Evaporation and Vapourization Quiz

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Questions and Answers

Which factor does NOT affect the boiling liquid side surface coefficient?

Cleanness of the heating surface

Viscosity of fluid

Heat content of condensate (correct)

Velocity of flow of milk

In the heat balance equation, what does the term 'F.hF' represent?

Heat content of concentrate

Enthalpy of steam

Heat content of vapor

Heat content of feed (correct)

According to the material balance, which equation accurately represents the relationship among feed, output rate of concentrate, and rate of vapour production?

F = L + V (correct)

F = L - V

L = F + V

V = L - F

When applying the balance equations, if the solids in vapour are taken as zero, what simplification occurs?

FxF = LxL Signup and view all the answers

What is the relationship between the feed rate and the rate of vapour production based on the provided equations?

V = F[1 - (xf/xL)] Signup and view all the answers

Which process is described as a faster method of producing vapors?

Vapourization Signup and view all the answers

What mainly influences the rate of evaporation?

Surface area of the liquid Signup and view all the answers

Under what condition does evaporation typically occur?

At normal room temperature Signup and view all the answers

How is vapourization visually detected?

By observing the formation of bubbles Signup and view all the answers

What condition controls the process of vapourization?

Variation in pressure above the liquid Signup and view all the answers

When will evaporation cease in a closed system?

When vapour pressure equals the equilibrium value Signup and view all the answers

Which factor has the least effect on the escape rate of surface molecules during evaporation?

Color of the liquid Signup and view all the answers

What enhances the removal of water from a liquid product by evaporation?

Adding heat and removing saturated air Signup and view all the answers

What is a main characteristic of falling film evaporators compared to rising film evaporators?

They operate at a lower temperature difference. Signup and view all the answers

Which of the following characteristics is true for rising film evaporators?

They have more fouling problems. Signup and view all the answers

Which material is primarily used for constructing evaporator bodies and tubes due to its corrosion resistance?

AISI-SS-316 Signup and view all the answers

What is a typical design consideration for evaporators operating under vacuum?

They are designed for an external pressure of 0.1 N/mm². Signup and view all the answers

How does the presence of static head affect rising film evaporators?

It leads to a change in boiling point due to hydrostatic head. Signup and view all the answers

Which of the following statements about falling film evaporators is correct?

They operate under vacuum and allow for the processing of heat sensitive products. Signup and view all the answers

What type of head is typically designed for the bottom of the evaporator drums?

Conical head Signup and view all the answers

What is a common issue faced by rising film evaporators compared to falling film evaporators?

Inability to handle heat sensitive products. Signup and view all the answers

What is a characteristic of a dry condenser?

Cooling water and non-condensables are handled by separate pumps. Signup and view all the answers

How much cooling water is needed for a jet condenser compared to the weight of vapor?

20 to 50 times the weight of vapor. Signup and view all the answers

What is a notable advantage of a counter flow type condenser?

Lower quantity of water is required. Signup and view all the answers

What causes an increase in the quantity of air removed from the evaporator system?

Leaks in the system. Signup and view all the answers

What type of condenser maintains vacuum while allowing condensate to escape?

Barometric leg condenser. Signup and view all the answers

What pressure corresponds to a hydrostatic head of 10.35 meters of water?

Atmospheric pressure. Signup and view all the answers

What is the specific volume of vapor at a boiling point of 60 oC?

7.7 m3/kg. Signup and view all the answers

Which type of condenser is preferred for milk evaporating operations due to its cost-effectiveness?

Jet condenser. Signup and view all the answers

What is the effect of increasing the number of effects in a multiple-effect evaporator on heat flux per effect?

It decreases the heat flux per effect. Signup and view all the answers

What is the primary concern when evaluating the cost-effectiveness of multiple-effect evaporation systems?

The balance between steam economy and equipment cost. Signup and view all the answers

When can the evaporation capacity of a system be significantly reduced?

When the solution being evaporated has a significant boiling-point rise. Signup and view all the answers

Which calculation is NOT typically required for a multiple-effect evaporator system?

The pressure in the liquid space of the last effect. Signup and view all the answers

What method is indicated as convenient for solving equations related to multiple-effect evaporators?

Trial and error. Signup and view all the answers

What is the correct first step in calculating for triple-effect evaporators?

Determine the boiling point in the last effect. Signup and view all the answers

What primarily affects the values needed for calculations in a multiple-effect evaporator?

Physical properties like enthalpies of liquid and vapors. Signup and view all the answers

What happens when evaporation processes exceed five or seven effects in terms of economy?

They are rarely economical due to added complexity and cost. Signup and view all the answers

Study Notes

Evaporation & Vapourization

Evaporation and vapourization occur when molecules gain enough energy to escape from a solution as vapour.
The rate of escape depends on temperature, pressure, surrounding temperature, surface area and heat propagation rate.
Evaporation continues in a closed container until the air is saturated with molecules.
Removal of water from a liquid is enhanced by adding heat and removing saturated air.

Evaporation Types

Vapourization: A faster process that depends on the pressure above the liquid, the amount of solute and the boiling point being reached.
Evaporation: A relativity slow process dependent on temperature differences and vapour pressure differences between air and liquid.
Vapourization produces visual bubbles rising from the surface.
Evaporation usually isn't visually detectable.

Falling Film vs Rising Film Evaporators

Rising Film: Has longer residence time and requires more temperature difference. Less overall heat transfer coefficient.
Falling Film: Shorter residence time and requires less temperature difference. Overall heat transfer coefficient is higher. It's used for heat sensitive products, with less fouling.

Evaporator Design

Evaporator bodies and tubes are usually made of stainless steel to prevent corrosion.
Evaporator Drums operate under vacuum conditions.
Calandria is designed as a tube heat exchanger and based on steam pressure.
Conical head, calandria and vapour drum are connected by flanged joints.
Large openings must be reinforced with compensating rings.

Heat & Material Balance

Heat Balance Equation: F.hf + S.Hs = L.hL+ V.H + Shc (neglecting heat losses)
Material Balance Equation: F= L+V
Solid Material Balance Equation: FxF = LxL + Vy
Capacity Equation based on Solid Material Balance: V = F[ 1- (xf/ xL)]

Condensers

Dry Condensers use two pumps, one for cooling water and one for non-condensable gases.
Wet Condensers: remove condensate and non-condensables together.
Jet Condensers: mix the water and vapour, often used in milk evaporation.
Counter-Flow Condenser: Requires less cooling water.
Air Removal: Air removal from the evaporator system is essential for efficient operation.

Barometric Leg Condenser

It's designed to remove water and condensate without losing vacuum.
The liquid in the leg creates a hydrostatic head that balances the vacuum.

Multiple-Effect Evaporators

Increases steam economy but reduces heat flux per effect.
The complexity of the equipment leads to increased heat losses and decreased capacity.
Boiling-point rise reduces the capacity of the system.

Multiple-Effect Evaporator Calculations

Determining the area of the heating surface, the steam supply rate and the amount of vapour leaving each effect are the main calculations.
Calculation methods are based on material and heat balances, and heat transfer equations.

Triple-Effect Evaporator Calculations

Step 1: Determine the boiling point in the last effect.
Step 2: Determine the total amount of vapor evaporated by material balance.
Step 3: Estimate temperature drops in each effect and calculate the boiling point within each effect.

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Description

Test your knowledge on evaporation and vapourization processes. This quiz covers the principles of how molecules escape from solutions, the factors affecting the rate of evaporation, and the differences between vapourization and evaporation. Additionally, it explores various types of evaporators.