GATE-CH-1992-5-d-ht-2mark

The advantage of backward-feed multiple effect evaporators over forward-feed units is that

• heat sensitive materials can be handled

• there is no additional cost of pumping

• most concentrated liquor is at highest temperature

• equal heat transfer coefficients exist in various effects

GATE-CH-1993-11-b-ii-ht-1mark

The capacity of a multiple-effect evaporator when compared with a single-effect evaporator both operating with significant boiling point rise at the same terminal temperatures and surface area in each effect equal to surface area of single-effect evaporator

• decreases

• increases

• remains the same

GATE-CH-1998-1-14-ht-1mark

A multiple effect evaporator as compared to a single effect evaporator of the same capacity has

• Lower heat transfer area

• Lower steam economy

• Higher steam economy

• Higher solute concentrations in the product

GATE-CH-2003-14-ht-1mark

A dilute aqueous solution is to be concentrated in an evaporator system. High pressure steam is available. Multiple effect evaporator system is employed because

• total heat transfer area of all the effects is less than that in a single effect evaporator system

• total amount of vapor produced per kg of feed steam in a multiple multieffect system is much higher than in a single effect

• boiling point elevation in a single effect system is much higher than that in any effect in a multieffect system

• heat transfer coefficient in a single effect is much lower than that in any effect in a multieffect system

GATE-CH-2014-21-ht-1mark

Steam economy of a multiple effect evaporator system is defined as

• kilogram of steam used per hour

• kilogram of steam consumed in all the effects for each kilogram of steam fed

• kilogram of steam used in all the effects for each kilogram of water vaporized per hour

• kilogram of water vaporized from all the effects for each kilogram of steam fed to the first effect

GATE-CH-2000-22-ht-5mark

An aqueous solution of a solute is concentrated from 5% to 20% (mass basis) in a single-effect short-tube evaporator. The feed enters the evaporator at a rate of 10 kg/s and at a temperature of 300 K. Steam is available at a saturation pressure of 1.3 bar. The pressure in the vapor space of the evaporator is 0.13 bar and the corresponding saturation temperature of steam is 320 K. If the overall heat transfer coefficient is 5000 W/(m\(^2\).K), calculate the:

(a) steam economy (%). {#1}

(b) heat transfer surface area (m\(^2\)). {#2}

	Enthalpy (kJ/kg)	Heat of vaporization (kJ/kg)
Saturated steam (1.3 bar; 380 K)	-	2000
Saturated steam (0.13 bar; 320 K)	2200	-
Feed (5% ; 300 K)	80	-
Concentrated liquor (20% ; 325 K)	400	-

Boiling point elevation is 5 K.

GATE-CH-2004-60-ht-2mark

It is desired to concentrate a 20% salt solution (20 kg of salt in 100 kg of solution) to a 30% salt solution in an evaporator. Consider a feed of 300 kg/min at 30^oC. The boiling point of the solution is 110^oC, the latent heat of vaporization is 2100 kJ/kg, and the specific heat of the solution is 4 kJ/(kg.K). The rate at which heat has to be supplied (in kJ/min) to the evaporator is

• \(3.06\times 10^5\)

• \(6.12\times 10^5\)

• \(7.24\times 10^5\)

• \(9.08\times 10^5\)

GATE-CH-2011-38-ht-2mark

An aqueous sodium chloride solution (10 wt%) is fed into a single effect evaporator at a rate of 10000 kg/h. It is concentrated to a 20 wt% sodium chloride solution. The rate of consumption of steam in the evaporator is 8000 kg/h. The evaporator capacity (kg/h) and economy are

• 5000, 0.625

• 10000, 0.625

• 5000, 1.6

• 10000, 1.6

GATE-CH-1987-14-iii-ht-2mark

10,000 kg/h of an aqueous feed containing 1% dissolved solids is to be concentrated to 20% solids, in a single effect evaporator. The feed enters at 25\(^\circ \)C. The steam chest is fed with saturated steam at 110\(^\circ \)C. The absolute pressure maintained in the evaporator is such that the water will boil at 55\(^\circ \)C. The heat transfer area available is 50 m\(^2\). The boiling point elevations are as follows: feed: 0.2\(^\circ \)C; 20% solution: 15\(^\circ \)C
The overall heat transfer coefficient, under normal operating conditions is 2500 W/m\(^2\).\(^\circ \)C. Estimate the heat load (in kW) on the condenser, assuming no subcooling of condensate.

• Answer