Name 409 refrigeration and air conditioning

NAME-409 Marine Engineering -II
Cdre M Muzibur Rahman, (E), psc, PhD, BN
Refrigeration and Air Conditioning
Cdre Muzib, psc, PhD
1
The subject of refrigeration and air conditioning has evolved out of human need for food and
comfort.
What is Refrigeration?
Refrigeration may be defined as the process of achieving and maintaining a temperature
below that of the surroundings, the aim being to cool some product or space to the required
temperature.
What is Air Conditioning:
Air Conditioning refers to the treatment of air so as to simultaneously control its temperature,
moisture content, cleanliness, odor and circulation, as required by occupants, a process, or
products in the space.

It is impossible to extract an amount of heat (QH) from a hot reservoir and use it all to do
work (W). Some amount of heat (QC ) must be exhausted to a cold reservoir. This
precludes engine.
This is sometimes called the "first form" of the second law, and is referred to as the
Kelvin-Planck statement of the second law.
Second Law of Thermodynamics
Cdre Muzib, psc, PhD 2

It is not possible for heat to flow from a colder body to a warmer body without
any work having been done to accomplish this flow. Energy will not flow spontaneously
from a low temperature object to a higher temperature object. This precludes a
perfect refrigerator. The statement is also applicable to air conditioners and heat pumps,
which embody the same principles.
This is the "second form" or Clausius statement of the second law.
Second Law of Thermodynamics

Second Postulate: 2nd Law of Thermodynamics

Reversed Carnot cycle

The performance of refrigerators is expressed in terms of coefficient
of performance (COP):
Performance of Refrigeration System
Under the same operating condition:
 The COP improves by 2 to 4% for each °C rise of evaporating temperature or
each °C fall of condensing temperature.

Type of refrigeration process:
a. Vapour compression refrigeration: Here moving part is the compressor
which sucks the refrigerant (vapour) from evaporator and compresses it to
the high pressure to provide work input to have cooling effect.
b. Vapour absorption refrigeration: Here, the process of suction and
compression are carried out by two different devices called as the absorber
and the generator. Thus the absorber and the generator replace the
compressor in the vapor absorption cycle. The absorbent enables the flow of
the refrigerant from the absorber to the generator by absorbing it. In this
case the flow is maintained by a pump.

Vapour Compression Refrigeration Systems
The basis of modern refrigeration is the ability of liquids to absorb enormous
quantities of heat as they boil and evaporate.
Professor William Cullen of the University of Edinburgh demonstrated this in 1755
by placing some water in thermal contact with ether under a receiver of a vacuum
pump. The evaporation rate of ether increased due to the vacuum pump and water
could be frozen. This process involves two thermodynamic concepts, the vapour
pressure and the latent heat.
A liquid is in thermal equilibrium with its own vapor at a pressure called the
saturation pressure, which depends on the temperature alone. If the pressure is
increased for example in a pressure cooker, the water boils at higher temperature.
The second concept is that the evaporation of liquid requires latent heat during
evaporation. If latent heat is extracted from the liquid, the liquid gets cooled. The
temperature of ether will remain constant as long as the vacuum pump maintains a
pressure equal to saturation pressure at the desired temperature. This requires the
removal of all the vapors formed due to vaporization.
If a lower temperature is desired, then a lower saturation pressure will have to be
maintained.

Vapour compression cycle:
Assumptions

Vapour Compression Refrigeration cycle

P-h Diagram

A flash tank is a pressure vessel, wherein the refrigerant liquid and vapour are
separated at an intermediate pressure. The refrigerant from condenser is first
expanded to an intermediate pressure corresponding to the pressure of flash tank, Pi
using a low side float valve. The float valve also maintains a constant liquid level in
the flash tank. In the flash tank, the refrigerant liquid and vapour are separated. The
saturated liquid is fed to the evaporator after throttling it to the required evaporator
pressure, Pe using an expansion valve. Depending upon the type of the system, the
saturated vapour in the flash tank is either compressed to the condenser pressure or
throttled to the evaporator pressure. In the absence of flash tank, the refrigerant
condition at the inlet to the evaporator would have been considerably with high
vapour quality which would reduce the refrigeration effect.

Ton Refrigeration
It is defined as the heat of fusion absorbed by
melting 1 ton (1000 kg) of pure ice at 0 °C (32 °F) in
24 hours. It is equivalent to the consumption of one
ton of ice per day and originated during the transition
from stored natural ice to mechanical refrigeration.
A refrigeration ton is approximately equivalent to
12,000 BTU/h or 3.51685 kW or 211 kJ/min.
Air-conditioning and refrigeration equipment capacity
in the U.S. is often specified in "tons" (of
refrigeration). Many manufacturers also specify
capacity in BTU/h, especially when specifying the
performance of smaller equipment.

Condensers:
Based on the external fluid, condensers can be classified as:
a) Air cooled condensers
b) Water cooled condensers, and
c) Evaporative condensers
Air-cooled condensers:
In air-cooled condensers air is the external fluid, i.e., the refrigerant
rejects heat to air flowing over the condenser. Air-cooled condensers
can be again of natural convection type or forced convection type.
Natural convection type
Forced convection type

Water Cooled Condensers:
In water cooled condensers water is the external fluid. Depending upon the
construction, water cooled condensers can be further classified into:
1. Double pipe or tube-in-tube type
2. Shell-and-coil type
3. Shell-and-tube type
Double Pipe or tube-in-tube type:
Double pipe condensers are normally used up to 10 TR
capacity. Figure aside shows the schematic of a double
pipe type condenser. In these condensers the cold water
flows through the inner tube, while the refrigerant flows
through the annulus in counter flow. Headers are used
at both the ends to make the length of the condenser
small and reduce pressure drop. The refrigerant in the
annulus rejects a part of its heat to the surroundings by
free convection and radiation. The heat transfer
coefficient is usually low because of poor liquid
refrigerant drainage if the tubes are long.

Shell-and-coil type:
These condensers are used in systems up to 50 TR capacity. The water flows through
multiple coils, which may have fins to increase the heat transfer coefficient. The refrigerant
flows through the shell. In smaller capacity condensers, refrigerant flows through coils while
water flows through the shell. When water flows through the coils, cleaning is done by
circulating suitable chemicals through the coils.

Shell-and-tube type:
This is the most common type of condenser used in systems from 2 TR upto thousands of TR capacity.
Here, the refrigerant flows through the shell while water flows through the tubes in single to four passes.
The condensed refrigerant collects at the bottom of the shell. The coldest water contacts the liquid
refrigerant so that some subcooling can also be obtained. The liquid refrigerant is drained from the
bottom to the receiver. There might be a vent connecting the receiver to the condenser for smooth
drainage of liquid refrigerant. The shell also acts as a receiver. Further the refrigerant also rejects heat to
the surroundings from the shell. The most common type is horizontal shell type. Vertical shell-and-tube
type condensers are usually used with ammonia in large capacity systems so that cleaning of the tubes is
possible from top while the plant is running.

Evaporative condensers
Here, both air and water are used to extract heat from the condensing refrigerant. It combines the features of a cooling
tower and water-cooled condenser in a single unit as follows:
 Used in medium to large capacity systems
 Normally cheaper compared to water cooled condensers
 Used in places where water is scarce. Since water is used in a closed loop, only a small part of the water evaporates.
Make-up water is supplied to take care of the evaporative loss. The water consumption is typically very low, about 5
percent of an equivalent water cooled condenser with a cooling tower.
 Since condenser has to be kept outside, this type of condenser requires a longer length of refrigerant tubing, which calls
for larger refrigerant inventory and higher pressure drops. Since the condenser is kept outside, to prevent the water from
freezing, when outside temperatures are very low, a heater is placed in the water tank.
 When outside temperatures are very low it is possible to switch-off the water pump and run only the blowers, so that the
condenser acts as an air cooled condenser.
Another simple form of condenser used normally in older type cold storages is called as atmospheric condenser. The
principle of the atmospheric condenser is similar to evaporative condenser, with a difference that the air flow over the
condenser takes place by natural means as no fans or blowers are used. A spray system sprays water over condenser
tubes. Heat transfer outside the tubes takes by both sensible cooling and evaporation, as a result the external heat transfer
coefficient is relatively large. The condenser pipes are normally large, and they can be either horizontal or vertical. Though
these condensers are effective and economical they are being replaced with other types of condensers due to the problems
such as algae formation on condenser tubes, uncertainty due to external air circulation etc.

Refrigerants
Refrigerant is a substance or mixture, usually a fluid, used in a heat pump and refrigeration
cycle. In most cycles it undergoes phase transitions from a liquid to a gas and back again.
Fluorocarbons, especially chlorofluorocarbons, became commonplace in the 20th century, but
they are being phased out because of their ozone depletion effects. Other common
refrigerants used in various applications are ammonia, sulfur dioxide, and non-
halogenated hydrocarbons such as propane.
The ideal refrigerant would have favorable thermodynamic properties, be noncorrosive to
mechanical components, and be safe, including free from toxicity and flammability.
It would not cause ozone depletion or climate change. Since different fluids have the desired
traits in different degree, choice is a matter of trade-off.
The desired thermodynamic properties are a boiling point below the target temperature, a
high heat of vaporization, a moderate density in liquid form, a relatively high density in
gaseous form, and a high critical temperature.

Refrigerants may be divided into three classes according to their manner of
absorption or extraction of heat from the substances to be refrigerated:
Class 1: This class includes refrigerants that cool by phase change (typically boiling),
using the refrigerant's latent heat.
Class 2: These refrigerants cool by temperature change or 'sensible heat', the
quantity of heat being the specific heat capacity x the temperature change. They are
air, calcium chloride brine, sodium chloride brine, alcohol, and similar nonfreezing
solutions. The purpose of Class 2 refrigerants is to receive a reduction of temperature
from Class 1 refrigerants and convey this lower temperature to the area to be air-
conditioned.
Class 3: This group consists of solutions that contain absorbed vapors of liquefiable
agents or refrigerating media. These solutions function by nature of their ability to
carry liquefiable vapors, which produce a cooling effect by the absorption of their
heat of solution. They can also be classified into many categories.

Desired properties of a Refrigerant:
1. Vapor density: To enable use of smaller compressors and other equipment the refrigerant should have
smaller vapor density.
2. Enthalpy of vaporization: To ensure maximum heat absorption during refrigeration, a refrigerant should
have high enthalpy of vaporization.
3. Thermal Conductivity: Thermal conductivity of the refrigerant should be high for faster heat transfer during
condensation and evaporation.
4. Dielectric strength: In hermetic arrangements, the motor windings are cooled by refrigerants vapor on its
way to the suction valve of the compressor. Therefore, dielectric strength of refrigerant is important property
in hermetically sealed compressor units.
5. Critical temperature: In order to have large range of isothermal energy transfer, the refrigerant should
have critical temperature above the condensing temperature.
6. Specific heat: To have minimum change in entropy during the throttling process, the specific heat should
be minimum. For this, liquid saturation line should be almost vertical.
7. Leak tendency: The problems with leakage are wearing out of joint or the material used for the fabrication
of the system. A denser refrigerant will have fewer tendencies to leak as compared to higher density
refrigerant. The detection of leaks should be easy to loss of refrigerant. Leakage can be identified quickly if
the refrigerant has distinct color or odour.
8. Toxicity: The refrigerant used in air conditioning, food preservation etc. should not be toxic in nature as
they will come into contact with human beings. Refrigerants will affect human health if they are toxic.
9. Cost of refrigerants: The quantity of refrigerant used in industries is very less. The cost of the refrigerants
is generally high when compared to other chemicals in the industry. Cost Per kg R22 Tk 800-1000, R134A Tk
1200-1800, R503 Tk 3000 – 4000.
10. Availability: Refrigerants should be available near the usage point. It must be sourced and procured
within a short period to enable the user in case of leaks, maintenance schedules etc.

Primary and secondary refrigerants
Primary refrigerants are those fluids, which are used directly as working fluids. When
used in compression or absorption systems, these fluids provide refrigeration by
undergoing a phase change process in the evaporator.
Secondary refrigerants are those liquids, which are used for transporting thermal
energy from one location to other. Commonly used secondary refrigerants are the
solutions of water and ethylene glycol, propylene glycol or calcium chloride. These
solutions are generally called brines. If the operating temperatures are above 0oC,
then pure water can also be used as secondary refrigerant. Brines are used at sub-
zero temperatures. Unlike primary refrigerants, the secondary refrigerants do not
undergo phase change as they transport energy from one location to other. An
important property of a secondary refrigerant is its freezing point. The temperature
at which freezing of a brine takes place depends on its concentration. The
concentration at which a lowest temperature can be reached without solidification is
called as eutectic point.

Vapor Absorption Refrigeration

There are several common combinations of absorbent-refrigerants:
• Water and Ammonia
• Lithium Nitrate and Ammonia
• Lithium Bromide and Water
• Lithium Chloride and Water

Simple Absorption System:
1) Condenser: Just like the vapor compression cycle, the refrigerant enters the condenser at high pressure
and temperature and gets condensed. The condenser is of water cooled type.
2) Expansion valve or restriction: When the refrigerant passes through the expansion valve, its pressure
and temperature reduces suddenly. This refrigerant then enters the evaporator.
3) Evaporator: The refrigerant at very low pressure and temperature enters the evaporator and produces the
cooling effect. In the vapor compression cycle this refrigerant is sucked by the compressor, but in the vapor
absorption cycle, this refrigerant flows to the absorber that acts as the suction part of the refrigeration cycle.
4) Absorber: The absorber is a sort of vessel consisting of water that acts as the absorbent, and the previous
absorbed refrigerant. Thus the absorber consists of the weak solution of the refrigerant (for example:
ammonia) and absorbent (e.g. water). When ammonia from the evaporator enters the absorber, it is absorbed
by the absorbent due to which the pressure inside the absorber reduces further leading to more flow of the
refrigerant from the evaporator to the absorber. At high temperature water absorbs lesser ammonia, hence it
is cooled by the external coolant to increase its ammonia absorption capacity.
5) Pump: When the absorbent absorbs the refrigerant strong solution of refrigerant-absorbent is formed. This
solution is pumped by the pump at high pressure to the generator. Thus pump increases the pressure of the
solution to about 10bar.
6) Generator: The refrigerant-ammonia solution in the generator is heated by the external source of heat.
This is can be steam, hot water or any other suitable source. Due to heating the temperature of the solution
increases. The refrigerant in the solution gets vaporized and it leaves the solution at high pressure. The high
pressure and the high temperature refrigerant then enters the condenser, where it is cooled by the coolant,
and it then enters the expansion valve and then finally into the evaporator where it produces the cooling
effect. This refrigerant is then again absorbed by the weak solution in the absorber.

Ammonia-Water Absorption Refrigeration
Absorption refrigeration systems
have important commercial and
industrial applications.
The principal components of an
ammonia-water absorption
system are shown in the figure.
Absorber
coolant

The left-side of the schematic
includes components familiar
from the discussion of the
vapor-compression system:
evaporator, condenser, and
expansion valve.
Only ammonia flows through
these components.
Absorber
coolant

►The right-side of the
schematic includes
components that replace
the compressor of the
vapor-compression
refrigeration system:
absorber, pump, and
generator. These
components involve
liquid ammonia-water
solutions.
Absorber
coolant

A principal advantage
of the absorption
system is that – for
comparable
refrigeration duty – the
pump work input
required is intrinsically
much less than for the
compressor of a
vapor-compression
system.
Absorber
coolant

►Specifically, in the absorption
system ammonia vapor coming
from the evaporator is absorbed
in liquid water to form a liquid
ammonia-water solution.
►The liquid solution is then
pumped to the higher operating
pressure. For the same
pressure range, significantly less
work is required to pump a liquid
solution than to compress a
vapor (see discussion of Eq.
6.51b).
Absorber
coolant

►However, since only
ammonia vapor is allowed to
enter the condenser, a
means must be provided to
retrieve ammonia vapor from
the liquid solution.
►This is accomplished by
the generator using heat
transfer from a relatively
high-temperature source.
Absorber
coolant

Ammonia-Water Absorption
Refrigeration
►Steam or waste heat
that otherwise might go
unused can be a cost-
effective choice for the
heat transfer to the
generator.
►Alternatively, the heat
transfer can be provided
by solar thermal energy,
burning natural gas or
other combustibles, and
in other ways.
Absorber
coolant

Example -1
Refrigerant-134a is the working fluid in an ideal compression refrigeration
cycle. The refrigerant leaves the evaporator at -20oC and has a condenser
pressure of 0.9 MPa. The mass flow rate is 3 kg/min. Find COPR and
COPR, Carnot for the same Tmax and Tmin , and the tons of refrigeration.
Using the Refrigerant-134a Tables, we have
1
2
2 2
1
1 2
2 1
1
3
3
2
1
238.41
278.23
900
20
0.9456 43.79
0.9456
1.0
3
900
0
s
s
o
o
s
s
State
State kJ
kJ
h Compressor exit
h
Compressor inlet kg
kg
P P kPa
kJ
T C
s T C
kJ
kg K s s
x
kg K
State
Condenser exit
P kPa
x

 
 
  
 
 
 
   
 

 
  
 
  



3 4
4
4 1
3
4 3
4
101.61 0.358
0.4053
20
0.3738
.0
o
State
kJ
h x
Throttle exit
kg
kJ
s
kJ T T C
s kg K
kg K h h


 



 
 

  
 
 

 
 

  Cdre Muzib, psc, PhD 53

1 4 1 4
, 2 1 2 1
( )
( )
(238.41 101.61)
(278.23 238.41)
3.44
L
R
net in
Q m h h h h
COP
W m h h h h
kJ
kg
kJ
kg
 
  
 




The tons of refrigeration, often called the cooling load or
refrigeration effect, are
1 4
( )
1
3 (238.41 101.61)
min 211
min
1.94
L
Q m h h
kg kJ Ton
kJ
kg
Ton
 
 

,
( 20 273)
(43.79 ( 20))
3.97
L
R Carnot
H L
T
COP
T T
K
K


 

 


Another measure of the effectiveness of the refrigeration cycle is how
much input power to the compressor, in horsepower, is required for each
ton of cooling.
The unit conversion is 4.715 hp per ton of cooling.
, 4.715
4.715
3.44
1.37
net in
L R
W
Q COP
hp
Ton
hp
Ton




7. An ideal vapor-compression refrigerant cycle operates at steady state
with Refrigerant 134a as the working fluid. Saturated vapor enters the
compressor at -100C, and saturated liquid leaves the condenser at 280C.
The mass flow rate of refrigerant is 5 kg/min. Determine (a) The
compressor power, in kW (b) The refrigerating capacity, in tons. (c) The
coefficient of performance.
8. A vapor-compression refrigeration system circulates Refrigerant 134a at
rate of 6 kg/min. The refrigerant enters the compressor at -100C, 1.4 bar,
and exits at 7 bar. The isentropic compressor efficiency is 67%. There are
no appreciable pressure drops as the refrigerant flows through the
condenser and evaporator. The refrigerant leaves the condenser at 7 bar,
240C. Ignoring heat transfer between the compressor and its surroundings,
determine (a) The coefficient of performance. (b) The refrigerating capacity,
in tons. (c) The irreversibility rates of the compressor and expansion valve,
each in kW (d) The changes in specific flow availability of the refrigerant
passing through the evaporator and condenser, respectively, each in kJ/kg.

Air Conditioning

Classification of air conditioning systems:
Based on the fluid media used in the thermal
distribution system, air conditioning systems
can be classified as:
1. All air systems
2. All water systems
3. Air- water systems
4. Unitary refrigerant based systems

In ‘all air system’ air is used as the media that transports
energy from the conditioned space to the A/C plant. In
these systems air is processed in the A/C plant and this
processed air is then conveyed to the conditioned space
through insulated ducts using blowers and fans. This air
extracts (or supplies in case of winter) the required
amount of sensible and latent heat from the conditioned
space. The return air from the conditioned space is
conveyed back to the plant, where it again undergoes the
required processing thus completing the cycle. No
additional processing of air is required in the conditioned
space.
All air systems can be further classified into:
1. Single duct systems, or
2. Dual duct systems.
1. All air systems:

1. All air systems: Fig.36.2. A constant
volume, Single duct,
single zone system
This system is called as a single duct system as there is only one supply duct, through which either hot air or cold
air flows, but not both simultaneously. It is called as a constant volume system as the volumetric flow rate of
supply air is always maintained constant. It is a single zone system as the control is based on temperature and
humidity ratio measured at a single point. Here a zone refers to a space controlled by one thermostat. However, the
single zone may consist of a single room or one floor or whole of a building consisting of several rooms. A
separate sub-system controls the amount of OD air supplied by controlling the damper position.

In all water systems the fluid used in the thermal distribution
system is water, i.e., water transports energy between the
conditioned space and the air conditioning plant. When cooling is
required in the conditioned space then cold water is circulated
between the conditioned space and the plant, and hot water is
circulated through the distribution system when heating is required.
Since only water is transported to the conditioned space, provision
must be there for supplying required amount of treated, outdoor air
to the conditioned space for ventilation purposes. Depending upon
the number of pipes used, the all water systems can be classified
into a 2-pipe system or a 4-pipe system.
2. All water systems:

2. All water systems:
Fig.36.6: A two-pipe, all water system

In air-water systems both air and water are used for providing required
conditions in the conditioned space. The air and water are cooled or
heated in a central plant. The air supplied to the conditioned space
from the central plant is called as primary air, while the water
supplied from the plant is called as secondary water. The complete
system consists of a central plant for cooling or heating of water and
air, ducting system with fans for conveying air, water pipelines and
pumps for conveying water and a room terminal. The room terminal
may be in the form of a fan coil unit, an induction unit or a radiation
panel. Figure in next slide shows the schematic of a basic air-water
system. Even though only one conditioned space is shown in the
schematic, in actual systems, the air-water systems can
simultaneously serve several conditioned spaces.
3. Air-water systems:

3. Air-water systems:

Fan Coil Unit (FCU) / Air Handling Unit (AHU)

Unitary refrigerant based systems consist of several separate air conditioning
units with individual refrigeration systems. These systems are factory assembled
and tested as per standard specifications, and are available in the form of
package units of varying capacity and type. Each package consists of
refrigeration and/or heating units with fans, filters, controls etc. Depending upon
the requirement these are available in the form of window air conditioners, split
air conditioners, heat pumps, ductable systems with air cooled units etc.
4. Unitary refrigerant based systems:
Window
Type:

Split Type

Selection of a suitable air conditioning system
depends on:
1. Capacity, performance and spatial requirements
2. Initial and running costs
3. Required system reliability and flexibility
4. Maintainability
5. Architectural constraints
Selection criteria for air
conditioning systems

Psychrometric Chart

From psychrometric chart we normally find following
parameters:
1. Dry bulb temperature lines: These lines are vertical. Generally, the
temperature range on psychrometric chart is from -6ºC to 45ºC. The dry bulb
temperature lines are drawn with difference of every 5ºC and up to the saturation
curve. The values of dry bulb temperatures are also shown on the saturation
curve.
2. Specific humidity or moisture content lines: These lines are
horizontal and uniformly spaced. Generally, moisture content range of these lines
on psychrometric chart is from 0 to 30 g / kg of dry air. The moisture content
lines are drawn with a difference of every 1 g (0.001 kg) and up to the saturation
curve.
3. Dew point temperature lines: These lines are horizontal and non-uniformly
spaced. At any point on the saturation curve, the dry bulb and dew point
temperatures are equal. The values of dew point temperatures are given along
the saturation curve of the chart. Cdre Muzib, psc, PhD 75

4. Wet bulb temperature lines: These lines are inclined straight lines and non-
uniformly spaced. At any point on the saturation curve, the dry bulb and wet bulb
temperatures are equal. The values of wet bulb temperatures are generally given
along the saturation curve of the chart.
5. Enthalpy (total heat) lines: The enthalpy lines are inclined straight lines and
uniformly spaced. These lines are parallel to the wet bulb temperature lines, and
are drawn up to the saturation curve. Some of these lines coincide with the wet
bulb temperature lines also. The values of total enthalpy are given on a scale
above the saturation curve.
6. Specific volume lines: These lines are obliquely inclined straight lines and
uniformly spaced. They are drawn up to the saturation curve. The values of
volume lines are generally given at the base of the chart.
7. Relative humidity lines: Relative humidity lines are curved and follow the
saturation curve. Generally, these lines are drawn with values of relative humidity
10%, 20%, 30% etc. and up to 100%. The saturation curve presents 100%
relative humidity.

Fig 1: Dry bulb temperature lines
Fig 2: Specific Humidity lines

Fig 3: Dew Point temperature lines
Fig 4: Wet Bulb temperature lines

Fig 5: Enthalpy (Total Heat) lines

Fig 6: Specific Volume lines
Fig 7: Relative Humidity lines

Cooling Load Calculation
components of a cooling load
1- Internal cooling loads
A- People
B- Electric Lighting
C- Power Equipment
and Appliances
2- External Cooling Loads
A- Solar Heat Gain through Fenestration Areas.
B- Conduction Heat Gain through Fenestration Areas.
C- Conduction Heat Gain through Roofs and External
Walls.
D- Conduction Heat Gain through Interior Partitions,
Ceilings and Floors.
3- Loads from Infiltration and Ventilation

Comfort Zone: 22-27 ℃ and 40-60% RH

Quick load calculation for offices
For offices with average insulation and lighting, 2/3 occupants and 3/4 personal
computers and a photocopier, the following calculations will suffice per hour:
Heat load (BTU) = Length (ft.) x Width (ft.) x Height (ft.) x 4
Heat load (BTU) = Length (m) x Width (m) x Height (m) x 141
For every additional occupant add 500 BTU.
Detailed Calculation of Cooling Load (per hour):
Step One -Calculate the area in square feet of the space to be cooled, and multiply
by 31.25
Area BTU = length (ft.) x width (ft.) x 31.25
= Length(m) X Width(m)X337
Step Two - Calculate the heat gain through the windows. If the windows don’t
have shading multiply the result by 1.4
North window BTU = Area of North facing windows (m. sq.) x 165
South window BTU = Area of South facing windows (m. sq.) x 870
Add the results together.
Total window BTU = North window + South window Cdre Muzib, psc, PhD 85

Step Three - Calculate the heat generated by occupants, allow 400 BTU per
person.
Occupant BTU = number of people x 400
Step Four - Calculate the heat generated by each item of machinery - copiers,
computers, ovens etc. Find the power in watts for each item, add them
together and multiply by 3.5
Equipment BTU = total equipment watts x 3.5
Step Five - Calculate the heat generated by lighting. Find the total wattage for
all lighting and multiply by 4.25
Lighting BTU = total lighting watts x 4.25
Step Six - Add the above together to find the total heat load.
Total heat load BTU = Area BTU + Window BTU + Occupant BTU + Equipment
BTU + Lighting BTU
Step Seven - Divide the heat load by the cooling capacity of the air
conditioning unit in BTU, to determine how many air conditioners are needed.

Example:
1. Room Area BTU = Length(m) X Width(m)X337
= 50 X 30 X337 =505500
2.Window Size and Position
South window BTU=window L(m)XW(m)X870
North Window BTU=window L(m) XW(m)X165
If blinds on the windows multiply by 1.5
If no windows, ignore this.
3.Occupants
Total occupants BTU= No.of occupants X 400 = 15 X 400=6000
4.Equipment
Add all the watts for servers, switches, Routers and multiply by 3.5
Equipment BTU= Total watts for all equipment X 3.5
= 100000 X 3.5=350000
5.Lighting
Take the total wattage of the lighting and multiply by 4.25
Lighting BTU = Total wattage for all lighting X 4.25 =1000 X4.25=42500
Total Heat Load=( Room Area BTU + Windows BTU + Total Occupants BTU
+ Equipment BTU + Lighting BTU)
= 505500 + 0 + 6000 + 350000 + 42500 =904000 BTU
=(904000)/12000=75.33 TR (1 TR=12000 BTU/hr)
Refrigeration plant capacity= 75.33 TR

Problem 1: Assume that the outside air temperature is 32°C with a
relative humidity φ = 60%. Use the psychrometric chart to determine the
specific humidity ω [18 gm-moisture/kg-air], the enthalpy h [78 kJ/kg-
air], the wet-bulb temperature Twb [25.5°C], the dew-point temperature
Tdp [23°C], and the specific volume of the dry air v [0.89m3/kg]. Indicate
all the values determined on the chart.

Problem 2: Assume that the outside air temperature is 8°C. If the air in a room is at 25°C with
a relative humidity φ = 40%, use the psychrometric chart to determine if the windows of that
room which are in contact with the outside will become foggy.
Solution: Air in
contact with windows
will become colder until
the dew point is
reached. Notice that
under the conditions of
25°C and 40% relative
humidity the dew point
temperature is slightly
higher than 10°C. At
that point the water
vapor condenses as the
temperature
approaches 8°C along
the saturation line and
the windows will
become foggy.

Problem 3: Outside air at 35°C and 60% relative humidity is to be conditioned by cooling
and heating so as to bring the air to within the "comfort zone". Using the Psychrometric
Chart neatly plot the required air conditioning process and estimate (a) the amount of
moisture removed [11.5g-H20/kg-dry-air], (b) the heat removed [(1)-(2), qcool = 48kJ/kg-
dry-air], and (c) the amount of heat added [(2)-(3), qheat = 10kJ/kg-dry-air].

Problem 4: Hot dry air at 40°C and 10% relative humidity passes through an evaporative
cooler. Water is added as the air passes through a series of wicks and the mixture exits at
27°C. Using the psychrometric chart determine (a) the outlet relative humidity [45%], (b)
the amount of water added [5.4g-H20/kg-dry-air], and (c) the lowest temperature that could
be realized [18.5°C].

Solved Problem 10.4:: Hot dry air at 40°C and 10% relative humidity passes through an
evaporative cooler. Water is added as the air passes through a series of wicks and the
mixture exits at 27°C. Using the psychrometric chart determine (a) the outlet relative
humidity [45%], (b) the amount of water added [5.4g-H20/kg-dry-air], and (c) the lowest
temperature that could be realized [18.5°C].
This type of cooler is extremely
popular in hot, dry climates, and
is popularly known as a Swamp
Cooler. An interesting application
of using a swamp cooler is to cool
drinking water in extremely hot
environments.

Name 409 refrigeration and air conditioning

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