fUNDAMENTALS OF hvac

AIR CONDITIONING and VENTILATION SYSTEMS
NME 515
Heating, Ventilating, and Air
Conditioning
ANALYSIS and DESIGN
Engr. Charlton Inao, ME
TESDA TVET NC Level 3 RAC-CRE Certified
17/19/2020 Engr. Charlton Inao

INTRODUCTION
Engr. Charlton Inao, ME
TESDA TVET NC Level 3 RAC-
CRE Certified
2
Reference: Heating, Ventilating, and Air Conditioning
ANALYSIS and DESIGN by F Mc Quiston, and JD Parker

1-2 UNITS AND DIMENSIONS
In HVAC computations as in all engineering work,
consistent units must be employed. A unit is a specific
quantitative measure of a physical characteristic in
reference to a standard.

7
EXAMPLE 1-1
Describe the quantity of pressure equal to 1000 N/m2 in
suggested SI terminology.
SOLUTION
1000 can be described by the prefix kilo (k). The N/m2
has the special name of pascal.
Therefore,
1000N/m2 = 1 kilo pascal 1 kPa

8
EXAMPLE 1-2
A certain corkboard has a thermal conductivity of 0.025
Btu/(hr-ft-F). Convert this quantity to the equivalent SI
value.
SOLUTION
According to the conversion factor given in the inside
front cover, to convert from Btu/(hr-ft-F) to W/(m-C)
you must divide the given value by 0.5778.

9
EXAMPLE 1-2
.

10
Because the prefix centi is to be avoided if possible
(Table 1-3), the quantity is expressed as shown. Notice
that the final quantity is not expressed to any more
significant figures than the original quantity, in this case
two.

11
The relationship between temperature on the Kelvin
Scale and temperature on the Celsius scale is,
For example, 100 C is identical to 373.15 K.

12
In the English Engineering system the unit of
temperature is the degree Fahrenheit. When the
thermodynamic absolute temperature is needed, the
temperature is specified in degrees Rankine (R). The
relationship between the Fahrenheit scale and the
Rankine scale is

13
Since both the Rankine and Kelvin scales are absolute
scales, absolute zero is identical on each scale. The
relationship between the Rankine scale and the Kelvin
scale is
For example, 900 R is identical in temperature to 500 K.
The relationship between the four temperature scales is
given in Fig. 1-1. Consistent units must always be
employed in physical computations.

14
The relationship between the Btu and the ft-Ibf is

16
EXAMPLE 1-3
A system is known to contain 100 Btu of thermal energy
and 30,000 ft-lbf of mechanical energy. What is the total
energy (mechanical plus thermal) contained by the
system in Btu?

17
SOLUTION
Use Eq. 1-4 as a conversion factor.

18
SOLUTION
In this example notice that an equation (Eq.1-4) was
changed to a conversion factor by simple algebra. Also
note that the answer was rounded off to three
significant figures to match the accuracy of the given
data.

19
Since 778.28 (ft-lbf)lBtu is equivalent to unity, it can be
placed in the appropriate term in such a way as to
cancel the undesired units, and yet not change the true
value of the physical quantity represented by the term.
In Example 1-3, 30,000 ft-lbf of energy is identical to
38.5 Btu of energy.

20
From Table 1-2 we see that the derived unit of
energy in the SI system is the joule, which is
equivalent to one newton-meter. The derived
unit of power in the SI system is the watt,
which is equivalent to one joule per second.

21
EXAMPLE 1-4
The specific heat of air at normal conditions is
approximately equal to 0.241 Btu/(1bm-F). Express this
value ofthe specific heat of air in SI units, joule per
kilogram, and degree.

22
SOUTlON
From the conversion factor given in the inside cover, the
following relationship is true

1-3 FUNDAMENTAL CONCEPTS
23
Heating
Heating is the transfer of energy to a space or to the air
in a space by virtue of a difference in temperature
between the source and the space or air. This process
may take different forms'.such as direct radiation and
free convection to the space, direct heating of forced
circulated air, or through heating of water that is
circulated to the vicinity of the space and used to heat
the circulated air.

24
Heat transfer, which is manifested in a rise in
temperature of the air, is called sensible heat
transfer.

25
The rate of sensible heat transfer can be related to the
rise in temperature of an air stream being heated by

26
The specific volume and the volume How rate of the air
are usually specified at the inlet conditions. Note that
the mass flow rate of the air th, equal to the volume
flow rate divided by the specific volume, is considered
not to change between inlet and outlet. The specific
heat is assumed to be an average value.

27
EXAMPLE 1-5
Determine the rate at which heat must be added in
Btulhr to a 3000 cfm air stream to change its
temperature from 70 to 120 F. Assume an inlet air
specific volume of 13.5 ft3/Ibm and a specific heat of
0.24 Btu/(Ibm-F).

28
SOLUTION
The heat being added is sensible as it is contributing to
the temperature change of the air stream. Eq. 1-5
applies.

29
Humidifying
The transfer of water vapor to atmospheric air is
referred to as humidification. Heat transfer is associated
with this mass transfer process; however, the transfer of
mass and energy are manifested in an increase in the
concentration of water in the air-water vapor mixture.

30
The latent energy required in a humidifying process can
be calculated if the rate at which water is being
vaporized and the enthalpy of vaporization (latent
enthalpy) are known. The relation is

31
EXAMPLE 1-6
It is desired to add 0.0 I Ibm of water vapor to each
pound of perfectly dry air flowing at the rate of 3000
cfm using saturated (liquid) water in the humidifier.
Assuming a value of 1061 Btu/lbm for the enthalpy of
vaporization of water, estimate the rate of latent energy
input necessary to perform this humidification of the air
stream.

32
SOLUTION
Since the rate ofwater addition is tied to the mass ofthe
air, we must determine the mass flow rate of the air
stream. Let us assume that the specific volume of the
air given in Example 1-1 is a suitable.value to use in this
case; then

33
SOLUTION
and the latent heat transfer

PROBLEMS
34
1-1. Write the following quantities in the suggested SI
terminology.
(a) 11,000 newton per square meter
(b) 12.000 watts
(c) 1600 joule per kilogram
(d) 101,101 pascal
(e) 12,000 kilowatt
(f) 0.012 meter
(g) 0.0000012 second

PROBLEMS
35
1-2. Convert the following quantities from English to SI
units.
(a) 98 Btu/(hr-ft-F)
(b) 0.24 Btul(lbm-p)
(c) 0.04 Ibml(ft-hr)
(d) 1050 Btu/lbm
(e) 1.0 ton (cooling)
(f) 14.7 Ibf/in.2

PROBLEMS
36
1-3. Convert the following quantities from SI to English
units.
(a) 120 kPa
(b) 100 W/(m2-C)
(c) 0.8 W/(m2-C)
(d) 10-6 (N-s)/m2
(e) 1200 kW
(f)1000 kJ/kg

PROBLEMS
37
1-4. The kinetic energy of a flowing fluid is proportional
to the velocity squared divided by two. Compute the
kinetic energy per unit mass for the following velocities
in the unIts indicated.
(a) velocity of 100 ft/sec; English units
(b) velocity of 60 m/s; SI units
(c) velocity of 400 ft/sec; SI units
(d) velocity of 500 m/s; English units

PROBLEMS
38
1-5. The potential energy of a fluid is proportional to
the elevation of the fluid. Compute the potential energy
for the elevations given below per unit mass.
(a) elevation of 200 ft; English units
(b) elevation of 70 m; SI units
(c) elevation of 120 ft; SI units
(d) elevation of 38 m; English units

PROBLEMS
39
1-6. A gas is contained in a vertical cylinder with a
frictionless piston. Compute the pressure in the
cylinder for the following cases.
(a) piston mass of 20 Ibm and area of 7 in2
(b) piston mass of 10 kg and diameter of 100 mm

PROBLEMS
40
1-7. A pump develops, a total head of 50 ft of water
under a given operating condition. What pressure is the
pump developing in SI units and tenninology?

PROBLEMS
41
1-8. A fan is observed to operate with a pressure
difference of 4 in. of water. What is the pressure
difference in SI units and terminology?

PROBLEMS
42
1-9. Compute the Reynolds number
for 21 C water flowing in a standard 2-in. pipe at a
velocity of 1 m/s using SI units. Tables B-3b and D-1
will be useful. What is the Reynolds number in English
units?

TABLE B-3b

TABLE D-1

PROBLEMS
45
1-10. Compute the thermal diffusivity
of the following substances in SI units.
(a) saturated liquid water at 38 C
(b) air at 47 C and 101.3 kPa
(c) saturated liquid refrigerant 22 at 38 C
(d) saturated liquid refrigerant 12 at 38 C

PROBLEMS
46
1-11. Compute the Prandtl number
for the following substances in SI units.
(a) saturated liquid water at 10 C
(b) saturated liquid water at 60 C
(c) air at 20 C
(d) saturated liquid refrigerant 22 at 38 C

PROBLEMS
47
1-12. Compute the heat transferred from water as it
flows through a heat exchanger at a steady rate of 1
m3/s. The decrease in temperature of the water is 5 C
and the mean bulk temperature is 60 C. Use SI units.

PROBLEMS
48
1-13. Make the following volume and mass flow rate
calculations in SI units. (a)Water flowing at an average
velocity of 2 m/s in nominal 2
1
2
-in., type L copper
tubing. (b) Standard air flowing at an average velocity
of 4 m/s in a 0.3 in. diameter duct.

PROBLEMS
49
1-14. A room with dimensions of 3 ×10 × 20 m is
estimated to have outdoor air brought in at an
infiltration rate of
1
4
volume change per hour. Determine
the infiltration rate i m3/s.

PROBLEMS
50
1-15. Air enters a heat exchanger at a rate of 5000
cubic feet per minute at a temperature of 50 F and
pressure of 14.7 psia. The air is heated by hot water
flowing in the same exchanger at a rate of 11,200
pounds per hour with a decrease in temperature of 10
F. At what temperature does the air leave the heat
exchanger?

PROBLEMS
51
1-16. Water flowing at a rate of 1.5 kg/s through a heat
exchanger heats air from 20 C to 30 C flowing at a rate
2.4 m3/s. The water enters at a temperature of 90 C
and the air is at 0.1 MPa. At what temperature does the
water leave the exchanger?

PROBLEMS
52
1-17. Air at a mean temperature of 50 F flows over a
thin-wall 1-in. O.D. tube, 10 feet in length, which has
condensing water vapor flowing inside at a pressure of
14.7 psia. Compute the heat transfer rate if the average
heat transfer coefficient between the air and tube
surface is 10 Btu/(hr-ft2-F).

PROBLEMS
53
1-18. Repeat Problem 1-17 for air at 10 C, a tube with
diameter 25 mm, a stream pressure of 101 kPa, and a
tube length of 4 m, and find the heat transfer
coefficient in SI units if the heat transfer rate is 1250 W.

PROBLEMS
54
1-19. Air at 1 atm and 76 F is flowing at the rate of
5000 cfin. At what rate must energy be removed, in
Btu/hr, to change the temperature to 58 F assuming
that no dehumidification occurs?

PROBLEMS
55
1-20. Air flowing at the rate of 1000 cfm and with a
temperature of 80 F is mixed with 600cfm of air at 50 F.
Use Eq. 1-5 to estimate the final temperature of the
mixed air. Assume cp = 0.24 Btu/(Ibm-F) for both
streams.

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