# Heat Transfer Operations Questions and Answers – Scraped Surface Heat Exchangers – Design and Operations

«
»

This set of Heat Transfer Operations MCQs focuses on “Scraped Surface Heat Exchangers – Design and Operations”.

1. What can Scraped Surface Heat Exchangers do?
a) Adds turbulence to the core of the fluid
b) Can handle large pressure drops
c) Removes the layer of fluid at surface and mixes with bulk
d) Use high pumping power

Explanation: Scraped Surface Heat Exchangers are normally used in operations where the designer wants to avoid excessive pressure drop and high turbulence at the core, so it removes the fluid from the surface gently and mixes with the bulk.

2. We use Scraped Surface Heat Exchangers when the fluid in operation is very viscous.
a) True
b) False

Explanation: Scraped Surface Heat Exchangers are normally used in operations where the designer wants to use highly viscous fluids where high turbulence at the core cannot be achieved, so it removes the fluid from the surface gently and mixes with the bulk.

3. Scraped Surface Heat Exchangers are used for fluids which are generally heat ________
a) Sensitive
b) Insensitive
c) Resistant
d) Insulator

Explanation: Scraped Surface Heat Exchangers are best suited for heat sensitive liquids that is those liquids which cannot handle high temperatures, for example, food products or pharmaceuticals, this is because of the low temperature at the interface between the liquid and heat transfer surface.

4. The maintenance cost of the Scraped Surface Heat Exchanger is usually low because of one of the following factor. What is it?
a) It removes the fouling formed at the surface
b) No motors are used
c) No pumps are used
d) High viscous fluids are used

Explanation: As the scrapers of the Scraped Surface Heat Exchanger remove fluid from the surface, any deposits on the surface caused by fouling is also removed, thus making sure that any kind of contamination of the feed is reduced to a minimum.

5. The number of blades is a Scraped Surface Heat Exchanger does not depend on one of the following factor, which one is it?
a) Capital Cost
b) Rotation speed
c) Fouling Factor
d) Viscosity

Explanation: The number of blades is decided after considering the capital cost, rotation speed of the blades, and the viscosity of the fluid. It does not fully depend on the fouling factor as if the fouling factor increases we can alter the rotation speed rather than number of blades.

6. Scraped Surface Heat Exchanger that actually don’t touch the surface are also known as _______ Surface Heat Exchanger.
a) Scratched
b) Wiped
c) Scraped
d) Swiped

Explanation: Those exchangers whose blades are not completely in contact with the surface, but rather move very close to it are be known as wiped surface heat exchangers, and are usually preferred as the wear and tear of the surface or the blades are significantly reduced.

7. It is usually preferred to use wiped surface heat exchangers because of reduced wear and tear of the blades and surface.
a) True
b) False

Explanation: Those exchangers whose blades are not completely in contact with the surface, but rather move very close to it are be known as wiped surface heat exchangers, and are usually preferred as the wear and tear of the surface or the blades are significantly reduced.

8. There are two types of Surface renewal heat exchangers, which one of the following is correct?
a) Scraped and Wiped Surface
b) Scraped and Swiped Surface
c) Scraped and Polished Surface
d) Scraped and Scratch Surface

Explanation: Those exchangers whose blades are not completely in contact with the surface, but rather move very close to it are be known as wiped surface heat exchangers, and are usually preferred as the wear and tear of the surface or the blades are significantly reduced.

9. For a concentrating process, setting the setup_________ is the preferred arrangement.
a) Vertically
b) Horizontally
c) Inclined
d) Reclined

Explanation: When operation involves evaporation or concentration of fluid, vertical arrangement is best suited for easy removal of vapour.

10. Which one of the following expression is correct for the rotary Reynolds Number in Scraped Surface HE?
a) $$\frac{D^2 N\rho}{\mu}$$
b) $$\frac{DN\rho}{\mu}$$
c) $$\frac{D^2 N\mu}{\rho}$$
d) $$\frac{N\rho}{\mu}$$

Explanation: There were several attempts to make standard correlation for Scraped surface heat exchangers but there isn’t any successful correlation, but usual correlations are sometimes used such as the rotary Reynolds number defined as $$\frac{D^2 N\rho}{\mu}$$.

11. If the diameter of the rotating blades in 30mm, agitator speed is 20 RPM and the fluid in the vessel has density 990kg/m3 and viscosity 2.5×10-2 kg/ms. What is the value of Rotary Reynolds Number?
a) 11.88
b) 12.88
c) 12.33
d) 11.55

Explanation: We know that the rotary Reynolds Number is defined as $$\frac{D^2 N\rho}{\mu}$$, hence Re = $$\frac{D^2 N\rho}{\mu}$$ = 0.032×20/60×990/(2.5×10-2) = 11.88.

12. Why do we calculate rotary Reynolds Number and not use Seider-Tate equation?
a) Because of no turbulence in the flow
b) Because no correlations have yet been found for this setup
c) Because the former equation involves moving fluid and not the rotating ones
d) Two fluids are not involved

Explanation: There has been thorough research carried out to account for the analytical basis of this setup but till now nothing has been achieved. Having regard to the complexity of the geometry particularly of the blade assembly, and the several interacting factors, only empirical relations help us to design the equipment which are derived from experience.

13. Evaporation of as fluid to complete dryness can never be performed in a Scraped Surface Heat Exchanger setup because?
a) The blades will tear faster
b) The statement is wrong, this operation can be carried out
c) Vessel will have high fouling
d) Agitators will burnout

Explanation: The statement is wrong, evaporation to complete dryness is one of the important applications of Scraped Surface Heat Exchangers as the constant scratching from the surface aggregates the solid particles.

14. The industry that extensively uses Scraped Surface Heat Exchangers is?
a) Food industry
b) Petrochemical industry
c) Oil industry
d) Steel Plants