Article Contents
Affinity law for centrifugal pump is far necessary for a mechanical engineer as well as chemical engineer. Centrifugal pump is very common used pump in all type of industries like pharmaceutical , chemical and speciality chemicals. In this article we study classification of centrifugal pump in basic, pump affinity laws spreadsheet, pump power calculation and some basic information about pump and its classification.
Affinity law for centrifugal pump
There are some laws which is useful for a chemical or process engineer in design of different types of centrifugal pumps like vertical centrifugal pump or horizontal centrifugal pump.
If you don’t what are the different types of pump and working principle of centrifugal pump you can read our article on it.
There are three pump affinity laws spreadsheet, like flow verses rotation of a centrifugal pump impeller , head verses rotation per minutes (RPM) of a centrifugal pump impeller and pump power calculation verses pump RPM (RPM of impeller).
You can read our article on characteristic curve of centrifugal pump.
Affinity laws for pump are as follows,
- Pump flow verses pump impeller RPM
- Pump head verses pump impeller RPM
- Pump power verses RPM of pump impeller
Pump flow verses pump impeller RPM
This is first affinity law of pump which describe the relation between pump capacity and impeller rotation per minutes. This relation is useful for selection of RPM for required flow of centrifugal pump.
First affinity law of pump state that flow or capacity of a pump is directly proportional to the impeller RPM. It means if we want to increase the pump flow we need to increase RPM of impeller.
On the basic of this law we can add RPM controi in pump (VFD) to increase or decrease the pump capacity or flow of pump.
Q1 = Q2*(RPM1/RPM2)
Q1 is pump flow at RPM1
Q2 is pump flow at RPM2.
Pump head verses pump impeller RPM
We know that pump head is nothing but hight at which liquid want to pump. So Second law of pump state that pump head is directly proportional to the square of impeller Rotation per minute ( RPM).
As compare to pump head we need to increase more rpm if is there any kind of change in head if pump.
Characteristic curve of centrifugal pump
H1 = H2 ( Square of RPM1/ Square of RPM2)
where,
H1 is nothing but head in meter at RPM1.
H 2 is head in meter at RPM 2.
Power of pump impeller verses pump impeller rotation
This is third and final law of pump. This state that pump power is directly proportional to the qube of pump impeller rotation per minute.
It mean if we want to decrease power required to pump the liquid we need to decrease the pump impeller RPM.
P1 = P2 * ( cube of RPM 1/ Cube of RPM 2 ).
where,
P1 is the power required to pump the liquid for required head and flow at RPM1.
P2 power required to pump the liquid at RMP2.
Above as long clearly say that if we change any thing from RPM, Head, Flow or power to motor other things also changes.
Example is we increase the Impeller RPM from 2800 to 1400, then head, flow and power requirement also get chages.
How to calculate power required to pump the liquid????
Power required to pump the liquid in case of centrifugal pump is depend upon factors like head and flow.
What is head? Head is height where we want to pump the liquid.
there are some types of pump head.
- 1) Suction head
- 2) Discharge head
- 3) Dynamic head
Flow is nothing amount of liquid which is to be pump for particular application.
if we know the pump flow and pump head we can easily calculate power required to pump the liquid.
Power in KW = Pump head * Pump flow /360
Using above formula we can easily get power required to pump.
But above formula is applicable for 100% pump efficiency.
Pump power = (head * flow /360) / pump efficiency
Pump power calculation example
Example No 1.
Calculate the power required to pump water at head 10 meter. Assuming pump efficiency 50%.
Applications of affinity law of centrifugal pump
The main application are as follows.
- We can calculate head change if there any changes in flow of pump.
- We can calculate pump flow if the any change in pump head.
- We can reduce or increase the pump RPM if we want to kore less flow.
Conclusion
Above article clear that how affinity law for centrifugal pump is useful in design of different types of centrifugal pumps. If we increase the RPM of motor then there will be changes in pump flow and pump head.
Hope you enjoy this article. I need your comments on it. Thank you ……..
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