Types of Process Variables and Degree of Freedom in Process

Those variables whose change in value affects the performance of the total manufacturing process are called process variables. and process variables are of different types. Process variables are measured and controlled for the following reasons. 

1. Improvement in the quality of the product. 

2. Increase in the quality of the product. 

3. Saving raw materials. 

4. To test the product. 

5. In cost calculation.

Example: Temperature, Pressure, Flow rate, Physical-chemical and electrical properties of the substance, etc.

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Types of Process Variables

Process variables are mainly of three types which are as follows.

1. Control Variable

The variable which directly indicates the desired value or desired form of the product is called the control variable.

Example: The temperature of water at the outlet of a controlled variable heater for a water heater is 0 °C.

2. Manipulated Variable

Those variables whose controlled variable is maintained at the desired value. Can affect the response of any process variable and can also be maintained comfortably.

3. Load Variable

Variables other than the manipulated and control variables of a process are called load variables.

What are Process Dynamics Elements in Chemical Engineering?

What is the degree of freedom in process engineering?

The number of independently acting automatic controllers on a system or process may not exceed the number of degrees of freedom.

The process degree of freedom can be defined as follows.

n = nv - ne

Where

n = No. of a degree of freedom

nv = No. of the variable of the system

ne = No. of defining the equation of the system

The number of degrees of freedom in each process is fixed.

A heat exchanger is shown in the figure in which there are four variables.

Where

C = Outlet temperature

Vc = Input temperature

W = Water flow rate

M = Steam flow rate

Thus nv = 4, ne = 1 

Thus degree of freedom = nv - ne 

= 4 - 1 

=3

Note: The number of independent automatic controllers for a system cannot exceed the number of degrees of freedom.

Suppose three controllers are installed in the above system. If the inlet temperature V, water flow rate W, and heat rate Rm are controlled, the outlet will automatically be controlled.

>Difference Between Physical and Block Diagrams

For a chemical process, if changes have to be made at the appropriate rates, then modifications have to be made, then for such a process, we solve with the Gibbs phase rule.

n = nc - np + 2

Where

nc = number of components

np = number of phases

n = number of chemical degrees of freedom

In this way, the outlet temperature automatically constant controller cannot be established by specifying all three degrees of freedom.

In the isothermal process. n = nc – np+1