Unit 8
Unit 8
Unit 8
10.1 Introduction
Objectives
10.2 Definition and Principle of Air-conditioning
10.2.1 Definition
10.2.2 Principle of Air-conditioning
10.2.3 Air Quantity
10.2.4 System Capacity
10.2.5 Classification of Air-conditioningS y s t m
10.3 Psychrometrics, Human Comfort and Ventilation
10.3.1 Definitions of T e r n Used in Air-Conditioning
10.3.2 PsychrometricChart .
10.3.3 Psychrometric Processes
10.3.4 Human Comfort
10.3.5 Ventilation
10.4 Sources of Heat Gain in Buildings
10.4.1 Heat transmission (Trans Gain)
10.4.2 Solar Gain
10.4.3 Solar and Trans Gain
10.4.4 Infiltration Gain
10.4.5 Ventilation Gain
10.4.6 Water Vapout Flow
10.4.7 Occupancy Load
10.4.8 Lights and Appliances
10.4.9 Product and Prxcss Load
10.4.10 Miscellaneous System Load
10.5 Sensible Heat Factor, Humidity Control and Part Load Operation
10.5.1 Sensible Heat Factor
10.5.2 Practical Air-conditioning Cycle
10.5.3 Humidity Control and High Latent Meat Applications
10-5.4 Part Load Control
10.6 Refrigeration Cycle and Refrigerants
10.6.1 Need for Refrigeration Equipment
10.6.2 Refrigerants
10.6.3 Refrigeration Cycle
10.6.4 Type of Air-conditioning Systems
10.7 System Components
10.7.1 Air Handling Equipmenh
10.7.2 Filters
10.7.3 RefrigerationEquipment
10.7.4 Auxiliary Equipment
10.8 Sheet Metal Ducts
10.8.1 Types of Systems
10.8.2 Duct Accessories
10.8.3 Air Outkts
10.8.4 Duct Design Methods
10.9 Water Piping
10.10 Physical Requirements
10.11 Su~llmary
10.1.2Answers to SAQs
Elements of Electrical &
Air-conditioningDesign 10.1 INTRODUCTION
Years ago comfort air-conditioning was considered a luxury and its application was
limited mainly to process industries wherein maintaining a specific condition (temperature
and humidity) is essential for the process. With the growing awareness that comfort
air-conditioning leads to increased efficiency in workshops, factories and offices; results ii?
neatness and cleanliness in hospitals, hostels and health resorts etc. and with the overall
growth of indigeneous air-conditioning industries, more and more buildings are nowadays
air-conditioned as a standard practice. It is, therefore, necessary for the people involved in
planning and designing stmctures for offices, hotels, factories, hospitals, etc. to have a
basic knowledge of air-conditioning and its requirements/demands. In this unit, we
introduce you to the theory and practice of air-conditioning systems, refrigeration, air
handling and auxiliary equipments as well as their requirements to be taken care of while
planning and designing the structure and its services.
Objectives
At the end of this unit, you should be able to I
10.2.1 Definition
Air-conditioning is described as the simultaneous control of temperature, humidity,
motion and purity of atmospheric air in a confined space. Contrary to the popular thinking
that air-conditioning means cooling, it includes winter heating also. Let us now see how
this is done in practice.
10.2.2 Principle of Air-conditioning
Consider a room that requires to be air-conditioned. This is a confined space well isolated
from the surrounding (out-side) atmosphere. We have to maintain a specific temperature
and humidity which are called inside design conditions, in the room, irrespective of the
changes in outside temperature and humidity which are called outside or ambient
conditions. You are aware that there is no control over the ambient conditions and it
changes from time to time and season to season. But the inside conditions of an
air-conditioned room is more or less constant throughout the year and at all times. Any
air-conditioning system shall take care of the maximum requirements and work
satisfactorily even at the worst conditions. Hence whenever we mention outdoor design
conditions, it always refers to the highest temperature and humidity during summer or
highest temperature and highest humidity in monsoon (in some case the combined effect
of temperature and humidity during monsoon is more than that in summer) and lowest
temperature and humidity in winter. For simplicity, let us assume that the inside design
temperature is lower than the outside temperature (which is called outside design
condition). However, the logic applied to cooling type of air-conditioning also applies to
heating type of air-conditioning. Let us also assume that the air-conditioning system is in
operation for quite some time and steady state conditions inside the conditioned space
have been achieved.
As the inside temperature is lower than the outside temperature, there is a continuous flow
of heat from outside into the room There are also certain internal heat sources which add
heat to the room Thus the temperature of air within the room tends to rise. As it is our
intention to maintain the specified design inside conditions, the heat that enters into the
room and generated within the room has to be removed on a continuing basis. We use air
itself to convey this heat from the room to the outside. This heat transfer is achieved by
constantly supplying air at a temperature lower than the design inside condition. This cool
air absorbs or picks up the heat mentioned above and in the process its temperature rises .
1
till it r aches the inside design temperature. This air is then removed or exhausted from the
room. e temperature of cold air and its quantity depends upon the inside design
condit' ns and the quantity of heat to be removed. Production of cool air is the job of
air-co itioning or refrigeration equipment, the details of which we will see later in this
Air-conditioning and
V entilatior,
I
m designed shall not only be effective but also he economical. As the inside
erature is lower thzn the outside temperature, the exhaust air from the
room is also at a temperature lower than the atmospheric air. If this air is
d re-used, the load on the system will be much lower than that, when only
air is cooled and supplied to the room. Of course, in order to maintain the air
described level, required amount of atmospheric air (fresh air or ventilation
ary to be added to the recirculated air. This is the basic principle of
ng. This process is shown in block diagram given in the Figure 10.1.
Return air ( 3 )
d
-
I 1
L
(1 1 1 (5 1
C
-
Cooler -
-c
C
(4 I
Room ( 2 I
L
I Table 10.1
er a room from which Q kcal of heat per hour is required to be removed and
erature rise of (Tr - T,,) O C , where Tris the room inside
e supply air temperature.
Then the wkight of air required to be circulated per hour is given by
Elements of Electrical &
I Gr-conditionine Desieu
where
WJ,= weight of supply air required to be circulated in kg/hr, and
S = specific heat of air in kcaVkgPC.
If V,, is the specific volume of supply air at its temperature and humidity, in cubic meters
per kg, then the quantity of supply air is given by
Note : As you are aware, the total heat removed from the room consists of sensible heat ~d latent
heat. L) in the above formulais only the sensible heat portion of the total heat. Sensible and latent
heats will be discussed in the next chapter.
SAQ 1
i) Define Air-conditioning. What are the classifications of air-conditioning.
ii) What do you understand by the terms
a) outdoor design conditio~ls
b) Indoor or space conditions '!
iii) Explain the basic principle of air-conditioning with the help of a simple block
diagram.
iv) Define TON and KILdOWATTas applicable lo air-conditioning.
v) Calculate the volume of air required to be circulated in a conditioned room
given the following:
a) air-conditioning load = 40 kW(sensible),
b) rcwm temperature = 25 OC,
c) supply air temperature = 15 "C,
d) specific volume of supply air = 0.88 cumkg, and
e) specific heat of dry air = 0.24kcalkgl OC.
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Normal trnospheric air without any moisture or water vapour present in it.
t
16 is the 'xture of dry air and water vapour. The atmospheric air usually contains some
water v our and hence except in some extreme cases, the atmospheric air is moist air.
Dew Yo t Temperature
at which condensation of moisture begins when the air is cooled.
of water vapour present per unit weight of dry air. This is usually expressed in
the dry bulb temperature of the air, it can hold only a certain maximum
vapour. If water vapour in excess of this maximum is added an equal
vapour present in the air condenses back into water. When air contains
amount of water vapour it is said to be saturated. At saturation, the dry
e, the wet bulb temperature and the dew point temperature are one and the
tio of mass of actual water vapour present in a unit volume of air to that of the
at the same dry bulb temperature.
Elements of Electrical & Enthalpy
Air-conditioningDesign
A thermal property indicating the quantity of heat in the air above an arbitrary datum. The
unit is kcalkg. The total heat of moist air consists of two components, viz. latent heat and
sensible heat.
Sensible Heat
This is related to the dry bulb temperature. It is the amount of heat required to raise the
temperature of air above the datum or reference temperature. This is given by the
expression
H, = 0.24 T,, + 0.45 W T, kcalkg
where,
0.24 is the specific heat of dry air in kcalkg.
0.45 is the specific heat of steam (water vapour) at atmospheric pressure in kcalkg.
W = weight of water vapour in kg per kg of dry air (specific humidity).
T,, = dry bulb temperature of air.
Latent Heat
It is the amount of heat absorbed by W kg of water at dew point temperature to become
water vapour at dew point temperature and is given by
H, = W x L,, kcalkg
where,
L,, = latent heat of evaporation of water at the corresponding dew point
temperature. and
W = specific humidity in kgkg
10.3.2 Psychrometric Chart
Psychrometrics is the branch of thermal science involving thermodynamic properties of
moist air and the effect of atmospheric moisture on material and human comfort.
Psychrometric process is the method of controlling the thermal properties of moist air.
Psychrometric chart shows the inter-relation of all important properties of moist air. The
actual psychrometric chart in MKS units is given as Annexure 3. To understand the
features of a psychrometric chart a skeleton chart is given in Figure 10.2.
Dry b u l b t e m p e r a t u r e
Figure 10.2 :Psychrometric Chart
Explanation on Psychrometric Chart
1) Vertical lines in the chart represent dry bulb temperatures.
ns. The total heat of the syteill before ar:3 attrr Lire nlixll~greinaii~the sarrle and Air-conditioning and
condition of the mixture lies on the straight line connecting the points itpresented Ventilation
conditions of air. Though this can be theoretically proved, it is not necessary for
s stage. Let us assume M, kg of dry air inltially at point Pi with a specific
of L), kcalkg. is mixed with M, kg ot cur initially at point P , with a specific
of Q2kcalkg, the final condition of the mixture lies on a point on the straight line
g points P I and P, and has a specific enthalpy.
t
On e psychrometric chart the condition of the mixture can be easily found by the
foll ing method :
and P2 be tlie initial conditions of air and P, the condition of mixture. Let M, and
e nlasses of air at conditions P, and P, respectively. Connect point P I and P, by a
inz. Divide the lines such that P, P, : P, P , = M , : M I .
Dry bulb
Figure 10.6: Mixing of Air
ocesses commonly encountered in any air-conditioning system are a combination of
cesses described above. Depending upon the change that takes place these
es are classified as
i) cooling with dehumidification,
ii) heating, with huniidification,
1 iii) cooling with humidification,
iv) heating with dehumidification, and
v) mixing of air.
e process are shown in the Psychrometric Chart in Figure 10.7.
I
Dry bulb
Figure 10.7 :Psychrometric Chart