Transfer of Energy/Heat

It Can Only Go One Way!

absolute temperature scale - 0 Kelvin PV=nRT - gas law - Charles, Boyle?

HOT COLD

How Do We Measure?

Basic Heat, One British Thermal Unit (Btu):

One Btu is the amount of heat required to raise thetemperature of one pound of water one degree F.

One pound ofwater

(about a pint)

One Btu

A Little More to It

ICECOLD

WATERSTEAM

HOT WATER

970 Btu144 Btu 1 Btu / oF

“for each pound of H2O”

How many Btus would it take to turn 5lbs of ice at 32F into 5lbs of steam at 212F?

In addition to sensible heat (a change in the air temperature that is felt or “sensed”), cooling systems must also account for latent heat gains.

Latent heat is the energy that is required to change a solid to a liquid or a liquid to a gas; no temperature change occurs during this process.

Latent Heat

Steam is comprised of sensible and latent heat. The latent heat portion is equal to the heat energy used to change the

water from liquid to vapor at 212oF. No change in temperature is “sensed”.

The sensible heat input of the flame heats the water. Some of this increases the temperature of the water and some is used for latent heat, the heat of changing the water from

liquid to gas.

Sensible (dry)heat

Cooking

Steam

People

Latent Heat

BTU’s per hour 12000 BTU’s/hr = 1 ton (often expressed as

refrigeration capacity - why?) MBH = 1000 BTU’s/hr

Let’s Melt Some IceRate of Heat Transfer:

Latent - 350 Btu/hrSensible - 350 Btu/hr

Three Ways Heat is Lost or Gained from a Building

Conduction Convection Radiation

Conduction

Transfers heat in a chain-like manner A building’s heat is conducted to the outdoors

through the solid surfaces of walls, roofs, floor slabs, glass doors, and windows

Q = (U)(A)(T)

Q = heat gain or loss in (Btu/hr)U = heat transfer coefficient (Btu/ft2/oF)A = surface area (ft2)T = temperature differential

Conduction

Heat moves from hot to cold--molecules transfer heat from one to another as heat moves through them.

10o F 68o F

Clapboards Plaster

Studs

U = 1/R

Convection

Transfers heat as warm molecules actually move from one place to another

In buildings, heat is convected from the interior to the outdoors by air that leaks, or infiltrates, through cracks around windows and doors and by the exhaust and ventilation air that moves between the interior and exterior of the building

Convection

Cold air infiltrates into a space by physically moving from outdoors to indoors.

Cold air

Crack 68o F

10o F

Door

Radiation

Solar energy penetrates the building wall by passing through transparent glass surfaces.

Glass

Transfers heat by electromagnetic waves.

Heat is radiated onto buildings by the sun during the day.

Why does it snow on the top of high mountains in Africa?

Why Are We So Concerned About the Movement of

Heat?ASHRAE worries for us:

American Society of Heating, Refrigerating, and Air Conditioning Engineers

Research Standards Development Education

80% Comfort Zone

100% RH

70%

RH

50%

RH

30%

RH

60 70 80 90oF

winter

summer

Acceptable ranges of operative temperature and humidity for persons clothed in typical summer and winter clothing, at light, mainly sedentary activity. (ASHRAE 55-1981)

What We Are Up Against

WINTER SUMMER WET-BULB99% 97.5% 1% 2.5% 1% 2.5%

Atlantic City 10 13 92/74 89/74 78 77Long Branch 10 13 93/74 90/73 78 77Newark 10 14 94/74 91/73 77 76New Brunswick 6 10 92/74 89/73 77 76Paterson 6 10 94/74 91/73 77 76Phillipsburg 1 6 92/73 89/72 76 75Trenton 11 14 91/75 88/74 78 76Vineland 8 11 91/75 89/74 78 76

How Much Cooling Do We Need?

Quick Load Calculation Conduction Convection Radiation Equipment People Lighting

Add it all up and nowyou know how muchcooling you will need

Total

When is a Watt Not a Watt200 watts electrical capacity

Vs.

135 watts cooling load

35 watts

100 watts

100 watts

100 watts

Conversion Factor(Watts) (3.41) = Btu/hr

Conference Room Heating Up

8 incandescents (100 watts) turned on 15 people enter the room

How many tons of cooling will we need to meet the load?How many CFM of system air (cubic feet per minute)?

Q(Btu/hr) = 1.08 (CFM) (T)- sensible heat only-

Psychrometric Chart 80% 60%

40%

20%

20%

40%

60%

80%

80o

Wet bulb

Dry bulb

Humidity

70o

60o

50o

100o 120040o

80o

60o

Moisture

Enthalpy

Psychrometric Chart

heating andhumidifyingevaporative

cooling

humidifyingonly

sensibleheating only

chemicaldehumidifyingdehumidifying

only

cooling anddehumidifying

sensiblecooling only

Boiling To The Rescue!

Water 212 F Alcohol 152 F Sulfur Dioxide 14 F Refrigerant 12 - 22 F Ammonia - 28 F Refrigerant 22 - 41 F Nitrogen - 320 F

Represents boiling point at atmospheric pressure

So That’s Why its Called An Evaporator

R- 22 Air Flow

Refrigerant Properties

Condensing pressure

Evaporating pressure

Enthalpy

Liquid

Gas

1

2

3

4

Pressure

Mixedcondition

constant

temperature

Condensing pressure

Evaporating pressure

Enthalpy

Liquid

Gas

1

2

3

4

Pressure

Mixedcondition

1 2

34

compressor

evaporator

condenser

throttlingvalve

Putting All The Pieces Together

Both Sides Water

Sensible and Latent Heat

Moist warm airMostly latent heat

Condensed moistureon coils

Dryer cool airSensible heat

Condensate

Fan Coil Cooling

Dry air (mostly oxygen and nitrogen) contains only sensible heat. The moisture in the air (water vapor)

contains both sensible and latent heat.

Rooftop Unit

Split System

ROOF

SPACE

Airside: Yellow loop Chilled water: Green loop Refrigeration: Blue loop Heat rejection: Red loop

75o

55o 55o

80o 55o 55o

45o 45o

50o 120o

38o 100o

95o 95o

85o 85o

CHWValve

ExpansionValve

CWValve

TerminalUnit

Transfer of Energy/Heat

OfficeSpace

CoolingTower

CHW Coil

Evaporator Condenser

Fan Pump Compressor Pump

The best systems: Provide comfortable environment Have relatively low operating costs Will operate inexpensively after normal business

hours Are easily operated and maintained Will allow expansion or reconfiguration Operate quietly Can be cost effectively purchased and installed

System Configuration

System Configurations

Central Plant Central chillers, central fans Central chillers, floor-by-floor fans

Unitary Self-contained package units per floor

HVAC Systems

Refrigeration Air Handling Room Air Distribution and Control Heating Systems

Refrigeration

Central Packaged/floor-by-floor Distributed (heat pumps)

CentralAdvantages Low operating cost Low maintenance cost Noise removed (NC30-35)

Disadvantages Expensive after hours Lack of redundancy

Heat

Packaged

Advantages Low after hours Flexible Reliable Cost allocation

Disadvantages Noise Higher operating

costs

Distributed

Advantages Flexible Low after hours Redundancy

Disadvantages Noise Expensive to

maintain and operate Poor IAQ

Air Handling

Central fans Floor-by-floor fans

Central - Air HandlingAdvantages Low energy Low maintenance

cost Noise No loss of space

Disadvantages Reliability Flexibility After-hours costs are

high VSD

X

X

X

O

Floor by Floor Fans

Advantages Flexible Reliable Low after hours Cost allocation

Disadvantages Noise Floor space High operating cost

Floor by Floor Fans with Central Chillers

Advantages Flexible Reliable Low after hours Cost allocation

Disadvantages Noise Floor space High operating cost

Room Air Distribution and Control

Constant volume with reheat Variable air volume (VAV) Fan-powered variable air volume

Constant Volume with Reheat

Advantages Air motion Indoor air quality

Disadvantages Operating expenses Zoning

55o

75o

Ther

mos

tat

Variable Air Volume

Advantages Low operating

expenses Zoning

Disadvantages Poor IAQ Lack of air motion Noise

55o

55o

Ther

mos

tat

Fan Powered Variable Air Volume

Advantages Air motion Indoor air quality Operating costs

Disadvantages Operating costs Noise

Ther

mos

tat

55o

66o

80o

Temperature

Full Shut OffC

FM

Parallel FanPoweredC

FM

Temperature

CF

M

Temperature

Series Fan PoweredVAV

VAV

Room Air DistributionVAV Box Supply Air

Return Air

Heating Systems

Induction units Baseboard radiation/convectors Fan-powered VAV with heat

Induction UnitsAdvantages Comfort Indoor air quality

Disadvantages Noisy Unattractive Takes up space Operating expense

RadiationAdvantages Comfort Operating Cost

Disadvantages Zoning Unattractive Takes up space

FPVAV with Heat

Advantages Not visible Indoor air quality Zoning

Disadvantages Noisy Comfort

First Class Office Building Standards

HVAC At least 1 zone per 1000 sq.ft. Individual control at office level Air flow - average 1.1 CFM/sq.ft. Typically 350 to 400 sq. ft. ton Supplemental cooling 70o to 76o

Flexible/reliable Low operating cost 3 to 6 watts per sq.ft. sensible cooling