all-air commercial HVAC systems

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• A fluid, usually water, carries energy away from the cooling coil

(heat exchanger) in the air handler to a chiller or chillers

• Chillers remove energy from that liquid, lowering its

temperature, so that it can be returned to the air handler for

additional cooling of the airstream

• Energy removed by the chiller is carried by water through

piping to a cooling tower, or the chiller may be built into or have

a remote air-cooled condenser

• Since water can transport relatively large amounts of energy

economically, chillers and cooling towers may be located

remotely from the individual air handlers

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Energy and fluid flow

• Centrifugal pumps are most often used to circulate the liquid through the piping.

• Cooling towers and condensers are located outdoors on the ground or on the roof, where the energy can ultimately be rejected to the atmosphere.

• The net flow of energy in cooling a space is from the space through the return duct to the air handler to the chiller and then to the cooling tower, where it is rejected to the atmosphere.

• In the case of space heating, a fluid brings energy from a boiler to the air-handler heating coil.

• The fluid is usually hot water or steam. Alternatively, the water circulating to the air handler may be heated using boiler steam.

• The steam-to-water heat exchanger used for this purpose is called a converter.

• The fuel for the boilers may be natural gas, Liquefied Petroleum Gas (LPG), fuel oil, or a solid fuel such as coal or wood.

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System selection and arrangement• Large buildings with variable needs in different zones can be served well with a

central system, in which most HVAC equipment is located in one or more

mechanical rooms.

• The energy and moisture addition or removal, the ventilation, and the removal of

pollutants can be accomplished by the equipment in the mechanical room, which

are normally outside the conditioned area, in a basement, on the roof, or in a

service area at the core of the building.

• Mechanical rooms reduce the noise, spills, and mechanical maintenance that

might otherwise occur in occupied spaces.

• Plenum systems make use of the existing ceiling cavity space to collect the return

and relief air, and thereby reduce construction costs and minimize coordination

problems

• Relief or flue air is exhausted out of the building as required for ventilation

purposes, building pressure control purposes, or to control contamination from

processes.

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Equipment normally found in the central mechanical room includes:

• Fans or air handlers for moving air with associated dampers and filters

• Pumps for moving heated or chilled water and appropriate control valves

• Heat exchangers for transferring energy from one fluid stream to another

• Flow measuring and control devices

• Chillers and furnace or boiler equipment

• Where cooling must be furnished to building spaces, there must always be some way to

reject the energy to the surroundings.

• Lakes and rivers are sometimes used as an energy sink.

• Where energy exchange is direct from the refrigerant to the air, the outdoor unit is simply

called the condensing unit.

• A zone is a conditioned space under the control of a single thermostat.

• A thermostat is a control device that senses the space temperature and sends a

correcting signal if that temperature is not within some desired range.

• Sometimes zone humidity may also be controlled by a humidistat.

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Central plant

• Large installations such as college campuses, military

bases, and research facilities may best be served by a

central station or central plants, where chillers and boilers

provide chilled water and hot water or steam through a

piping system to the entire facility, often through

underground piping.

• Diversity factor is the ratio of the actual maximum demand

of a facility to the sum of the maximum demands of the

individual parts of the facility.

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• As the distance over which energy must be transported increases, the cost of moving that energy tends to become more significant compared to costs of operating the chillers and boilers.

• As a general rule, smaller systems tend to be the most economical if they move the energy as directly as possible.

• For example, in a small heating system the air will most likely be heated directly in a furnace and transported through ducts to the controlled space.

• Likewise, in the smaller units the refrigerating system will likely involve a direct exchange between the refrigerant and the supply air (D-X) System.

• Installations where energy must be moved over greater distances, liquid (or steam) transport system will probably be used.

• Water with a high specific heat and density, and steam, with a high enthalpy of vaporization, can carry greater quantities of energy per unit volume than air.

• Not only can pipe sizes be much smaller than ductwork, but the cost of power to move steam or liquid is much less than that for air.

• The transfer of energy from fluid to air involves extra heat exchangers and drops in temperature, which are not required in the direct exchange from refrigerant to air or from combustion gases to air.

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Types of all-air systems

• An all-air system is one in which everything required in the conditioned space – heating, humidification, cooling, dehumidification – may be furnished to the space by air

• Some systems require no heating and some require only perimeter heating by baseboard, reheat coils, or radiant panels. It is common to refer to cooling systems with such heating provisions as all-air systems

• In most large commercial systems, liquid is used to transfer energy between the boilers or furnaces and chillers and the air handlers

• However, it is air that transfers the energy and the ventilation between the air handlers and the conditioned spaces

• The ductwork arrangement between the air handler and the conditioned space determines the type of an all-air system

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Single-Zone System• The simplest all-air system is a supply unit (air handler) serving

a single zone.

• The air-handling unit can be installed either within a zone or remote from the space it serves and may operate with/without ductwork.

• A single-zone system responds to only one set of space conditions.

• It is limited to applications where reasonably uniform temperatures can be maintained throughout the zone.

• Next slide shows a schematic of the air handler and associated dampers and controls for a single-zone constant-volume all-air system.

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• In this system, the room thermostat maintains the desired temperature in the zone by controlling the temperature of the air being supplied to the zone.

• The discharge thermostat takes a signal from the zone thermostat and opens/closes the appropriate valve on heating/cooling coil to maintain desired room temperature.

• Because the heating valve is normally open (NO) and direct acting (DA) and the zone thermostat is direct acting, an increase in room temperature will cause the hot water valve to close to a lower flow condition.

• The cold water valve will be closed as long as there is a call for heat.

• When cooling is required, the hot water valve will be closed and the cooling water valve will respond in the proper direction to the thermostat.

• The outside dampers have a motor to drive them from a closed position when the fan is off to the desired open position with the fan running.

• The dampers in the recirculated airstream are manually adjustable in this case. They are often set to operate in tandem with the outside air dampers and with the exhaust or relief dampers should they be present.

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Functioning of the single-zone system

Reheat System• Reheat system is a modification of the single-zone constant-volume system.

• Its purpose is to permit zone or space control for areas of unequal loading, or to provide heating or cooling of perimeter areas with different exposures.

• It is an excellent system in which low humidity needs to be maintained.

• As the word reheat implies, the application of heat is a secondary process, being applied to either preconditioned (cooled) primary air or recirculated room air.

• A single low-pressure reheat system is produced when a heating coil is inserted in the zone supply.

• The more sophisticated systems utilize higher pressure duct designs and pressure-reduction devices to permit system balancing at the reheat zone.

• The medium for heating may be hot water, steam, or electricity.

• Conditioned air is supplied from a central unit at a fixed cold air temperature sufficiently low to take care of the zone having the maximum cooling load. The zone control thermostats in other zones activate their reheat units when zone temperatures fall below the desired level.

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Variable-Volume System (VAV)

• The Variable-volume system compensates for variations in cooling requirement by regulating (throttling) the volume of air supplied to each zone.

• Air is supplied from a single-duct system and each zone has its own damper.

• Individual zone thermostats control the damper and the amount of air to each zone.

• Some VAV systems have fan-powered terminal units.

• In fan-powered units, as air flow is reduced from the main duct by damper action, more return air from the room is drawn into the box by the fan and mixed with the primary cold air supply to give a constant air flow into the room.

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Dual-Duct System• In dual-duct (double-duct) system, the central equipment

supplies warm air through one duct run and cold air through another.

• Temperature in an individual space is controlled by mixing the warm and cool air in proper proportions.

• Some form of regulation is incorporated into the system to maintain a constant flow of air.

• Without this regulation, the system is difficult to control because of the wide variations in system static pressure that occur as load patterns change.

• This system provides great flexibility in satisfying the loads and in providing prompt and opposite temperature response.

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Multizone System• Multizone central units provide a single supply duct for each zone

• Zone control is obtained by mixing hot and cold air at the central unit in response to room or zone thermostats.

• Air for each zone is at the proper temperature to provide zone comfort as it leaves the equipment.

• The system conditions zones by a blow-through arrangement having heating and cooling coils in parallel downstream from the fan.

• The use of multiple duct runs and control systems can make initial costs of this system high compared to other all-air systems.

• Obtaining very close control of this system may require a larger capacity in refrigeration and air-handling equipment, increasing both initial and operating costs.

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Air-and-water systems

• In all-air systems, the spaces within a building are cooled solely by air supplied from the central air-conditioning equipment.

• In an air-and-water system, both air and water are distributed to each space to perform the cooling function.

• Cooling water is furnished to carry away most sensible energy from the conditioned space.

• Because water has a larger cp and r compared to air, space

required for distribution pipes is much less than that required for ductwork to accomplish the same cooling task.

• Consequently, less building space needs to be allocated for the HVAC distribution system.

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All-Water Systems

• All-water systems are those with fan-coil, unit ventilator, or

valance-type room terminals, with unconditioned ventilation air

supplied by an opening through the wall or by infiltration.

• Cooling and dehumidification are provided by circulating chilled

water or brine through a finned coil in the unit.

• Heating is provided by supplying hot water through the same or

a separate coil using water distribution from central equipment.

Electric heating or a separate steam coil may also be used.

• Humidification is not practical in all-water systems unless a

separate package humidifier is provided in each room.

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Heat pump systems

• Any refrigeration system is a heat pump in the sense that energy is moved from a low temperature source to a higher temperature sink.

• In HVAC, the term heat pump often defines a system in which refrigeration equipment is used to both heat and cool.

• The thermal cycle is identical to that of ordinary refrigeration; however, in most heat pump systems a reversing valve permits flow reversal of refrigerant leaving the compressor such that the evaporator and condenser roles are switched.

• In some applications both the heating and cooling effects obtained in the cycle can be utilized at the same time. Tremendous energy savings can occur since the heat pump often provides more energy for heating than is required to operate the system.

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Closed-loop and ground-coupled systems• In some cases a building may require cooling in interior zones while

heating in exterior zones. The needs of the north zones of a building may also be different from those of the south.

• In such cases, a closed-loop heat pump system may be a good choice.

• In the ideal case, the loads from all zones will balance and there will be no surplus or deficiency of energy in the loop.

• If cooling demand is such that more energy is rejected to the loop than is required for heating, the surplus may be rejected to the atmosphere by a cooling tower.

• In case of a deficiency, an auxiliary boiler may make up the difference.

• The earth itself is a near-ideal source or sink for heat pumps. Using a closed-loop system with piping buried in the ground, circulating water either picks up energy for heating or loses energy.

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Heat Recovery Systems• Large buildings often have heating and cooling occurring at the same

time. Redistribution of heat energy within a structure can be accomplished through the use of heat pumps of the air-to-air or water-to-water type.

• Because of introduction of outdoor ventilation air, it is necessary to exhaust significant quantities of air from large buildings.

• In the heating season considerable savings can be realized if the heat energy from the exhaust air can be recovered and used in warming the exterior parts of the structure.

• In a similar manner energy can be saved when outdoor temperatures are high by precooling ventilation air using the cooler air exhausted from the building.

• All these systems may also be effective during the cooling season, when they function to cool and perhaps dehumidify the warm incoming ventilation air.

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Chapter 4: (Homework reading)Comfort and Health - Indoor Environmental Quality

Environmental factors that affect a person's thermal balance and therefore influence thermal comfort are:

1. The dry bulb temperature of the surrounding air

2. The humidity of the surrounding air

3. The relative velocity of the surrounding air

4. The temperature of any surfaces that can directly view any part of the body and thus exchange radiation

5. Personal variables

(i) activity, measured in met

(ii) clothing, measured in clo

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met and clo

• The energy generated by a person's metabolism varies with that person's activity.

• met indicates metabolic rate per unit of body surface area of a sedentary person (seated, quiet).

• 1 met = 18.4 Btu/(hr-ft2) = 58.2 W/m2

• clo indicates the insulating value of a single equivalent uniform layer over the whole body

• 1 clo = 0.880 (F-ft2-hr)/Btu = 0.155 (m2-C)/W

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Mean radiant and operative temperature

• Operative temperature is the average of the mean radiant and ambient air temperatures, weighted by their respective heat transfer coefficients.

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Heat stress index and wet bulb globe temperature

In enclosed environments,

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Heat stress index is the ratio of the total evaporative heat loss

required for thermal equilibrium to the maximum evaporative heat

loss possible for the environment, multiplied by 100, for steady-

state conditions, and with skin temperature held constant at 95 F.

Wet bulb globe temperature twbg is an environmental heat stress

index that combines the dry bulb temperature tdb, naturally

ventilated wet bulb temperature tnwb, and the globe temperature tg.

ASHRAE Thermal Sensation Scale

• ASHRAE Standard 55-1992 gives the conditions for an

acceptable thermal environment. Most comfort studies

involve use of the ASHRAE thermal sensation scale.

• This scale relates words describing thermal sensations felt

by a participant to a corresponding number. The scale is:

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Figure 4-1

Acceptable ranges of operative

temperature and humidity for

people in typical summer and

winter clothing during light and

primarily sedentary activity

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Basic concerns of IAQ

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Common contaminants 1. Carbon dioxide and other common gases

2. Radon

3. Volatile Organic Compounds (VOC’s)

4. Mycotoxins (Mold Poisons)

5. Particulate Matter

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Methods to control contaminants1. Source elimination or modification

2. Use of outdoor air

3. Space air distribution

4. Air cleaning

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Contaminant concentration in a space in SS condition

• The effectiveness Eoa with which outdoor air is used can be expressed as the fraction of the outdoor air entering the system that is utilized.

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• The occupied zone is the region within an occupied space

between the floor and 72 in. (1800 mm) above the floor

and more than 2 ft (600 mm) from the wall or fixed air-

conditioning equipment.

• Perfect mixing of supply air with room air does not occur,

and some fraction S of the supply air rate bypasses

and does not enter the occupied zone.

• If R is the fraction of return air rate that is recirculated,

the effectiveness Eoa with which outdoor air is used can

be expressed as the fraction of the outdoor air entering

the system that is utilized.

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Problem on steady state contaminant concentration

• Carbon dioxide is being generated in an occupied space

at the rate 0.25 cfm (0.1181/s) and outdoor air with a C02

concentration of 200 ppm is being supplied to the space

at the rate of 900 cfm (0.425 m3/s). What will be the

steady-state concentration of C02 in ppm if complete

mixing is assumed?

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