HVAC - Lecture 11B

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Heating and Air Conditioning I

Principles of Heating, Ventilating and AirConditioning

R.H. Howell, H.J. Sauer, and W.J. Coad

ASHRAE, 2005

basic textbook/reference material

For ME 421

John P. Renie

Adjunct ProfessorSpring 2009


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Chapter 7Nonresidential Load Calculation Heat Gain through Interior Surfaces

Whenever a conditioned space is next to a space at a different

temperatureheat gain

If no heat sources(tbto)5 F

Floor in contact to ground or basementno effect on cooling load

estimate


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Chapter 7Nonresidential Load Calculation Heat Sources in Conditioned Spaces

PeopleSee Table 7-14

Lightingoften the major component of space loadtherefore

accuracy is needed. Different from electrical paper.

Convective portion immediate picked up by A/C system

Radiative is absorbed and re-released by wall, floors, furniture, etc,

after a time lageven when lights have been turned off. Instantaneous rate of heat gain given by

Special allowance factorTable 7-15


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Chapter 7Nonresidential Load Calculation Table 7-14


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Chapter 7Nonresidential Load Calculation Figure 7-15


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Chapter 7Nonresidential Load Calculation Light Heat Components

Heat to space loadlight heat directly contributing to the spaceheat gain.

Heat to return loadlight heat released into the above-ceiling

cavity picked up by the return air

Return air plenum equations


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Chapter 7Nonresidential Load Calculation Typical Return Air PlenumHeat balance equations


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Chapter 7Nonresidential Load Calculation Other components to heat gain

Power equipmentTable 7-16


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Hooded and unhooded equipmentTable 7-18


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Load diversity


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Ventilation and infiltration air

Moisture transfer through permeable building materials

Miscellaneous sources of heat

Temperature rise in fan stream due to fan inefficiency

Temperature rise in fan stream as a consequence of air staic and

velocity pressure Temperature rise from heat generated by motor and drive

inefficiencies

Air leakage into/ out of ducting


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Chapter 7Nonresidential Load Calculation Description of Radiant Time Series

Based on the assumption of steady-periodic conditions (the designdays weather, occupancy, and heat gain coefficients are identical

for those of preceding daysrepeat on a 24 hour cyclical basis.)

Cooling loads must address two time-delay effects

Delay of conductive heat gain through opaque exterior surfaces

Delay of radiative heat gain conversion to cooling loads Overview (see Figure 7-6)

Solar radiationRTS method same as TFM and TETD/TA

Differencescomputation of conductive heat gain, the splitting of all

heat gains into radiant and convective portions, and the conversion of

radiant heat gains into cooling loads.

Time series multiplicationto account for time delaysradiant timefactors and conduction time factors derived from heat balance

equations

Used for comparing the time delay impact of one construction to

another


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RTS Procedure

After summing to get the cooling load each hour, peak hour to

select equipmentdo this for each design month to find peak.


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Chapter 7Nonresidential Load Calculation Conduction Time Series


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Chapter 7Nonresidential Load Calculation Conduction Time Series - continued


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Chapter 7Nonresidential Load Calculation Conduction Time Factors Table 7-28 (walls) and 7-29 (roofs)


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Chapter 7Nonresidential Load Calculation Conduction Time Factors Table 7-28 (walls) and 7-29 (roofs)


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Chapter 7Nonresidential Load Calculation Thermal Properties Table 7-30

ExampleWall 1made up of F01, F09, F04, I02, F04,G01, F02


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Chapter 7Nonresidential Load Calculation Cooling Load Calculations Using RTS

Instantaneous cooling load is the rate that heat is convected intothe zone air at a given point in time. Complicated by radiative heat

transfer and the time dependency (time lag or dampening) that is

associated. Convective heat gain is immediately a cooling load.

Heat balance procedures in RTS rely on radiative-convective split

to determine contribution of internal loads to the radiative

exchange between surfaces (see Table 7-27)

The radiant time series method converts the radiant portion of

hourly heat gains to hourly cooling loads using radiant time factors,

the coefficients of the radiant time series. Current cooling loads

based on the current and past heat gains.

Two different radiant time series are used Solardirectly transmitted solar heat gainsfloor and furnishing only

Nonsolarall other types of heat gainsuniformly distributed on all

internal surfacesdiffuse solar heat gain and direct solar heat gain

from windows with inside shading


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Chapter 7Nonresidential Load Calculation Cooling Load Calculations Using RTs


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Improved accuracy and less subjective inputs


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Chapter 7Nonresidential Load Calculation Heating Load Calculations

Same except ... Credit for solar or internal heat gains not included

Thermal storage of building structure ignored

Worst case load based upon

Design interior and exterior conditions

Infiltration and/or ventilation

No solar effect

No heat gain from lights, people and appliances

Account taken for night-time setback

HVAC - Lecture 11B

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