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COOLING OF ELECTRONIC EQUIPMENT TEST 2
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Question 1 of 15
1. Question
1 pointsA 0.15W small cylindrical resistor mounted on a circuit board is 0.5 in. long and has a diameter of 0.15 in. The view of the resistor is largely blocked by the circuit board facing it, and the heat transfer from the connecting wires is negligible. The air is free to flow through the parallel flow passages between the PCBs as a result of natural convection currents. If the air temperature in the vicinity of the resistor is 130°F, determine the surface temperature of the resistor.
Correct
194°F
Incorrect
194°F

Question 2 of 15
2. Question
1 pointsThe components of an electronic system dissipating 150 W are located in a 1mlong horizontal duct whose cross section is 15 cm x 15 cm. The components in the duct are cooled by forced air, which enters at 30°C at a rate of 0.4 m³/min and leaves at 45°C. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 25°C, determine the heat transfer from the outer surfaces of the duct to the ambient air by natural convection and
Correct
31.7 W,
Incorrect
31.7 W,

Question 3 of 15
3. Question
1 pointsThe components of an electronic system dissipating 150 W are located in a 1mlong horizontal duct whose cross section is 15 cm x 15 cm. The components in the duct are cooled by forced air, which enters at 30°C at a rate of 0.4 m³/min and leaves at 45°C. The surfaces of the sheet metal duct are not painted, and thus radiation heat transfer from the outer surfaces is negligible. If the ambient air temperature is 25°C, determine the average temperature of the duct.
Correct
40°C
Incorrect
40°C

Question 4 of 15
4. Question
1 pointsConsider a hollowcore PCB that is 15 cm high and 20 cm long, dissipating a total of 30 W. The width of the air gap in the middle of the PCB is 0.25 cm. The cooling air enters the core at 30°C at a rate of 1 L/s. Assuming the heat generated to be uniformly distributed over the two side surfaces of the PCB, determine the temperature at which the air leaves the hollow core .(ANSWER UPTO ONE DECIMAL)
Correct
56.4°C
Incorrect
56.4°C

Question 5 of 15
5. Question
1 pointsConsider a hollowcore PCB that is 15 cm high and 20 cm long, dissipating a total of 30 W. The width of the air gap in the middle of the PCB is 0.25 cm. The cooling air enters the core at 30°C at a rate of 1 L/s. Assuming the heat generated to be uniformly distributed over the two side surfaces of the PCB, determine the highest temperature on the inner surface of the core. .(ANSWER UPTO ONE DECIMAL)
Correct
67.6°C
Incorrect
67.6°C

Question 6 of 15
6. Question
1 pointsA transistor with a height of 0.25 in. and a diameter of 0.2 in. is mounted on a circuit board. The transistor is cooled by air flowing over it at a velocity of 400 ft/min. If the air temperature is 140°F and the transistor case temperature is not to exceed 175°F, determine the amount of power this transistor can dissipate safely(answer upto two decimal)
Correct
0.15 W
Incorrect
0.15 W

Question 7 of 15
7. Question
1 pointsAn array of power transistors, each dissipating 2 W of power, is to be cooled by mounting them on a 20cm x 20cm square aluminum plate and blowing air over the plate with a fan at 30°C with a velocity of 3 m/s. The average temperature of the plate is not to exceed 60°C. Assuming the heat transfer from the back side of the plate to be negligible, determine the number of transistors that can be placed on this plate.
Correct
9
Incorrect
9

Question 8 of 15
8. Question
1 pointsWater enters the tubes of a cold plate at 95°F with an average velocity of 60 ft/min and leaves at 105°F. The diameter of the tubes is 0.25 in. Assuming 15 percent of the heat generated is dissipated from the components to the surroundings by convection and radiation, and the remaining 85 percent is removed by the cooling water, determine the amount of heat generated by the electronic devices mounted on the cold plate.
Correct
263 W
Incorrect
263 W

Question 9 of 15
9. Question
1 pointsA 6W chip having a surface area of 0.5 cm² is cooled by immersing it into FC86 liquid that is maintained at a temperature of 25°C. Using the boiling curve in Figure , estimate the temperature of the chip surface
Correct
82°C
Incorrect
82°C

Question 10 of 15
10. Question
1 pointsA 3W chip having a surface area of 0.2 cm2 is cooled by immersing it into FC86 liquid that is maintained at a temperature of 45°C. Using the boiling curve in Figure , estimate the temperature of the chip surface
Correct
108°C
Incorrect
108°C

Question 11 of 15
11. Question
1 pointsConsider a 15–cm x 15–cm multi layer circuit board dissipating 22.5 W of heat. The board consists of four layers of 0.1mmthick copper (k = 386 W/m · °C) and three layers of 0.5mmthick glass–epoxy (k = 0.26 W/m · °C) sandwiched together, as shown in Figure below. The circuit board is attached to a heat sink from both ends, and the temperature of the board at those ends is 35°C. Heat is considered to be uniformly generated in the epoxy layers of the PCB at a rate of 0.5 W per 1cm 15–cm epoxy laminate strip (or 1.5 W per 1cm x 15–cm strip of the board). Considering only a portion of the PCB because of symmetry, determine the magnitude and location of the maximum temperature that occurs in the PCB. Assume heat transfer from the top and bottom faces of the PCB to be negligible.(answer in one decimal)
Correct
55.5°C
Incorrect
55.5°C

Question 12 of 15
12. Question
1 pointsA 15–cm x 18cm doublesided circuit board dissipating a total of 18 W of heat is to be conductioncooled by a 1.2mmthick aluminum core plate (k = 237 W/m · °C) sandwiched between two epoxy laminates (k = 0.26 W/m · °C). Each epoxy layer has a thickness of 0.5 mm and is attached to the aluminum core plate with conductive epoxy adhesive (k = 1.8 W/m · °C) of thickness 0.1 mm. Heat is uniformly generated on each side of the PCB at a rate of 0.5 W per 1cm x 15–cm epoxy laminate strip. All of the heat is conducted along the 18cm side since the PCB is cooled along the two 15–cmlong edges. Considering only part of the PCB board because of symmetry, determine the maximum temperature rise across the 9cm distance between the center and the sides of the PCB
Correct
10.1°C
Incorrect
10.1°C

Question 13 of 15
13. Question
1 pointsThe components of an electronic system are located in a 1.2mlong horizontal duct whose cross section is 10 cm x 20 cm. The components in the duct are not allowed to come into direct contact with cooling air, and so are cooled by air flowing over the duct at 30°C with a velocity of 250 m/min. The duct is oriented such that air strikes the 10cmhigh side of the duct normally. If the surface temperature of the duct is not to exceed 60°C, determine the total power rating of the electronic devices that can be mounted in the duct.
Correct
481 W
Incorrect
481 W

Question 14 of 15
14. Question
1 pointsThe components of an electronic system are located in a 1.2mlong horizontal duct whose cross section is 10 cm x 20 cm. The components in the duct are not allowed to come into direct contact with cooling air, and so are cooled by air flowing over the duct at 30°C with a velocity of 250 m/min. The duct is oriented such that air strikes the 10cmhigh side of the duct normally. If the surface temperature of the duct is not to exceed 60°C, What would your answer be if the duct is oriented such that air strikes the 20cmhigh side normally?
Correct
384 W
Incorrect
384 W

Question 15 of 15
15. Question
1 pointsA computer cooled by a fan contains six PCBs, each dissipating 15 W of power, as shown in Figure below . The height of the PCBs is 15 cm and the length is 20 cm. The clearance between the tips of the components on the PCB and the back surface of the adjacent PCB is 0.4 cm. The cooling air is supplied by a 20W fan mounted at the inlet. If the temperature rise of air as it flows through the case of the computer is not to exceed 10°C, determine the flow rate of the air the fan needs to deliver
Assumptions 1 Steady operating conditions exist. 2 Air is an ideal gas. 3 The computer is located at sea level so that the local atmospheric pressure is 1 atm. 4 The entire heat generated by the electronic components is removed by air flowing through the opening between the PCBs. 5 The entire power consumed by the fan motor is transferred as heat to the cooling air. This is a conservative approach since the fan and its motor are usually mounted to the chassis of the electronic system, and some of the heat generated in the motor may be conducted to the chassis through the mounting brackets.(answer upto three decimal)
Properties We use properties of air at 30°C since the air enters at room temperature, and the temperature rise of air is limited to just 10°C:
p=1.164 kg/m³ k=0.02588 W/m · °C
Cp=1005 J/kg · °C v=1.608 10^5 m²/s
pr=0.7282
Correct
Analysis Because of symmetry, we consider the flow area between the two adjacent PCBs only. We assume the flow rate of air through all six channels to be identical, and to be equal to onesixth of the total flow rate. Noting that the temperature rise of air is limited to 10°C and that the power consumed by the fan is also absorbed by the air, the total mass flow rate of air
through the computer can be determined fromQ=mCp(Tout Tin)
m=Q/Cp(Tout Tin)=(6 x 15 +20) J/s/(1007 J/kg · °C)(10°C)=0.01092 kg/s
Then the volume flow rate of air and the air velocity become
v=m/p=0.01092 kg/s/1.164 kg/m³=0.009381 m³/s=0.563 m³/min
Incorrect
Analysis Because of symmetry, we consider the flow area between the two adjacent PCBs only. We assume the flow rate of air through all six channels to be identical, and to be equal to onesixth of the total flow rate. Noting that the temperature rise of air is limited to 10°C and that the power consumed by the fan is also absorbed by the air, the total mass flow rate of air
through the computer can be determined fromQ=mCp(Tout Tin)
m=Q/Cp(Tout Tin)=(6 x 15 +20) J/s/(1007 J/kg · °C)(10°C)=0.01092 kg/s
Then the volume flow rate of air and the air velocity become
v=m/p=0.01092 kg/s/1.164 kg/m³=0.009381 m³/s=0.563 m³/min