03 classification hvac

8/11/2019 03 Classification HVAC

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International MSc Programme Sustainable Energy EngineeringInternational MSc Programme Sustainable Energy Engineering

THERMAL COMFORT AND INDOOR CLIMATE

Lecture:

- CLASSIFICATION OF HVAC SYSTEMS

Assist. Prof. Igor BALEN


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Cooling coil

Heat transfer

from air tocooling medium

Extended

surface coil

Drain Pain

Removes

moisture

condensed

from air

stream

Evaporator

Condenser

Expansion valveCompressor

Controls

Operation

principles


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Operation

principlesHeating coil

Heat transfer

from heating

medium to

air

Heat pump(condenser)

Furnace

Boiler

Electric resistance

Controls


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Operation

principles

Blower

Overcome

pressure drop

of system

Adds heat to airstream

Makes noise

Performs diffe-rently at different

conditions (air

flow and pressure

drop)


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Operation

principlesDuct system

(piping forhydronic

systems)

Distribute

conditioned

airRemove air

from space

Providesventilation

Makes noise

Affects comfort

Affects indoor

air quality (IAQ)


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Operation

principles

Diffusers

Distribute

conditioned

air within

room

Provides

ventilation

Makes noise

Affects comfort

Affects IAQ


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Operation

principles

Dampers

Change

airflow

amounts

Controls outside

air fraction

Affects building

safety


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Operation

principles

Filter

Removes

pollutants

Protects

equipment

Imposes substantial

pressure drop

Requires

maintenance


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Operation

principles

Controls

Makes

everything

work

Temperature

Pressure (drop)

Air velocity

Volumetric flow

Relative humidityEnthalpy

Electrical Current

Electrical cost

Fault detection


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HVAC system basic classification

1. Low-velocity (low-pressure)

- air flow velocity in ducts 2-8 (10) m/s

- pressure drop in ducts (external) 500-2000Pa

- ducts usually in rectangular shape; sides ratio 1:2 to 1:4,5

- comfort applications: hotels, theaters, museums, concert halls...

2. High-velocity (high-pressure)

- air flow velocity in ducts 10-30 m/s

- pressure drop in ducts (external) 1500-3500Pa

- ducts usually in round shape

- applications: bussiness/office buildings, buildings with limited space for

placing of the ducts ...


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HVAC system typology

- Three generic types of HVAC systems:

All-air system

Air-water system

All-water system


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1.All-air system

- low-velocity

- high-velocity

Single duct Dual duct

- warm air duct

- cold air duct

Constant Air

Volume (CAV)

Variable Air

Volume (VAV)

Single zone Multiple zone- with zone reheat coils

- with zone AHU

- with central AHU

Unitary- single-package

- split


Constant Air

Volume (CAV)

Variable Air

Volume (VAV)


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2.Air-water

- low-velocity

- high-velocity

Induction units Radiant

heating/cooling

Three-pipe

HSUP+CSUP

RET common

Two-pipe

SUPPLY+RETURN

Four-pipe

HSUP+HRET

CSUP+CRET

With zone

heating/cooling

coils

Fan coil units

With changeover

between hot and

cold medium

Without

changeover

Valve control

- one exchanger

Damper control

- two exchangers


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Unitary air conditioners- consist of one or more factory-made assemblies that normally include an

evaporator or cooling coil and a compressor and condenser combination.

- heating and cooling function, ventilation

- air-source unitary heat pump consisting of one or more factory-made

assemblies, which normally include an indoor conditioning coil,

compressor(s), and an outdoor coil must provide a heating function and

possibly a cooling function as well.

- water-source heat pump is a factory-made assembly that rejects or

extracts heat to and from a water loop instead of from ambient air.

- split system is a unitary air conditioner or heat pump having more than

one factory-made assembly (e.g., indoor and outdoor units)


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Unitary air conditioners

Rooftop HVAC system (single-package)

- limited to five or six floors because

duct space and available blower power

become excessive in taller buildings.


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Air-/water-sourced heat pump

Typical schematic of air-to-air heat pump

single-package reverse-cycle system

for both heating and cooling

single source of energy can supplyboth heating and cooling requirements

heat output can be as much as two to

four times that of the purchased

(electric) energy input (in kWh). vents and/or chimneys may be

eliminated


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Split system

- different types:- mono-split, multi-split, CRV, VRV

- new multi-split VRV with up to 50 indoor units connected to one outdoor

unit, up to 300m length and 50m height difference of refrigerant piping


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Central HVAC systems

- equipment room for a central system is normally located outside the

conditioned area - in a basement, penthouse, service area, or adjacent to

or remote from the building require space at a central location and a

potentially large distribution system- larger air-handling equipment is usually custom-designed and fabricated

to suit a particular application

- most of the components are available from many manufacturers

completely assembled or in subassembled sections that can be boltedtogether in the field

- specific design parameter must be evaluated to balance cost,

controllability, operating expense, maintenance, noise, and space

- close year-round control of temperature and humidity are possible


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All-air systems

Central single-duct, multiple-zone, constant air volume

- zone temperature or zone supply volume flow rate is controlled by

terminals

- reheat coils are used to control the temperature and/or relative humidity

Typical schematic of a system with zone reheat coils


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All-air systems

Central single-duct, multiple-zone, constant air volume

- system with zone air-handling units (AHU)


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All-air systems

Central single-duct, variable air volume

- controls temperature in a space by varying the quantity of supply air rather

than varying the supply air temperature

- VAV terminal unit at the zone varies the quantity of supply air to the space

- supply air temperature is held relatively constant: while supply air

temperature can be moderately reset depending on the season, it must

always be low enough to meet the cooling load in the most demanding

zone and to maintain appropriate humidity- greatest energy saving associated with VAV occurs at the perimeter

zones, where variations in solar load and outside temperature allow the

supply air quantity to be reduced

- potential problems with minimum outdoor air supply (increasing CO2concentration) and humidity control, where particular care should be taken

in areas where the sensible heat ratio (ratio of sensible heat to sensible

plus latent heat to be removed) is low, such as in conference rooms


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All-air systems

Central single-duct, variable air volume


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All-air systems

Variable air volume (VAV) box

- VAV box or a cooling VAV box is a terminal device in which the supply

volume flow rate is modulated by varying the opening of the air passage by

means of a single-blade butterfly damper

- pneumatic or direct digital control (DDC) of a damper

- minimum supply air rate 30% of design flow rate


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All-air systems

Central dual-duct, constant air volume

- separate warm (45C) and cold (12-16C) air duct

- temperature control by mixing warm and cold air in properproportion to satisfy the load of the space


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All-air systems

Central dual-duct, variable air volume

- may include single-duct VAV terminal units connected to the

cold air chamber


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All-air systems

Central dual-duct, variable air volume

- terminal units that function like a single-duct VAV cooling terminal unit

and a single-duct VAV heating terminal unit in one physical package


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All-air systems

Advantages:

The location of the central mechanical room for major equipment allows

operation and maintenance to be performed in unoccupied areas. In

addition, it allows the maximum range of choices of filtration equipment,vibration and noise control, and the selection of high quality and durable

equipment.

Keeping piping, electrical equipment, wiring, filters, and vibration and

noise-producing equipment away from the conditioned area minimizesservice needs and reduces potential harm to occupants, furnishings, and

processes.

These systems offer the greatest potential for use of outside air for

economizer cooling instead of mechanical refrigeration for cooling. Seasonal changeover is simple and adapts readily to automatic control.


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Advantages (continued):

A wide choice of zoning, flexibility, and humidity control under all

operating conditions is possible, with the availability of simultaneous

heating and cooling even during off-season periods. Air-to-air and other heat recovery may be readily incorporated.

They permit good design flexibility for optimum air distribution, draft

control, and adaptability to varying local requirements.

The systems are well suited to applications requiring unusual exhaust or

makeup air quantities (negative or positive pressurization, etc.).

All-air systems adapt well to winter humidification.

By increasing the air change rate and using high-quality controls, it ispossible for these systems to maintain the closest operating condition of

0.15 K dry bulb and 0.5% rh. Today, some systems can maintain

essentially constant space conditions.

All-air systems


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All-air systems

Disadvantages:

They require additional duct clearance, which reduces usable floor space

and increases the height of the building.

Depending on layout, larger floor plans are necessary to allow enoughspace for the vertical shafts required for air distribution.

Ensuring accessible terminal devices requires close cooperation between

architectural, mechanical, and structural designers.

Air balancing, particularly on large systems, can be more difficult.

Perimeter heating is not always available to provide temporary heat

during construction.


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Air-water systems

Central primary air, high-velocity system (induction units) or low-velocity

system (fan coils)


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Air-water systems

Central primary air, with fan coils


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Air-water systems

Central primary air and fan coils, separated


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Advantages:

Individual room temperature control allows the adjustment of each

thermostat for a different temperature at relatively low cost.

Separate heating and cooling sources in the primary air and secondarywater give the occupant a choice of heating or cooling.

Less space is required for the distribution system when the air supply is

reduced by using secondary water for cooling and high velocity air. The

return air duct is smaller and can sometimes be eliminated or combinedwith the return air system for other areas, such as the interior spaces.

The size of the central air-handling apparatus is smaller than that of an all

air system because little air must be conditioned.

Dehumidification, filtration, and humidification are performed in a central

location remote from conditioned spaces.

Air-water systems


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Air-water systems

Advantages (continued):

Ventilation air supply is positive and may accommodate recommended

outside air quantities.

Space can be heated without operating the air system via the secondarywater system. Nighttime fan operation is avoided in an unoccupied

building. Emergency power for heating, if required, is much lower than for

most all-air systems.

Components are long-lasting. Room terminals operated dry have ananticipated life of 15 to 25 years. The piping and ductwork longevity should

equal that of the building. Individual induction units do not contain fans,

motors, or compressors. Routine service is generally limited to

temperature controls, cleaning of lint screens, and infrequent cleaning ofthe induction nozzles.


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Air-water systems

Disadvantages:

For most buildings, these systems are limited to perimeter space;

separate systems are required for other areas.

More controls are needed than for many all-air systems.

Secondary airflow can cause the induction unit coils to become dirty

enough to affect performance. Lint screens used to protect these terminals

require frequent in-room maintenance and reduce unit thermal

performance.

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

This is a disadvantage 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 expense.

A low primary chilled water temperature is needed to control space

humidity adequately.


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Air-water systems

Disadvantages (continued):

The system is not appropriate for spaces with high exhaust requirements

(e.g., research laboratories) unless supplementary ventilation air is

provided.

Central dehumidification eliminates condensation on the secondary water

heat transfer surface under maximum design latent load. However,

abnormal moisture sources (e.g., from open windows or people

congregating) can cause condensation that can have annoying or

damaging results.

Energy consumption for induction systems is higher than for most other

systems due to the increased power required by the primary air pressure

drop in the terminal units. The initial cost for a four-pipe induction system is greater than for most

all-air systems.


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All-water systems

Chilled water systems

Two-pipe constant flow


with primary loop


with secondary loops


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All-water systems

Four-pipe fan coils

- separate hot and cold supply and return piping

- heating and cooling possible in different rooms at the same time


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Induction unit

- centrally conditioned primary air is supplied to the unit plenum at

medium to high pressure

- medium- to high-velocity air flows through the induction nozzles and

induces secondary air from the room through the secondary coil- secondary air is either heated or cooled at the coil, depending on the

season and the room requirement


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Fan coil

Ventilation Air

Cooling Coil

Filter

Heating Coil

Supply Air

Return Air

Drain pan

Fan

- heat, cool, move air by forced

convection through the conditioned

space, filter the circulating air, and

introduce outside ventilation air

- available in many configurations

(vertical floor mounted, horizontal

ceiling mounted, ...)


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Special systems

Dehumidification heat pump


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Special systems

All-water heat pump with thermal storage and optional solar collectors


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Special systems

OUTSIDE

RETURN

SUPPLY

EXHAUST

System with indirect evaporative cooling


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Special systems

System with dessicant rotary wheel


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Central vs. decentralized HVAC

03 classification hvac

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