REFRIGERATION+AIR COND.TECH.-W/2 ACCESS
8th Edition
ISBN: 9781337370042
Author: TOMCZYK
Publisher: CENGAGE L
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Chapter 26, Problem 21RQ
True or False: The two types of defrosts used with low-temperature fixtures are hot gas and electric defrost.
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A power transistor with three 0.8 mm diameter leads of a copper-silver alloy (k = 400 W/m-K) dissipates 3.5 Watts of
power. The printed circuit board (PCB) on which the transistor is mounted is maintained at 30 C. The air temperature is
20 C and the heat transfer coefficient for the leads is 60 W/m² K. The length of the leads from the transistor to printed
circuit board is 9.0 mm.
(a) Determine the maximum lead temperature neglecting all convection effects.
(b) Determine the maximum lead temperature if convection is present.
T=20C
h = 60 W/m²K
Air
Power
Transistor
Lead
k = 400 W/m K
TPCB = 30 C
9 mm
A window in a tall building experiences a horizontal wind flow parallel to the surface at a velocity of 8 m/s. The window
measures 2 m high, 1.5 m wide and 6.5 mm thick. The transition from the building wall to the window glass surface is
not smooth. The window casement that surrounds the edge of glass pane is raised 10 mm above the surface of the glass
and adjacent wall structure. Inside the building, the air temperature and heat transfer coefficient for the interior surface
are 21 °C and 9 W/m² K respectively. The window has a uniform thermal conductivity of 1.4 W/m K. If the outdoor air
temperature is 5 °C, determine the rate of convective heat loss from the window. Develop a thermal resistance network
for the window system. Please state and justify all assumptions. Neglect any conduction through the casement.
Assume the following air properties: kair =0.0248 W/m K, v= 14.20 x 106 m²/s, Pr = 0.712.
Outside Air
u=8m/s
T=5°C
casement
glass
Inside
2 m
T=21°C
h=9W/m² K
6.5 mm
1.5 m…
A thin electrical heater that dissipates 2 kW of thermal energy is position between two plane walls. The thickness of
each wall is shown in the figure below. The thermal conductivities of sections A and B are 3 W/m C and 0.8 W/m C
respectively. The electrical heater is assumed to be highly conductive (k=200 W/m C). The exterior surface of each wall
experiences convective conditions as summarized below.
(a) Construct the thermal circuit representing the steady state heat transfer for the system. Label all nodes, resistances
and relevant terms.
(b) Determine the temperate of the electrical heater.
T = 50 C
h = 200 W/m² C
A
10 cm
6 cm
B
Electrical
Heater
T_ = 20 C
h = 50 W/m² C
K₁ = 3 W/m²K
0.05 cm
k₁ = 0.08 W/m² K
Chapter 26 Solutions
REFRIGERATION+AIR COND.TECH.-W/2 ACCESS
Ch. 26 - The two broad categories of display cases are...Ch. 26 - How are conditions maintained in open display...Ch. 26 - What are mullion heaters?Ch. 26 - The three temperature ranges for refrigeration...Ch. 26 - The two methods for rejecting heat from...Ch. 26 - When the compressors are located in the equipment...Ch. 26 - When one large compressor is used, what desirable...Ch. 26 - Define a parallel compressor system and list five...Ch. 26 - Three disadvantages of parallel compressor systems...Ch. 26 - What is the difference between an even and an...
Ch. 26 - Exactly matching the refrigeration load with the...Ch. 26 - What is the function of the pressure transducer in...Ch. 26 - What is the function of the microprocessor-based...Ch. 26 - Explain how the oil separator, oil reservoir,...Ch. 26 - Large or small compressors on parallel compressor...Ch. 26 - A(n)_______varies the frequency of an...Ch. 26 - Prob. 17RQCh. 26 - Prob. 18RQCh. 26 - Three advantages of unitary, stand-alone...Ch. 26 - How is defrost accomplished when the equipment...Ch. 26 - True or False: The two types of defrosts used with...Ch. 26 - What special precautions should be taken with...Ch. 26 - Prob. 23RQCh. 26 - What special valve is used in a refrigerated air...Ch. 26 - Briefly explain what is meant by a surge-type...Ch. 26 - Briefly explain what is meant by a subcritical...Ch. 26 - Briefly explain what is meant by a transcritical...Ch. 26 - Prob. 28RQCh. 26 - Prob. 29RQCh. 26 - Prob. 30RQCh. 26 - Prob. 31RQCh. 26 - Prob. 32RQCh. 26 - Prob. 33RQ
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