hvac equipment presentation

8/6/2019 HVAC Equipment Presentation

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HVAC Equipment

MAE 406

Energy Conservation in IndustryStephen Terry


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HVAC Fundamentals

HVAC stands for Heating, Ventilation, and Air

Conditioning.

Sometimes see written as HVAC-R, which

includes Refrigeration HVAC important in residential, commercial, and

industrial plants


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Focus in this Course

Weve already looked at using boilers to provide

steam which is used by many large facilities,

including NCSU, for heating.

This lecture and the next will focus mostly onspace air conditioning and refrigeration.


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Method of Providing Cooling

Heat naturally moves from hot to cold,

Refrigeration systems must move heat from a cold

space to a hot space

This is accomplished using a refrigeration cyclemanipulating the pressure and temperature of a

refrigerant

This process also requires energy heat pump


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Refrigeration Cycle

hFrom Heating, Ventilating, and Air Conditioning Analysis andDesign, McQuiston and Parker, 3rd edition


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Cycle Efficiency MAE302

Depends on condenser and evaporator

temperatures & compressor isentropic efficiency

Usually discuss efficiency as COP or in kW/ton

Efficiency (COP) = Desired Output / Required Input= Qevaporator / Wcompressor

= Qevaporator/ (Qcondenser Qevaporator)

Note: 1 ton = 12,000 BTU/hr

1 kWh = 3,413 BTU


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Typical Operating Temperatures

Evaporator Temperature: between 35-45F

Operating colder than this can cause freezing /

frost on coils defrost cycle

Condenser Temperature: 80-90F for water cooled applications

110-140F for air cooled applicationsWhy?

Compressor isentropic efficiency, ~70%


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Types of Refrigeration Units

Air cooled DX unit

Uses air-cooled condenser

Usually cools air directly

by venting return / outside

air through ductwork

Packaged and split system

varieties.

Efficiency 1.2-1.4 kW/ton


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Advantages / Disadvantages of DX Unit

Advantages

Flexible system, can grow

with plant or building

Self-contained system

Relatively low first cost

Disadvantages

Efficiency is poor

Maintenance can be

expensive with many units


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Types of Refrigeration Units

Central Water Chiller

Refrigeration cycle cools

water that circulates

around building to AHUs

Chiller can be water or

air-cooled

Requires piping and

pumps

Efficiency 0.6 kW/ton to

1.0 kW/ton


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Advantages / Disadvantages of Chiller

Disadvantages

Requires cooling tower

and pumps / pipes

Must operate at lowest

temperature required for

entire system

Must maintain chilled

water system and coolingtower water system with

chemicals

Advantages

Great energy efficiency

Central unit reduces

maintenance costs

Can provide space

conditioning and process

cooling


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Refrigerants

R-134a and R-22 are commonly used today

Older chillers use R-11/R-12 Ozone depleters

Since R-11 not made any more, older chillers must

be retrofitted if plants supply of refrigerant runsout.

Food manufacturers will often use NH3 in a

double stage system.


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Double Stage System

Useful for largesystems with two

zones

Ammonia ischeap&efficient,

but is toxic!

Condenser good

for heat recovery

to cleaning water.


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Cooling Towers

Most often used to provide condenser for chillers.

Can also be used to cool process equipment,

especially air compressors.

Water is sprayed / falls through a media while airis drawn through.

Evaporation of water represents most of

refrigeration effect

Outlet temperature depends on wet-bulb temp.


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Cooling Towers

Efficiency: 0.05 kW/ton

Typical temperatures:

90 F in / 70-80 F out

Fans usually are 2-50 hp

Can use multiple cells

Two speed motors /

VSDs


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Absorption Chillers

Uses low pressure steam to generate chilled water!

Chilled water temps: 40-50 F

Refrigerant is a Li-Br salt. Steam is used to

concentrate salt like a compressor is used tocompress vapor

Efficiencies: 1-2 Btu cooling / Btu of steam

Requires a large cooling tower Systems are 2x the cost of regular chiller, so it is

not economical unless steam is free


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Air Distribution System Single Zone

Rooftop Unit

Most common type of system seen in smaller

industrial plants

Utilizes an air cooled direct expansion unit to

provide cold refrigerant to the cooling coil System may have multiple compressors to stage

cooling

May have a gas pack or steam coil for heating

Often utilizes an economizer with relief venting


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Single Zone Roof-top Unit Schematicfilters

Return air from

room

Heating Coil or

Gas PackEconomizerdamper

Condenser

Compressors

fan

Cooling

Coil

Supply air to

room


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Economizers

Compares outdoor air temperature / enthalpy to

indoor conditions. Three modes of operation:

Free cooling outside air temperature less than supply

air temperature use mixture of outdoor air and return

air, deactivate compressors

Economizer cooling outdoor air cooler than indoor,

but above supply air temperature. Use all outdoor air

and cool it to supply temperature

No savings outdoor air hot and system uses all

return air (except minimum outside air)


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Outdoor Air Requirements

Must supply minimum outside air to space to

remove CO2 and contaminants, especially in

newer buildings.

Can utilize some air-to-air heat recovery in coldclimates

ASHRAE standard is 15 cfm per person

Can be varied for classrooms or auditoriums if a

CO2 sensor is used to determine fresh air required.


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Multiple Zones

Dual Deck System

Utilizes ductwork with hot air and ductwork with cold

air, which is mixed in proportion to maintain conditions

Energy inefficient since hot and cold air are mixed.


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Multiple Zones

Variable air volume (VAV)

Utilizes sensors to vary flow of air and therefore

cooling through unit. Can use electric strip heater for

heating purposes


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Psychrometrics

Used to determine actual cooling / heating

requirements, considering both temperature and

humidity.

Refer to psychrometric chart in thermodynamicsbook or ASHRAE handbook


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Definitions Sensible Heat

Sensible heating heating or cooling that is done

to change temperature only, no change in

moisture. Example:

Determine sensible heat to raise the temperature of 1,000 lb/hr of air from

70 to 110 F.

Sensible Heat = 1,000 lb/hr x 0.24 BTU/lb- F x (110-70 F)= 9,600 BTU/hr


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Latent Heat

Latent heat is heat added in the form of moisture.

Evaporative cooling is an example water

sprayed into dry air causes liquid to evaporate to

vapor, removing heat in the process. Moisture content of air specified by dew-point

temperature or by specific humidity. Lines of

constant dew-point are horizontal on chart

Note that relative humidity is not horizontal


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Enthalpy

A measure of sensible and latent heat

Is a true measure of energy required to change

state.

Can be used to determine actual cooling loads,including latent heat removal.


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Example

Find the heat removed from 10,000 lb/hr (2,222

cfm) of air starting at 95F and 75F dew-point to

55F and saturated.Solution:

h 95/75 = 44 BTU/lb

h 55/55 = 23 BTU/lb

Q removed = 10,000 lb/hr x (44 23 BTU/lb)

= 210,000 BTU/hr

= 17.5 tons of cooling


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Air Washer Systems

Uses a water spray to spray cold chilled water into

an air stream.

Air is first saturated along wet-bulb line (constant h)

Once saturation is reached, saturated air is cooled by

conduction to exit temperature

Air is usually dehumidified in process

Direct contact heat exchanger

infinite surface area Must make provision for cleaning out dirt and grime

from chilled water

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