Chapter 8, Problem 8.69P

Interpretation Introduction

(a)

Interpretation:

A complete labelled flowchart for the given process is to be drawn with temperatures and specific enthalpies of each of the streams being specified.

Concept introduction:

A flowchart is the complete representation of a process through boxes or other shapes which represents process units and arrows that represents the input and output of the process. The flowchart must be fully labelled to infer important data about the process involved.

Specific heat capacity $\left(C_p\right)$ of a substance is the amount of heat needed for a unit mass of substance to raise its temperature by $1^{\circ }\text{C}$. It is temperature dependent and varies with temperature of the substance.

Specific enthalpy $\left(\hat{H}\right)$ of any substance at temperature $T^{\circ }\text{C}$ is given by:

$\hat{H}={\int }_{T_{ref}}^T{C}_{p}dT$ .....(1)

Here, $T_{ref}$ is the reference temperature taken for the calculations.

Interpretation Introduction

(b)

Interpretation:

The required feed rate of the steam in the first effect is to be calculated.

Concept introduction:

In a system, a conserved quantity (total mass, mass of a particular species, energy or momentum) is balanced and can be written as:

$input+generation-output-consumpumtion=accumulation$

Here, ‘ input’ is the stream which enters the system. ‘ generation’ is the term used for the quantity that is produced within the system. ‘ output’ is the stream which leaves the system. ‘ consumption’ is the term used for the quantity that is consumed within the system. ‘ accumulation’ is used for the quantity which is builds up within the system.

All the equations which are formed are then solved simultaneously to calculate the values of the unknown variables.

The equation for energy balance is:

$\Delta \dot{H}+\Delta {\dot{E}}_{\text{k}}+\Delta {\dot{E}}_{\text{p}}=\dot{Q}-{\dot{W}}_{\text{s}}$ .....(2)

Here, $\Delta \dot{H}$ is the change in the enthalpy of the system, $\Delta {\dot{E}}_{\text{k}}$ is the kinetic energy change of the system, $\Delta {\dot{E}}_{\text{p}}$ is the potential energy change of the system, $\dot{Q}$ is the net energy which is transferred to a system and ${\dot{W}}_{\text{s}}$ is the word done by the shaft.

Interpretation Introduction

(c)

Interpretation:

The production rate of fresh water and the salt concentration in the final brine solution are to be calculated. Also, the reason for not including the condensate from the first effect to the production rate of fresh water is to be stated.

Concept introduction:

In a system, a conserved quantity (total mass, mass of a particular species, energy or momentum) is balanced and can be written as:

$input+generation-output-consumpumtion=accumulation$

Here, ‘ input’ is the stream which enters the system. ‘ generation’ is the term used for the quantity that is produced within the system. ‘ output’ is the stream which leaves the system. ‘ consumption’ is the term used for the quantity that is consumed within the system. ‘ accumulation’ is used for the quantity which is builds up within the system.

All the equations which are formed are then solved simultaneously to calculate the values of the unknown variables.

The equation for energy balance is:

$\Delta \dot{H}+\Delta {\dot{E}}_{\text{k}}+\Delta {\dot{E}}_{\text{p}}=\dot{Q}-{\dot{W}}_{\text{s}}$ .....(2)

Here, $\Delta \dot{H}$ is the change in the enthalpy of the system, $\Delta {\dot{E}}_{\text{k}}$ is the kinetic energy change of the system, $\Delta {\dot{E}}_{\text{p}}$ is the potential energy change of the system, $\dot{Q}$ is the net energy which is transferred to a system and ${\dot{W}}_{\text{s}}$ is the word done by the shaft.

Interpretation Introduction

(d)

Interpretation:

The reason for the decrease in the pressure from one effect to the next is to be explained.

Concept introduction:

The saturation temperature of water at which it begins to boil and can exists in both liquid and vapor state, increases with increase in its pressure.

Interpretation Introduction

(e)

Interpretation:

The required feed rate of saturated steam at $4\text{ bar}$ to achieve the production rate of fresh water as calculated in part (c) when single-effect evaporator at $0.20\text{ bar}$ is used. The condition to determine which process out of the two is more economical is to be stated.

Concept introduction:

In a system, a conserved quantity (total mass, mass of a particular species, energy or momentum) is balanced and can be written as:

$input+generation-output-consumpumtion=accumulation$

Here, ‘ input’ is the stream which enters the system. ‘ generation’ is the term used for the quantity that is produced within the system. ‘ output’ is the stream which leaves the system. ‘ consumption’ is the term used for the quantity that is consumed within the system. ‘ accumulation’ is used for the quantity which is builds up within the system.

All the equations which are formed are then solved simultaneously to calculate the values of the unknown variables.

The equation for energy balance is:

$\Delta \dot{H}+\Delta {\dot{E}}_{\text{k}}+\Delta {\dot{E}}_{\text{p}}=\dot{Q}-{\dot{W}}_{\text{s}}$ .....(2)

Here, $\Delta \dot{H}$ is the change in the enthalpy of the system, $\Delta {\dot{E}}_{\text{k}}$ is the kinetic energy change of the system, $\Delta {\dot{E}}_{\text{p}}$ is the potential energy change of the system, $\dot{Q}$ is the net energy which is transferred to a system and ${\dot{W}}_{\text{s}}$ is the word done by the shaft.