HVAC Air Cooling

1. Department of Mechanical Engineering
Faculty of Engineering & Technology, Jamia Millia Islamia
Design of A/C system for a Multiplex
Under Supervision of Prof. J. A. Usmani
Project – I
BTM – 791
Submitted by :
Mahfooz Alam
Musawwir Alam
Md. Iftikhar
Nasir Aziz
Md. Abdul Khaliq

2. Introduction
The average summer temperatures experienced by most countries are increasing every year
and consequently the energy needs to provide air-conditioning is also increasing annually. The
HVAC industry has a challenging task of providing energy efficient technologies to satisfy
this growing demand with a minimum impact on global warming and ozone depletion.
The control of properties of air to suit the physiological requirements of the human body or to
assist in improvement of the quality of industrial process is known as air conditioning. This
requires the simultaneous control of temperature humidity ,air circulation(ventilation),outdoor,
dust content, bacteria content, ionic content, light, pressure etc. the art of air conditioning
developed only gradually from the predecessors arts of cooling , cleaning, heating and
ventilation. The human body produces certain amount of heat due to metabolism. The heat
produced depends on the degree of activity and the average temperature of healthy human
being is 37C.when the air temperature is lesser than the body surface temperature; the body
loses heat by convection and evaporation. If the DBT of the air is greater than the body
surface temperature, the convection effects will heat rather than cool the body.[1]
Fig.1 - Humidity Vs. temperature chart

3. The comfort air-conditioning systems are divided into three groups:
1. Summer-Air Conditioning System: The problems encountered in summer A/C are:
 To reduce the sensible heat
 To reduce water vapor content of the air by cooling and dehumidifying. The removal of water
vapor from the air is termed as dehumidification of air. The dehumidification of air is only
possible if the air cooled below the dew point temperature of the air.
2. Winter Air Conditioning System: The problem encountered in winter A/C is to increase the
sensible heat and water vapor content of air by heating and humidification. The addition of
water vapor to the air is termed as a humidification of air.
Fig.2 - Relative humidity Vs. temperature chart
3. Year Round Air Conditioning System: This system assures the control of temperature and
humidity of air in an enclosed surface throughout of the year. When the atmospheric
conditions are changing as per season. In many countries, both summer and winter are very
discomfort able. Under six conditions year round A/C system must be capable of maintaining
a specified temperature and humidity with the A/C spaces regardless of outside weather
conditions. In most of the A/C applications for industry the common problem is to control the
temperatures, humidity and air motion for maintaining the quantity of the product to perform a
specific industrial process, successfully.[1]

4. Fig. 3 – Building Orientation

5. Cooling Load calculation
Heating and cooling loads are the measure of energy needed to be added or removed from a
space by the HVAC system to provide the desired level of comfort within a space. Right-
sizing the HVAC system begins with an accurate understanding of the heating and cooling
loads on a space. Right-sizing is selecting HVAC equipment and designing the air distribution
system to meet the accurate predicted heating and cooling loads of the house. The values
determined by the heating and cooling load calculation process will dictate the equipment
selection and duct design to deliver conditioned air to the rooms of the house, right-sizing the
HVAC system. The heating and cooling load calculation results will have a direct impact on
first construction costs along with the operating energy efficiency, occupant comfort, indoor
air quality, and building durability.[2]
Fig.4 - Required cooling capacities for various building types

6. Fig.5 - Various load components

7. Sample Problem :
A school classroom is 6 m long, 6 m wide and 3 m high. There is a 2.5 m x 4 m window in the
east wall. Only the east wall/window is exterior. Assume the thermal conditions in adjacent
spaces (west, south, north, above and below) are the same as those of the classroom.
Determine the cooling load at 9:00 am, 12:00 noon on July 21.[3]
Other known conditions include:
Latitude = 40o
N
Ground reflectance = 0.2
Clear sky with a clearness number = 1.0
Overall window heat transmission coefficient = 7.0 W/m2
K
Room dry-bulb temperature = 25.5°C
Permissible temperature exceeded = 2.5%
Schedule of occupancy: 20 people enter at 8:00 am and stay for 8 hours
Lighting schedule: 300 W on at 8:00 AM for 8 hours
Exterior wall structure:
• Outside surface, A0
• Face brick (100 mm), A2
• Insulation (50 mm), B3
• Concrete block (100 mm), C3
• Inside surface, E0
Exterior window:
• Single glazing, 3 mm
• No exterior shading, SC = 1.0
Solution :
Cooling Load due to Exterior wall:
U =1/R = 0.643 W/m2
K
From Table A28-33A, find wall type 13.
From Table A28-32, CLTD9:00 = 9 and CLTD12:00 = 14 for east wall.
CLTD Corrected = CLTD + (25.5 - Ti) + (Tm - 29.4)
Ti = inside temperature
Tm = mean outdoor temperature
Tm = (maximum outdoor temperature) - (daily range)/2

8. At 9:00 am CLTD Corrected = 9 + (25.5 - 25.5) + (31 -9/2 - 29.4) = 6.1 K
At 12:00 am CLTD Corrected = 14 + (25.5 - 25.5) + (31 -9/2 - 29.4) = 11.1 K
Q = U A (CLTD)
Q = 0.643 (W/m2
K) x (6 x 3 - 4 x 2.5) m2
x 6.1 = 34 W (at 9 am)
Q = 0.643 (W/m2
K) x (6 x 3 - 4 x 2.5) m2
x 11.1 = 62 W (at 12 noon)
Layer Unit Resistance (R value)
m
2
K/W
A0 0.059
A2 0.076
B3 1.173
C3 0.125
E0 0.121
Total 1.554
Window conduction:
From Table A28-34, CLTD9:00 = 1 and CLTD12:00 = 5
CLTD Corrected = CLTD + (25.5 - Ti) + (Tm - 29.4)
CLTD corrected = 1 + 0 + (31 - 9/2 - 29.4) = -1.9 K (at 9 am)
CLTD Corrected = 5 + 0 + (31 - 9/2 - 29.4) = 2.1 K (at 12 noon)
Q = U A (CLTD)
Q = 7.0 W/m2
K x 4 x 2.5 m2
x (-1.9 K) = -133 W (at 9 am)
Q = 7.0 W/m2
K x 4 x 2.5 m2
x 2.1 K = 147 W (at 12 noon)
Solar load through glass:
From Table A28-35B, find zone type is A
From Table A28-36, find CLF = 576 at 9 am and CLF = 211 at 12 noon
Q = A (SC) (SCL)
Q = (2.5 x 4 m2
) x 1.0 x 576 = 5760 W (at 9 am)
Q = (2.5 x 4 m2
) x 1.0 x 211 =2110 W (at 12 noon)

9. People:
From Table A28-35B, find zone type is B
From Table A28-3, find sensible/latent heat gain = 70 W
From Table A28-37, find CLF9:00 (1) = 0.65 and CLF12:00 (3) =0.85
Q Sensible = N (sensible heat gain) CLF
Q Sensible = 20 x 70 W x 0.65 = 910 W (at 9 am)
Q Sensible = 20 x 70 x 0.85 = 1190 W (at 12 noon)
Q Latent = N (latent heat gain)
Q Latent = 20 x 45 = 900 W (at 9 am and 12 noon)
Lighting:
From Table A28-35B, find zone type is B
From Table A28-38, find CLF9:00 (1) = 0.75 and CLF12:00 (3) = 0.93
Q Lighting = W Full Fsa (CLF)
Q Lighting = 300 W x 1 x 1 x 0.75 = 225 W (at 9 am)
Q Lighting = 300 x 1 x 1 x 0.93 = 279 W (at 12 noon)
TOTAL COOLING LOAD:
Component 9.00 AM 12.00 Noon
Wall 34 62
Window conduction -133 147
Window solar transmission 5760 2110
People 910 1190
Lights 225 279
Total 6796 W 3788 W

10. Design of duct system:
1. Air should be conveyed as directly as possible to save space, power and material
2. Sudden changes in directions should be avoided. When not possible to avoid sudden changes,
turning vanes should be used to reduce pressure loss
3. Diverging sections should be gradual. Angle of divergence ≤ 20.
4. Aspect ratio should be as close to 1.0 as possible. Normally, it should not exceed 4.
5. Air velocities should be within permissible limits to reduce noise and vibration.
6. Duct material should be as smooth as possible to reduce frictional losses.[4]
Fig.7 - General layout of HVAC system

11. Fig. 8 – Ductwork

12. Fig. 9 – Types of Duct
Importance of HVAC:
As the outside weather changes, the environment in which your equipment is housed will
change as well. It is important to protect the equipment from over-heating, freezing, moisture,
etc. In addition to telecom shelters, HVAC units are also commonly used in data centers and
server rooms. Saving money is one of the main reasons a person should maintain their HVAC
unit. An HVAC unit that is running efficiently, as well as a home that is properly insulated,
means less money spent on electricity, heating and cooling costs. HVAC maintenance is also
important to prevent the need for major repairs or replacements.[4]
1. Healthy Air
A properly maintained HVAC unit will not only keep a home warm or cool, but it will prevent
problems with air quality. Clean filters and coils mean better breathing for an entire family.
An unmaintained HVAC unit is a breeding ground for dirt, mold and bacteria, all of which can
cause or worsen respiratory problems for those living home.

13. 2. System Life
The better a person maintains their HVAC unit, the longer that unit will be able to function
well and provide cooling and heating to a home or building. With proper HVAC maintenance,
a unit can last well over 10 years. Considering the amount of money a person spends to install
an HVAC unit, it makes sense they would want to keep that unit running well for as long as
possible.
3. Efficiency
As with the life of the system, a well-maintained HVAC unit will run more efficiently.
Research shows that dirty or unmaintained units need to work 20 percent harder to produce
the same amount of cooling or heating as a well-maintained machine. Less energy will be
expended when it is running, which means less stress on the components of the machine. The
less wear and tear on the machine means simple maintenance during the spring and fall
months. Maintenance will be much quicker and smoother if a person takes the preventive
steps to keep a system running in tip-top shape.
4. Less Emergency Repairs
Most HVAC units may need emergency repairs from time to time. Well-maintained units are
less likely to fail during the months of hard use (June through September and December
through March). Keeping a unit up-to-date on all inspections and maintenance checks means
less worry that the unit will break down when it is needed the most.
5.Parts Under Warranty
In the event of a problem, the part or component in question might still be under a warranty.
This is another important reason to maintain an HVAC unit on a regular basis. Keep a list of
all warranty expiration dates and check the parts prior to that date to ensure there are no
problems before it is too late. Parts under warranty can often be fixed at no cost, aside from
the labor charges that may arise if homeowners are unable to fix or replace parts alone.

14. For many obvious reasons, HVAC maintenance is important. Scheduling regular checks and
following through on fixing minor problems will help homeowners enjoy the comfort of a
system with the worry of it failing. A well-maintained unit is likely to have less serious
problems, meaning less hassle and unnecessary costs.

15. References:
[1] C.P. Arora, 2000, ‘Refrigeration and air conditioning’, Air Conditioning.
[2] http://nptel.ac.in/courses/112105129/pdf/R%26AC%2520Lecture%252035.pdf .
[3] A. Bhatia, ‘Cooling Load Calculations and Principles’, Continuing Education and
Development, Course No: M06-004.
[4] R.K Rajput, 2009, ‘A textbook of refrigeration and air conditioning’, Design of
Ducts, S. K. Kataria & Sons.