13
CHAPTER 2
HVAC FUNDAMENTALS
OVERVIEW
The HVAC Section of this book is organized into six chapters addressing the four
elements of HVAC systems:
ˍFundamentals (Chapter 2)
ˍOverall Systems (Chapter 3)
ˍHeating and Cooling Production (Chapters 4 and 5)
ˍDelivery Systems (Chapters 6 and 7)
Heat transfer and fluid flow are the governing principles for the design of HVAC
systems. Other information required in design includes the selection of weather date,
properties of materials, and performance of equipment and its components. Essentials of
these are included in these chapters to illustrate the design process. These data are by no
means complete, and should not be used in actual design of construction projects. The
Handbooks published by the American Society of Heating, Refrigerating and Air
Conditioning Engineers, Inc. (ASHRAE) should be used when designing actual
construction projects. ASHRAE is located at 1791 Tullie Circle, N.E., Atlanta, GA
30329.
HVAC Fundamentals is an introduction to criteria for HVAC system design and basics of
load calculations.
Section 2.1, “Environmental Comfort,” emphasizes the fact that temperature control,
while essential, is only one of many factors that affect human comfort in the built
product of mass (or mass flow), specific heat, and temperature difference. All the
equations in this section are similar except for the constants, which are specific to the
material under consideration.
Sections 2.4 through 2.7 cover basic heating and cooling load calculations. The method
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CHAPTER 2
HVAC FUNDAMENTALS
QUESTIONS AND ANSWERS
2.1 State the factors that affect environmental comfort and their general ranges
of values, where applicable.
2.2 Briefly describe the difference between sensible and latent heat.
2.3 For air at 75
q
F, 50% relative humidity, what will happen to relative humidity
if the air is cooled to 65
q
F? Cooled to 50
q
F?
2.4 How much water is present in 1 pound of air at 75
q
F, 50% relative humidity?
How much water is present in 1 pound of air at 95
q
F DB (dry bulb), 78
q
F WB
(wet bulb)?
2.5 What is the rate of removal of moisture required to reduce a 1,000-cfm
airstream from the higher to the lower conditions in problem 2.4?
Mass flow rate of 1,000 cfm can be determined from specific volume on the
psychrometric chart. At 95qF DB, 78qF WB, specific volume is approximately
16
2.6 If 40 lbs. per hour of water is allowed to evaporate into an airstream, what
will be the effect on the dry bulb temperature? What will be the effect on the
wet bulb temperature?
2.7 How much heat is required to warm 1,000 gallons of water from 60
q
F to
130
q
F?
2.8 How much heat is liberated when 1,000 gallons of water cools from 160
q
F to
140
q
F?
2.9 What is the heat liberation rate for a 500-gpm water flow cooling from 140
q
F
to 110
q
F?
2.10 A heating system load is 300,000 Btuh. How much heating water flow is
required to satisfy the load if the system is designed for a 20
q
F temperature
drop? How much flow for a 30
q
F drop?
2.11 Approximately how much steam flow would be required for a 300,000 Btuh
heating load?
2.12 Cooling system load is 48,000 Btuh sensible. How much chilled air is required
to satisfy the load if the system is designed for a 20
q
F temperature rise? How
much flow is required for 15
q
F rise?
2.13 Your client desires an interior temperature of 72
q
F for the design of his
HVAC system in St. Louis (close to the airport weather station). Assuming
that a 97.5% design will be satisfactory, what is the design temperature
difference?
2.14 Your client desires an interior temperature of 78
q
F for the design of her
HVAC system in St. Louis (close to the airport weather station). Assuming
that a 5% design will be satisfactory, what is the design temperature
difference?
2.15 What will be the U-factor for a wall constructed as follows?
2.16 What will be the heating load for a wall 10′ high by 500′ long that is
constructed as per the last description in Question 15 under conditions
specified in Question 13?
2.17 What will be the cooling load if the wall in Question 16 faces south? North?
West?
2.18 What will be the heating load for a 5′ × 5′ window constructed of single-pane
clear glass according to the criteria in Question 13? Assume U = 1.05.
Double-pane clear glass, U = 0.55?
2.19 What will be the July cooling load for a 5′ × 5′ west-facing window
constructed of single-pane clear glass according to the criteria in Question
13? Double-pane clear glass? Double-pane tinted glass with a shading
coefficient of 0.75?
2.20 What internal heat gain will result from each of the following in an office?
19
2.21 What minimum outside air quantity would be required for the room in
Question 2.20?
2.22 What heating load would result from the minimum outside air in Question
2.21 under the criteria of Question 2.13? What cooling load for inside
conditions of 75 ° F, 50% RH?
2.23 What is the Wind Chill Factor of an outdoor environment if the measured dry
bulb temperature is 40
q
F with a wind velocity of 50 mph?
2.24 If the Wind Chill Factor of the outdoor is said to be (-)20
q
C and the dry bulb
temperature is 0
q
C, what is the expected wind velocity?
2.25 Absolute humidity can be expressed in grains of water per lb. of air, or lbs. of
water per lb. of air. Write the equation relating the following variables using
both units for absolute humidity: airflow (cfm), heat flow (Btuh), absolute
humidity (grains water per lb. air), and lbs. water per lb. air.
With W expressed in grains of water per lb. of air,
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2.26 For St. Louis, Missouri, what will the percentage difference be in heating load
between 99% design dry bulb and 97.5% design dry bulb heating criteria?
Assume an interior space criteria of 72
q
F.
2.27 Referring to the prior question, will the percent difference in cooling load be
the same for 1% summer design condition vs. 2.5%? Explain why, or why not.
2.28 How much more air conditioning supply air will be needed in a room with a 50
sq ft single strength, clear window facing north compared with an identical
room facing west? Assume supply air temperature of 55
q
F; room temperature
75
q
F.
2.29 How much more air conditioning supply air will be needed in a room with a 50
sq ft single strength, clear window facing east compared with an identical room
facing west? Ignore conducted load. Assume supply air temperature of 55
q
F;
room temperature 75
q
F.
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2.30 How will the orientation (east vs. west) of the room in the previous question
affect the overall building load?
2.31 How much moisture will a supply air quantity of 500 CFM absorb in warming
from 55
q
F, saturated to 75
q
F, 50% relative humidity? How many people
would liberate this amount of humidity?
2.32 If there is no latent load for the supply air in the previous question, what will
be the room humidity? If there are 15 people in the room, what will be the
relative humidity? (Assume that the sensible load still results in a 75
q
F room.)
2.33 How much heating load (Btu per year) would be saved by changing from R-4
to R-12 insulation to a 10,000 sq ft wall with original U-factor of 0.15? Assume
the wall is located in a region with four months of winter, average outside
temperature 30
q
F, inside temperature 70
q
F.
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2.34 If the net cost of heating energy is $6.00 per million Btu, what will the utility
cost saving be for the wall change in the prior problem?
2.35 How much heating load (Btu per year) would be saved by changing from R-4
to R-20 insulation in a 10,000 sq ft wall with an original U-factor of 0.15?
Assume the wall is located in a region with four months of winter, average
outside temperature 30
q
F, inside temperature 70
q
F.
The original wall’s average load for the winter would be U × A × TD, or 0.15 ×
2.36 If the net cost of heating energy is $6.00 per million Btu, what will the utility
cost saving be for the wall change in the prior problem?
2.37 What would heating load savings be for problem 2.33 in a different location
with average winter temperature of 40
q
F? What would annual heating savings
be at $6.00 per million Btu?
The original wall’s average load for the winter would be U × A × TD, or 0.15 ×
10,000 × (70 – 40) = 45,000 Btuh. The four month winter is 1/3 of a year, or (8,760
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2.38 Would savings in problem 2.33 be higher or lower if the facility were operated
round the clock, rather than regular business hours? Why?
2.39 A proposed building design has 10,000 sq ft of glass with shading coefficient
0.7, equally distributed north, south, east, and west. What is the total solar
load in tons July 21 at 4:00 pm?
2.40 What would the solar load in tons for the building in 2.39 be if the glass were
oriented 40% north, 40% south, 10% east, and 10% west?
Glass areas for the four orientations will be as follows:
2.41 What would the solar load in tons for the building in 2.39 be if all the glass
faced north? How could this be accomplished while maintaining views in all
four directions?
The following calculation is based on the equation Qsolar = A × SC × SHGF. SC is
2.42 If the initial, incremental cost of air conditioning equipment is $2,000 per ton,
how much less would the case in 2.41 cost, compared with the case in 2.39?