space heating load

8/13/2019 Space Heating Load

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Chapter 6. Space Heating Lo


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Heating Load:

1- Heat loss through walls, windows,etc

2- Heat to warm outside air (1) desired (2) infiltration

The ideal heating system would provide just enough heat to m

loss from structure.

Load is not a constant

Outdoor temp changes as well as wind velocity, sunlight (sola

heat gains, appliances, people.

Heating system must handle the coldest outside average tem

the year.

Most equipment operate at partial load most of the time

Unheated Rooms: To< Tu< Ti Assume steady state heat transfer

)( oi TTUAq =&


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The following Factors should be considered

performing heat load calculations:1. Is the heat capacity of the structure high or low ?

2. Is the structure insulated ?

3. Is the structure exposed to high wind ?

4. Is the ventilation or infiltration load high ?

5. Is there more glass area than normal in the structure ?

6. During what part of the day will the structure be used ?

7. What is the nature of occupancy ?

8. Will there be long periods of operation at reduced indoor tempe

9. What is the amplitude between maximum and minimtemperature in the locality ?

10. Are there local conditions that cause significant variattemperatures listed ?

11. What auxiliary heating devices will be in the building ?

12. What is the expected cost of fuel or energy ?


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Chapter 4 Design Temp for inside. Low end (70F at = 30%) of


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Representative inside air design tempera


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Air change method, ACH Air Chang

0aslowasbecanand0.25.0

hourperchangeairofnumberACH

))((

Re =

=

sidential

T

ACH

C

VACHQ&

Experience shows nominal values for air changes per h


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Experience shows nominal values for air changes per h


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Crack Method

Q=AC Pn

A= effective leakage area of the cracks

C= flow coefficients, which depends on the type of crack

of flow in the crackP=outside - inside pressure difference, Po-Pin=exponent that depends on the nature of the flow crack


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Crack method is complex but in general more accurat

We need to be able to calculate P to put in the equa

essureBuildingSw PPPP Pr++=

Pw=Pressure difference due to wind

Cp= function of angle of wind, Height and length to width ratio of bu

Ps=Pressure difference stack effect

Cd range from 1 for buildings with no doors in the stairways to abou

for modern office buildings

Pp=Pressure difference due to building pressurization

wwindPP =

cgaR

ghoPdC

sP =


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Figure 6.1 Windows and door infiltration Characteristic

Figure 6.2 Variation of wall averaged pressure coefficierise buildings

Figure 6.3 Variation of wall averaged pressure coefficiebuildings

Figure 6.4 Average roof pressure coefficients for a tall b

Figure 6.5 Pressure difference due to stack effect. Figure 6.6 Curtain wall infiltration for one room or one

Figure 6.7 Infiltration through cracks around a closed sw

Figure 6.8 Swinging door infiltration characteristics wit

Figure 6.9 Flow coefficient dependence on traffic rate

Supply Air for Space Heating Source Media for space heating


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Figure 6.1 Windows and doo

infiltration Characteristic


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Fig. 6.2 Variation of wall averaged pre

coefficients for low rise buildings


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Fig. 6.3 Variation of wall averaged pre

coefficients for tall buildings


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Fig. 6. 4 Average roof pressure

coefficients for a tall building

Fig. 6. 5 Pressure differe

stack effect.


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Fig.6.6 Curtain wall infiltration for

one room or one room floor

Fig. 6.7 Infiltration throug

around a closed swingin


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Fig. 6.8 Swinging door infiltration

characteristics with traffic

Figure 6.9 Flow coeff

dependence on traffic


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Procedure for calculating heat losses from a s

1. Select the design outdoor weather conditions: temperature, hum

direction, and speed.

2. Select the design indoor air temperature to be maintained

space when outside design conditions exist.

3. Estimate temperatures in adjacent un-heated spaces.

4. Select or compute the heat transmission coefficient (U fbuilding components through which heat losses are to be calcu

5. Determine all surface areas through which the heat is lost.

6. Compute heat transmission losses for each kind of wall, glass

and roof in the building by multiplying the heat transmission

each case by the area of the surface and the temperature diffe

indoor and outdoor air, or adjacent unheated space.

7. Compute heat losses from basement or grade-level slab floo

presented in Chapter 5.


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8. Select infiltration air quantities and compute the heat loinfiltration around doors and windows.

9. When positive ventilation using outdoor air is provided by an

air-conditioning unit, the energy required to warm the outdootemperature must be provided by the unit. The principle for calload component is identical to that for infiltration. If mechanicathe room is provided in an amount equal to the out-door air drunit, the unit must also provide for the natural infiltrationmechanical exhaust is used, and the outdoor ventilation air sup-exceeds the natural infiltration that would occur without vent

infiltration may be neglected.10. The sum of the transmission losses or heat transmitted through

walls, floors, ceiling, glass, and other surfaces, plus the energy athe cold entering air by infiltration or required to replace mechrepresents the total heating load.

11. In buildings that have a reasonably steady internal heat releasemagnitude from sources other than the heating system, a comp

heat release under design conditions should be made and dedtotal of the heat losses computed above.

12. Additional heating capacity may be required for intermibuildings to bring the temperature of the air, confining surfacecontents to the design indoor temperature within a specified tim


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Source Media for space heating

Hot water as heating media

q = mwcp (t2 t1)

where:

q = heating required, Btu/hr or W

mw = mass flow rate of hot water, lbm/hr or kg/s

cp = specific heat of water, Btu/lbm or kJ/(kg-C

t2 = water temperature leaving coil, F or C

t1 = water temperature entering coil, F or C

Assuming that cp is constantq = 500 Qgpm(t2 t1) English Units

q = 4.2 QL/s(t2 t1) SI units


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Steam as heating fluid

q = mv (i2 i1)

where:

q= heating required, Btu/hr or Wmv = mass flow rate of the vapor, lbm/hr or kg/s

i2 = enthalpy of the vapor leaving the coil, Btu/lbm o

i1 = enthalpy of the vapor entering the coil, Btu/lbm

If saturated vapor is the heating medium,

i2 i1 = the enthalpy of vaporization, i fg.


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Furnace Heating

In the case of a furnace where combustion gases

air directly, the heating value of the fuel and a fu

efficiency must be known. A general relation from

mfor Qfcan be found is

qf= mf(HV)= Qf(HV)where:

qf = heating required, Btu/hr or W

mf = rate at which fuel is used, lbm/hr or kg

Qf = Volume flow rate ft3/min or m3/s

HV = heating value of the fuel, Btu/lbm or kJ

= furnace efficiency


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space heating load

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