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Heat Load Calculation Spreadsheet

Introduction

When talking about thermal load on a building, it is understood that it is a phenomenon that tends to modify the interior temperature of the air or its content in humidity.

In this sense it is possible to establish a first classification of the thermal loads, according to their incidence:

• Sensitive thermal loads: those that originate in a variation in the temperature of the air.

• Latent thermal loads: those that originate from a variation in the absolute humidity of the environment (water content in the air).

On the other hand, the knowledge of the thermal loads is essential, as a preliminary step for the task of designing the interior air conditioning system of a building, dependence or premises.

Precisely in the design of an air conditioning system that calculates the thermal loads for the summer and winter design situations, dimensioning the installation for the most unfavorable situation.

Components of the thermal load

Depending on the origin, two large groups of thermal loads can be distinguished:

• Thermal loads from the outside environment of the building:

In turn, external thermal loads can be of various types:

– Loads through enclosures;

– Loads through glazed surfaces, windows and skylights;

– Loads introduced through ventilation;

– Loads owing an infiltration.

• Thermal loads generated inside the building:

In turn, the internal thermal loads can be of several types:

– Charges generated by people;

– Lighting loads;

– Loads generated by electrical equipment, computer …

– Other loads generated in the interior.

Calculation of the thermal load for cooling

This tutorial will focus on calculating the thermal loads to meet the cooling needs of a building, dependency or location.

The calculation of the thermal load of the cooling (Qr) is necessary to know the cooling capacity of the air conditioners that are used the uses, and in the last instance of its electrical energy consumption.

The total thermal load of the cooling (Qr) of a room is obtained from the expression:

Qr = Qs + Ql

Where:

Qs is the sensible thermal load (W);

Ql is the latent thermal load (W).

The following sections describe how to calculate the sensible and latent thermal loads that are transmitted to the premises, in order to sum them up and thus obtain the total thermal load.

2- Calculation of the sensitive thermal load

General Expression

For the calculation of the sensible thermal load (Qs) the following expression is used:

Qs = Qsr + Qstr + Qst + Qsi + Qsai

Where:

Qsr is the value of the sensitive charge due to solar radiation through the glazed surfaces (W);

Qstr is the sensitive charge by transmission and radiation through exterior walls and ceilings (W);

Qst is the load sensitive by transmission through walls, ceilings, floors and interior doors (W);

Qsi is the sensitive charge transmitted by external air infiltrations (W);

Qsai is the sensitive load due to internal inputs (W).

Therefore, the calculation of the sensitive load is based on calculating each of the different previous loads and adding them, thus obtaining the value of the total sensible load. And this is precisely what will be done in the next sections.

Charging by solar radiation through crystal “Qsr”

Solar radiation passes through the translucent and transparent surfaces and impinges on the interior surfaces of the room, heating them, which in turn increases the temperature of the indoor environment.

The thermal radiation charge through crystals and translucent surfaces (Qsr) is calculated as follows:

Qsr = S · R · F

Where:

Qsr is the thermal charge by solar radiation through glass, in W.

S is the translucent or glazed surface exposed to radiation, in m2.

R is the solar radiation that crosses the surface, in W / m2, corresponding to the orientation, month and latitude of the place considered.

As a reference are attached some links that provide values ​​of solar radiation:

F is the correction factor of the radiation depending on the type of glass used in the window, effects of shadows that may exist, etc. This value can be obtained from the tables included in the CTE-DB HE Energy Saving document. An extract is included that includes how to calculate the solar factor (F) of hollows and skylights:

Transmission load and radiation through exterior walls and ceilings “Qstr”

Transmission load and radiation transmitted through exterior bounding walls and ceilings (Qstr) is calculated as follows:

Qstr = K · S · (Tec-Ti)

Where:

Qstr is the transmission load through exterior walls and ceilings, in W.

K is the global coefficient of thermal transmission of the enclosure, also called thermal transmission, expressed in W / m2ºC.

S is the surface of the wall exposed to the temperature difference, in m2.

Ti is the interior design temperature of the room (° C)

Tec is the outside temperature of calculation on the other side of the room (ºC)

For the calculation of the coefficient of thermal transfer of the enclosure (K), the following links are attached:

As the design interior temperature (Ti) you can take the values ​​from the following table, which collects the design conditions for the temperature and relative humidity of the indoor air, according to the seasons of the year:

Interior design conditions

Finally, in order to obtain the value of the external temperature of calculation (Tec) it is in turn part of the so-called external design temperature (Te).

The design external temperature (Te) is calculated by taking into account the average temperature of the warmest month (Tme) and the maximum temperature of the warmest month (Tmax) of the place, from the following expression:

Design exterior temperature, Te = 0.4 · Tme + 0.6 · Tmax

To obtain the values ​​of the mean temperature of the warmest month (Tme) and the maximum temperature of the warmest month (Tmax) the following link is attached where this information can be obtained:

The calculation external temperature (Tec) will finally be calculated from the exterior design temperature (Te) and the orientation of the enclosure being considered, from the following table:

* Calculation outside temperature (Tec)

Transmission load through walls, ceilings, floors and interior doors “Qst”.

The transmission load through the internal enclosures of the premises that limit it with other rooms of the building (Qst) is calculated by applying the following expression:

Qst = K · S · (Te-Ti)

Where:

Qst is the transmission load through the inner enclosures, in W.

K is the global coefficient of thermal transmission of the enclosure, also called thermal transmittance, expressed in W / m2ºC.

S is the surface of the inner enclosure, in m2.

Te is the design temperature on the other side of the enclosure (ºC).

Ti is the interior design temperature of the room (° C).

As the design interior temperature (Ti), the values ​​indicated in Table 1 above can be taken, which includes the design conditions for the temperature and relative humidity of the indoor air, according to the seasons of the year.

Finally, in order to choose the exterior design temperature (Te) corresponding to the rooms of the building that limit with the place of calculation, the uses given to these rooms will be taken into account.

Load transmitted by external air infiltration “Qsi”.

The load transmitted by infiltration and external air ventilation (Qsi) is determined by the following expression:

Qsi = V · ρ · Ce, air · ΔT

Where:

Qsi is the thermal load by infiltration and ventilation of external air (W);

V is the flow of infiltrated air and ventilation (m3 / s);

Ρ is the density of the air, with a value of 1.18 kg / m3;

Ce, air is the specific heat of the air, of value 1012 J / kgºC;

ΔT is the temperature difference between the outdoor and indoor environment.

In Chapter 4 of tutorial no. 251 “Ventilation and Renovation of Indoor Air in Buildings” you can obtain the ventilation flow of premises and buildings depending on their use.

In this way, once the air ventilation flow rate of the exterior that enters the room and applying the previous formulation can be obtained the thermal load due to ventilation and infiltration.

Sensitive charge for internal contributions “Qsai”

The gain of the sensitive load due to the internal inputs of the premises (Qsai) is determined in turn as sum of the following types of loads that are generated within the same:

Qsai = Qsil + Qsp + Qse

Where:

Qsil is the value of the internal gain of sensible load due to the interior lighting of the room (W);

Qsp is the internal gain of sensible load due to the occupants of the premises (W);

Qse is the internal gain of sensible load due to the various apparatuses in the room, such as electrical appliances, computers, etc. (W).

• Sensitive light load (Qsil):

For the calculation of the sensible thermal load provided by the interior lighting of the establishment, the full power of the lighting lamps shall be deemed to be converted into sensible heat.

In the case of fluorescent or discharge lamps, the total power of all lamps shall be multiplied by 1.25 to consider the supplementary consumption of the reactances.

– Incandescent lamps:

Qsil, incandescent = n · PotLamp. incandescent

Where n is the number of incandescent type lamps placed.

– Fluorescent or discharge lamps:

Qsil, discharge = 1,25 · n · PotLamp. discharge

Where n is the number of fluorescent lamps placed.

The gain of light sensitive load will be obtained as the sum of the above:

Qsil = Qsil, incandescent + Qsil, discharge.

• Occupant Sensitive Load (Qsp):

To calculate the sensible load that each person (Qsp) contributes, it is necessary to know beforehand the different thermal loads that originate:

– Radiation: because the average body temperature is higher than the surrounding objects.

– Convection: since the surface of the skin is at a higher temperature than the air that surrounds it, creating small convection currents that contribute heat to the air.

– Driving: originated from the contact of the body with other elements around it.

– Breathing: which causes a supply of heat by the exhaled air, which is at a higher temperature. There is also a supply of water vapor which will increase the relative humidity of the air.

– Skin evaporation: this contribution of heat can be important in summer.

The load per occupation therefore has a sensitive component and a latent component, due to the latter as much to respiration as to perspiration. In both cases it will be necessary to take into account the number of occupants of the estancia.

Heat Load Calculation Spreadsheet

The table below shows the values ​​of latent and sensitive heat, in kcal / h, released by a person according to the activity and the temperature in the room:

Heat latent and sensitive detached by person

The expression to obtain the sensible heat of contribution by the occupation of the premises would be the following one:

Qsp = n · Sensitive, person

being:

N is the number of people expected to occupy the premises;

Sensitive, person is the sensible heat per person and activity that realizes, according to the table 3.

• Sensitive charging by electrical appliances (Qse):

For the calculation of the thermal load contributed by the machinery, equipment and other appliances present in the air-conditioned space of the premises, it will be considered that the integrated power of the machines and equipment present in that room will be transformed into sensible heat.

On the other hand, all equipment and appliances are considered not to work all at once, so it will affect a coefficient of simultaneity of 0.75 to the sum obtained from all powers.

Total Sensitive Load “Qs”.

The total sensible load (Qs) contributed to the premises is the sum of all the above:

Qs = Qsr + Qstr + Qst + Qsi + Qsil + Qsp + Qse

3- Calculation of the latent thermal load

General Expression

The following expression is used to calculate the latent thermal load (Ql):

Ql = Qli + Qlp

Where:

Qli is the latent load transmitted by external air infiltrations (W);

Qlp is the latent load due to occupation of the premises (W).

Therefore, the latent load calculation is based on calculating each of the different previous loads and adding them, thus obtaining the value of the total latent load. And this is precisely what will be done in the next sections.

Latent load transmitted by external air infiltrations “Qli”.

The latent load transmitted by infiltration and external air ventilation (Qli) is determined by the following expression:

Qli = V · ρ · Cl, water · Δw

Where:

Qli is the latent thermal load by external air ventilation (W)

V is the rate of infiltrated air and ventilation (m3 / s);

Ρ is the density of the air, with a value of 1.18 kg / m3;

Cl, water is the specific heat of water, 2257 kJ / kg;

Δw is the absolute humidity difference between the indoor and outdoor environment.

In Chapter 4 of tutorial no. 251 “Ventilation and Renovation of Indoor Air in Buildings” you can obtain the ventilation flow of premises and buildings depending on their use.

In this way, once the air ventilation flow rate of the exterior that enters the room and applying the previous formulation can be obtained latent heat load due to ventilation and infiltration in the room.

Latent load per occupation “Qlp”.

The latent load per occupancy of the premises (Qlp) is determined by multiplying the valuation of the latent heat emitted by the person-type and by the number of occupants predicted for the premises.

As shown, Table 3 shows the latent and sensible heat values, in kcal / h, released by a person according to the activity and the temperature in the room.

Heat Load Calculation Spreadsheet

The expression to obtain the latent heat of contribution by the occupation of the premises would be the following one:

Qlp = n · Clatente, person

being:

N is the number of people expected to occupy the premises;

Clatente, person is the latent heat per person and activity that realizes, according to table 3.

. Total latent load “Q1”.

The total latent load (Ql) contributed to the premises is the sum of all the previous ones:

Ql = Qli + Qlp