1. The density of the fluids, its viscosity and the thermal conductivity (K) is measured at ______
a) LMTD
b) Mean temperature
c) Median of the temperature
d) Square mean of the temperature
Explanation: The LMTD has limitations on its applicability such as there should be no change in specific heats, the overall heat transfer coefficient should be constant and negligible heat losses. Hence owing to these limitations, the density and other physical properties are measured at the Mean temperature of the fluids.
2. Which one of the following is true about a hairpin used in double pipe HE?
a) It can handle high pressure drops
b) It cannot handle high pressure drops
c) It is very resistant to fouling
d) It is very expensive
Explanation: One of the major advantages of hairpins is its U-shaped structure which makes it capable to handle very high pressure drops in the tube side, which is one of the contributing factors which make its use very common in industries. It is comparatively cheaper with respect to other designs.
3. To calculate the temperature difference in a double pipe heat exchanger, we use _____
a) LMTD
b) Mean temperature difference
c) Median of the temperature difference
d) Square mean of the temperature difference
Explanation: We can use mean temperature difference in place of LMTD but it is impossible to continuously measure the temperature difference along the whole length of the tube of the HE. Hence we use LMTD for a better approximation.
4. In a double pipe heat exchanger, in the inner side fluid enters at 15℃ and leaves at 65℃. The annulus has steam at 1atm. What is the value of LMTD?
a) 39℃
b) 66.7℃
c) 70℃
d) 56.35℃
Explanation: As steam is condensing at 100℃(Tvap at 1atm), we have LMTD = \(\frac{(100-15)-(100-65)}{Ln(\frac{100-15}{100-65})}\) = 56.35℃.
5. What do you understand by Rfo and Rfi?
a) Dirt factor and Fouling factor
b) Inner Tube outer surface fouling factor and inner surface fouling factor
c) Inner Tube outer surface fouling factor and outer surface fouling factor
d) Inner Tube outer surface fouling factor and Annulus tube inner surface fouling factor
Explanation: As the heat transfer takes place by conduction through only the inner tube of the HE, we are concerned about fouling only on this tube. Thus we calculate inner tube outer surface fouling factor and inner tube inner surface fouling factor respectively.
6. What is the unit of Fouling factor (Rf) in S.I. system?
a) m2K/Pa
b) mK/Pa
c) m2K / W
d) mK/W
Explanation: When a heat exchanger is used with a fouling liquid, it leaves traces/deposits on the surface of the separating wall which reduces the overall heat transfer coefficient of the HE. This extra factor which reduces it is specified as the dirt factor (RD) or the fouling factor (Rf), which is represented as \(\frac{1}{U_D} = \frac{1}{U_D} + R_D\), as U has dimension W/m2K, hence we have dimension of R as 1/U, i.e. m2K/W.
7. If the pressure drop for an operation is more than the pressure assigned to the equipment, then the equipment is perfect to be used for that operation.
a) True
b) False
Explanation: In a double pipe heat exchanger we can use the pressure drop less than or equal to the maximum pressure assigned to the equipment, else it may result in high maintenance cost for the equipment and early wear and tear of the HE.
8. Consider we have a Double Pipe Heat Exchanger, with the inner tube of diameter 20mm (neglect thickness) and outer tube of diameter 30mm. We have two fluids A & B (both with viscosity 2.5×10-5Pa-s), we desire to have their flow rates as 15Kg/s and 21Kg/s respectively. What is the Friction Factor (F) for this setup for the inner pipe?
a) 0.0086
b) 0.086
c) 0.0068
d) 0.068
Explanation: We have Re = GD/μ hence Re= 15×20×103/(2.5×10-5) = 12000.
Now the friction factor F can be calculated by F = 0.0035+\(\frac{0.264}{Re^{0.042}}\) = 0.0086.
9. Which of the following has the maximum Pressure Drop for a Heat Exchanger?
a) Counter-flow
b) Parallel Flow
c) Hairpins arrangement
d) Split Flow
Explanation: The arrangement with hairpins in the heat exchangers has higher pressure drop as the 180 degree turn increases the pressure drop.
10. How many times do we calculate pressure drop for a double pipe heat exchanger?
a) 1
b) 2
c) 3
d) 4
Explanation: We calculate the Pressure drop for both the annulus as well as the inner tube and verify it with the critical values whether operation at that pressure is possible or not.