epgep cooling load - eu standards

COOLING LOAD CALCULATION

PRINCIPLES II.

EN STANDARDS

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- Sensible load calculation of a space (MSZ EN 15255)

- Latent load calculation of a space (MSZ EN 15243)

Sensible load calculation of a space(MSZ EN 15255)

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The heat flow due to the solar radiation absorbed and the vault sky losses by the light components (opaque and transparent; thickness ≤ 120 mm) :

jeersr

w

jfkeersr

l

kfsl hUqlSAhUqISA )]/([)]/([

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1

⋅+⋅⋅+⋅+⋅⋅=Φ ∑∑==

where:

A - Area of surfaces [m2],

fS - Solar factor of each opaque components [-],

2fS - The window secondary solar factor; [-],

srI - Intensity of the solar radiation reaching the surfaces [W/m2],

erq - Heat load (convection part) [W/m2],

U - Thermal transmittance under steady state conditions [W/m2K],

eh - surface heat transfer coefficient [W/m2K].


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The heat flow due to the solar radiation absorbed and the vault sky losses by the opaque heavy component ( thickness >120 mm) is given by:

yeersr

h

yfsh hUqISA )]/([

1

⋅+⋅⋅=Φ ∑=

The solar radiation reaching the surface of the building envelope components is given by:

rdDssr IIIfI ++⋅= where

sf - the sunlit factor due to external obstructions, derived from EN ISO 13791;

DI - the direct component of the solar radiation reaching the surface;

dI - the diffuse component of the solar radiation reaching the surface;

rI - the reflected component of the solar radiation reaching the surface.


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Example to the instantaneous total solar radiation on the exposure of the wall (W/m2):

Hour Vertical west wall W/m2

4:00 05:00 226:00 557:00 808:00 1019:00 11710:00 12811:00 13512:00 150 13:00 36614:00 558 15:00 70316:00 77817:00 75618:00 60419:00 27120:00 0

Latent load calculation of a space(MSZ EN 15243)

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For the cooling coil condensation, a simple model is used. It assumes that one part of the air passing through the coil is not treated, and that the other part leaves at the dew point characteristics of the cooling coil temperature. This latter air flow is called here recirculated air flow. The caculation of recirculated air flow requirements depend on the type of the water control.

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Recirculated air flow

Control 1. Nonvariable water flow: In this case the cooled coil temperature is equal to the water inlet + 2K. The required recirculated air flow depends on the sensible cooling need and the cooling coil temperature.

)26(,

satpa

sensrecircm c

Qq

θ−⋅=

where:

recircmq , - required recircuated air mass flow rate [kg/s],

sensQ - sensible cooling need [W],

pac - mass heat of dry air [J/kg K],

satθ - cooled coil temperature [°C].

Control 2. Variable water flow: aifrecircm Aq ρ⋅⋅= 15,

where:

aiρ - internal air density [kg/m3],

fA - floor area [m2]


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On a given hour, 2 situations can occur: 1. At the start of the hour, the indoor humidity is higher than the saturation humidity for the

cooling coil. The condensation will occur, either during the whole hour either during one part of it.

2. At the beginning of the hour, the indoor humidity is lower than the saturation humidity for

the cooling coil. The cooling coil will be dry at the beginning, but condensation can occurbefore the end of the hour.


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For a duration of t, indoor air humidity (at the end):

)1(,,tB

startifini eAxx ⋅−−⋅+= [kg/s]

A and B are calculated as follows: If the cooling coil is wet:

startrecircmmsatrecircmimentrwet xqqxqaqxA −+⋅++⋅= )/()( ,,

)/()( , airecircmmwet VqqB ρ⋅+=

If the cooling coil is dry:

startmimentrdry xqaqxA −+⋅= /)(

)/( aimdry VqB ρ⋅=

xi - indoor humidity kg/kg dry air, xentr - entering air humidity kg/kg dry air, qm - entering air flow kg/s, xstart - indoor humidity at the beginning kg/kg, ai - internal humidity gains kg/s, ρai - density of indoor air kg/m3, xsat - saturated humidity at the cooling coil temperature kg/kg.


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Latent energy demand:

)(25001000 ,, satmoyirecircmlat xxqQ −⋅= [W]

xi,moy is proportional with xi,fin according to the fact that the cooling coil is wet or dry at the end of the hour.


Hungarian Standard for calculation ofcooling load (MSZ-04.140/4)

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Calculation of outdoor heat load:

Components of outdoor heat load:

üFe QQQ += && [W]

where:

FQ& - solar heat load through exterior building structures (wall, roof) into the space,

üQ& - solar heat load through windows into the space.

Solar heat load through exterior building structures (wall, roof):

ekvFF tkAQ ∆⋅⋅=& [W]

where:

FA - surface of wall or roof [m2]

k - thermal transmittance of the wall or roof

Km

W2

ekvt∆ - equivalent thermal difference ][K

The equivalent thermal difference depends on the following parameters:

- laying of the exterior structures (walls, roof), - quality of exterior surfaces (absortion and emission features of building envelope), - thermal parameters of exterior surfaces (thermal transmittance), - inside temperature.

Hungarian Standard for calculation ofcooling load (MSZ-04.140/4)

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Solar heat load through windows:

)]([ ieÜSRGÁÜÜÜ ttkZINNAQ −⋅+⋅⋅⋅⋅=& [W]

ahol

ÜA - surface of windows ][ 2m

ÜN - galzing factor (similar to SHGC coefficient – determine the size of direct

solar heat load that goes into the space) [-]

Á

N - shading factor [-]

SRGI - Intensity of solar radiant gain ]/[ 2mW

Z - reduction factor [-] (takes into account the heating capacity of building envelope )

Ük - thermal transmittance of the window

Km

W2

et - outdoor air temperature ][ C°

it - indoor air temperature ][ C°

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Thank you for your attention !

epgep cooling load - eu standards

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