The Theory Of Cooling Load Computation

Published Date: 02 Nov 2017

CHAPTER 3

METHODOLOGY

3.1 The theory of cooling*load computation

The*objectives*of this part*is to, estimate the amount of cooling load required to keep the room in the aircraft comfortable in the peak time of the day. Malaysia is a tropical country, where the weather is relatively stable over time. Temperature in Malaysia is between 25°C to 35°C.

The interior of an aircraft receive heat from a number of *sources. If the temperature and humidity of the air in the room is to be maintained at a suitable comfortable level, heat must be brought out. The amount of heat that was extracted is called the cooling load*. The cooling load* needs to be determining so that we can select the proper size of air conditioning unit and ducting size. It is also used to analyzed energy used and conservation.

The components that contributed to the room heat gain are:

Conduction through exterior structure.

Conduction through interior structure.

Solar*radiation through*glass.

Lighting*

People*

Equipment*

Heat from infiltration of outside air through openings*.

From the components above, we divided into two groups, which are external cooling load* and internal cooling load*.

External cooling load*

Conduction through exterior structure.

Conduction through interior structure.

Solar radiation through glass.

Heat from infiltration of outside air through openings.

Internal cooling load

Lighting

People

Equipment

There are several types of methods can be used to calculating the cooling*load* value such as transfer function*method*(TFM), cooling*load temperature difference*(CLTD) and total equivalent temperature*difference (TETD). In this thesis, the CLTD method is used.

3.1.1 Conduction*through exterior*structure

The conduction*heat gain*through the exterior walls, roof*and glasses are found from the*following equation:

Qex = U x A x CLTD*

Where;

Qex = Net room*conduction heat gain through*roof, wall or glass, in BTU/hr*

U = Overall*heat transfer coefficient for roof, wall*or glass, in*BTU/hr-ft²F

A*= Area of roof, wall or glass, in ft²

CLTD*= Correct value of cooling*load temperature*difference, in F.

The area of each component is found from the aircraft layout plan. The U values are found in Table…

The CLTD*is a temperature*difference that accounts*for the heat storage effect. Table……. Has the list of CLTD values for a few different roof and wall constructions. Table.. describes wall construction for table… list CLTD value for glass.

The value in the table is base on mean indoor design temperature*of 78°F* (25.6°C) and mean outdoor*temperature of 85°F (29.4°C), with dark colored walls and roofs. Due to the mean temperature for each places are different, we need to calculate the correct CLTD according to the right mean temperature at that place. The formula for correct cooling load temperature difference (CLTDc) are listed below.

For roof: CLTDc*= [(CLTD + LM*) x K + (78°F - Tr) + (To - 85°F)] x F

For wall: CLTDc*= (CLTD + LM*) x K + (78°F - Tr) + (To - 85°F)

For glass: CLTDc*= CLTD + (78°F – Tr) + (To - 85°F)

Where,

CLTD = Temperature from the table.

LM = Correction value for latitude and month from table value.

K = Correction thermal conductivity for color of surface;

1.0 = Dark color*or*industrial areas.

0.5 = Light*color*roof in real areas.

0.65 = Light color walls in real areas.

Tr = Actual mean room temperature, °F.

To = Actual mean outside temperature, °F.

3.1.2 Conduction*through interior*structure

The heat*that*flows from interior unconditioned spaces to the condition spaced through partitions, floor and ceiling can be found from equation:

Qi*= U x A x TD*

*Where;

Qi = Heat transfer rate through partition, floor or ceiling, in BTU/hr

U = Overall heat transfer for partition, floor or ceiling, in BTU/hr-ft²-F

A = Area of partition, floor or ceiling, in ft²

TD = Temperature difference between conditioned and unconditioned space, in °F.

If*the*temperature*of*the*unconditioned*space*is*not*known,*an* approximation often used is to assume that it is at 5°F less than the outdoor*temperature.

3.1.3 Solar*radiation*through*glass

Radiant energy*from the*sun passes through*transparent*materials such as glass*and becomes a heat*gain to*the*room.*Its*value varies with time, orientation*, shading*and*storage*effect. The*solar net heat*gain*can*be*found from*the following*equation:

Qs = SHGF*x A x SC*x CLF*

Where*;

Qs = Net solar*radiation heat gain through glass, in BTU/hr*

SHGF = Maximum*solar*heat*gain*factor, in BTU/hr-ft²

A*= Area*of*glass, in ft²

SC*= Shading*coefficient

CLF*= Cooling*load*factor for*glass

3.1.4 Lighting

Lighting is one of the factor contribute to heat gain in the cockpit. The equation for determining the heat gain is:

QL = 3.4 x W x BF x CLF*

Where;

QL = Net heat*gain from lighting, in BTU/hr

W*= Lighting capacity, in Watts*

BF*= Ballast*factor

CLF*= Cooling*loads*factor for lighting*

3.1.5 People

The heat gain from people is composed of two parts, sensible heat and the latent heat resulting from perspiration. Some of the sensible heat may be absorbed by the heat storage effect, but not the latent heat. The equations for sensible heat gain and latent heat gain comes from people are:

Qsen = qsen x n x CLF

Qla = qla x n

Where;

Qsen, Qla = Sensible and latent heat gain, in BTU/hr

Qsen, qla = Sensible and latent heat gain per person

n = Number of people

CLF = Cooling load factor for people

The rate of heat gain table Table.. list the rate of heat gain from people, according to their physical.

The heat storage effect factor CLF applied to the sensible heat gain from people. If the air condition system is shut down at right, however no storage should be include, and CLF = 1.0. Table… list the sensible hat cooling load factor for people.

3.1.6 Equipment

Heat gain from the equipment is a difficult factor to have. Sometimes the aircraft manufacture produces the data. For this analysis, this factor can be neglected, due to the data is difficult to obtain.

3.1.7 Heat from infiltration of outside air through openings

This factor cause due to air through cracks around windows or door results in both sensible and latent heat gain. In this analysis, this factor is been neglected, due to the data is difficult to obtain.

3.2 The theory of duct design

Ducting is a system used to distribute the air from any kind of system such as heating, cooling or air ventilation. The objective of the ducting is to deliver adequate amount of air at a static pressure equal or slightly greater than the total resistance offered by the duct system.

The ducting is design to handle the maximum air velocity without excessive friction loss. The standard friction resistance of duct is 1.0 to 0.2 inch of water per 100ft of straight duct. To reduce noise, sharp edges must be avoided. At the elbow part, the air is compressible for the common elbows, due to small radius. The pressure drop in the elbow is ten times (10x) than the straight part. Due to this, the radius of the elbow should be less than the duct width.

For the duct design, the procedures should be followed:

Study the layout plan of the aircraft and proposed the position of the outlet point.

Provide suitable amount of outlet within the space.

From the cooling load value, compute the air requirements (CFM) at each outlet section.

Select outlet size from the manufacturer.

Compute the size of main and branch duct by using one of the methods:

0Velocity reduction method*

0Equal friction method*

0Static regain method*

3.2.1 Velocity reducing method*

In this velocity reduction duct design method, the factor control is the0air0velocity in the duct. This is commonly used to prevent noise due to high air velocity. Usually, the duct velocity is determined before the size of the duct is selected. With the air velocity, the diameter of the duct is computed. Then, the shape size whether rectangular or round is selected and the relevant static loss is calculated accordingly.

3.2.2 Equal friction method

The principal of this method is to make the pressure loss per feet of the length is same for the entire system. By using this method, balancing is required for symmetrical layout in which all runs, the longest run will required considerable damper.

Usually practice is to select the velocity in the main duct near fan from the standpoint of noise for the particular application. Since the air required (CFM) is known,