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.Â
1. Improvement in the quality of the product.Â
2. Increase in the quality of the product.Â
3. Saving raw materials.Â
4. To test the product.Â
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 = 1Â
Thus degree of freedom = nv - neÂ
= 4 - 1Â
=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
