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Fundamentals of Food Process Engineering

Romeo T. Toledo

Chapter 11

Evaporation - all with Video Answers

Educators


Chapter Questions

18:15

Problem 1

A single-effect falling film type evaporator is used to concentrate orange juice from $14 \%$ to $45 \%$ solids. The evaporator utilizes a mechanical refrigeration cycle using ammonia as refrigerant, for heating and for condensing the vapors. The refrigeration cycle is operated at a high pressure of $200 \mathrm{psia}(1.379 \mathrm{MPa})$ and a low side pressure of $50 \mathrm{psia}(344.7 \mathrm{kPa})$. The evaporator is operated at a vapor temperature of $90^{\circ} \mathrm{F}\left(32.2^{\circ} \mathrm{C}\right)$. Feed enters at $70^{\circ} \mathrm{F}$ $\left(21.1^{\circ} \mathrm{C}\right)$. The ratio of insoluble to soluble solids in the juice is 0.09 and the soluble solids may be considered as glucose and sucrose in 70:30 ratio. Consider the $\Delta \mathrm{T}$ as the log mean $\Delta \mathrm{T}$ between the liquid refrigerant temperature and the feed temperature at one point and the hot refrigerant gas temperature and the concentrated liquid boiling temperature at the other point. The evaporator has a heat transfer surface area of $100 \mathrm{ft}^2\left(9.29 \mathrm{~m}^2\right)$, and an overall heat transfer coefficient of $300 \mathrm{BTU} /\left(\mathrm{h} \$ \mathrm{ft}^2 \${ }^{\circ} \mathrm{F}\right)$ or $1703 \mathrm{~W} /\left(\mathrm{m}^2 \$ \mathrm{~K}\right)$ may be expected. Calculate:
(a) The evaporator capacity in weight of feed per hour.
(b) Tons of refrigeration capacity required for the refrigeration unit based on the heating requirement for the evaporator.
(c) Additional cooling required for condensation of vapors if the refrigeration unit is designed to provide all of the heating requirements for evaporation.

Khoobchandra Agrawal
Khoobchandra Agrawal
Numerade Educator
01:14

Problem 2

Condensate from the heating unit of one effect in a multiple effect evaporator is flashed to the pressure of the heating unit in one of the succeeding effects. If the condensate is saturated liquid at $7.511 \mathrm{psia}(51.79 \mathrm{kPa})$ and the heating unit contains condensing steam at $2.889 \mathrm{psia}$ $(19.92 \mathrm{kPa})$, calculate the total available latent heat that will be in the steam produced from a unit weight of the condensate.

Hast Aggarwal
Hast Aggarwal
Numerade Educator
01:05

Problem 3

A single-effect evaporator was operating at a feed rate of $10,000 \mathrm{~kg} / \mathrm{h}$ concentrating tomato juice at $160^{\circ} \mathrm{F}\left(71.1^{\circ} \mathrm{C}\right)$ from $15 \%$ to $28 \%$ solids. The ratio of insoluble to soluble solids is 0.168 and the soluble solids may be assumed to be hexose sugars. Condensing steam at 29.840 psia (205.7 kPa) was used for heating and the evaporator was at an absolute pressure of 5.993 psia (41.32 kPa). It is desired to change the operating conditions to enable the efficient use of a vapor recompression system. The steam pressure is to be lowered to $17.186 \mathrm{psia}$ (118.37 $\mathrm{kPa}$ ). Assume there is no change in the heat transfer coefficient because of the lowering of the heating medium temperature.
Calculate:
(a) The steam economy for the original operating conditions.
(b) The capacity in weight of feed per hour under the new operating conditions.
(c) The steam economy of the vapor recompression system.
Express the steam economy as the ratio of the energy required for concentration of the juice to the energy required to compress the vapor assuming a mechanical efficiency of $50 \%$ for the compressor. Assume condensate from the heating element is added to the superheated steam to reduce temperature to saturation.

Aadit Sharma
Aadit Sharma
Numerade Educator