# How can I solve for flow rate and energy savings in a centrifugal fan system?

• Howlin
In summary, the conversation is about a question regarding a fan that needs to deliver a specific volume flow rate against a given system pressure. The conversation includes discussions about plotting the system characteristic, comparing the use of damper control and speed control, calculating the speed of the fan, and determining the kW savings. The solution involves using the affinity laws for centrifugal fans.
Howlin
Hi all
I have the following question that I require help with.

1. Homework Statement

A supply fan must deliver 2.5m3s of air against a system pressure of 500 Pa.The selected fir this has a performance characteristic as listed below when running at a speed of 1800 revs/min.
i) Plot the system characteristic and determine the actual operating point when the fan is connected to the system.

ii) Show using simple sketches the difference between using damper control and speed control to obtain the design flow rates of 2.5m3/s.

iii)Calculate the speed the fan should run at to deliver the design flow rate.

iv) Calculate the kW savings that can be made by using speed controls instead of damper control

Volume Flow Rate (m3/s) ......0.5 1 1.5 2 2.5 3
Pressure drop (Pa) .......800 850 820 740 600 400
Power(kW) ..........0.53 1.1 1.54 1.8 1.88 1.74

2. Homework Equations

Given none but equations I have used:

ΔP = c* qv2

## The Attempt at a Solution

i)
c = ΔP/qv2
c = 500/2.52
c = 80

ΔP1 = 80*.52 = 20
ΔP1 = 80*.1.02 =80
ΔP1 = 80*1.52 = 180
ΔP1 = 80*2.02 = 320
ΔP1 = 80*2.52 = 500
ΔP1 = 80*3.02 = 720

I graphed this data and got the following graph:

The lines intersect at approximately 2.6m3/s and 528 kPaI do not know how to work out ii)
but I think they are the following but I don't understand it

iii)
The equation to work out the speed of the fan below but I am unsure which values go in where.
N1 / N2 = F1/F2iv)
The equation to work out the kW savings
W1/ W2 = N1/N2

and work is
W = qv *ΔPa/η

I am unsure how to work this out.
Can anyone help me by nudging me in the right direction? Thank you in advance
Regards,
Stephen

## 1. What is a pump system characteristic?

A pump system characteristic is a graph that shows the relationship between the flow rate and the head (pressure) of a pump. It is used to determine the performance of a pump and to select the appropriate pump for a given application.

## 2. What are the key components of a pump system characteristic?

The key components of a pump system characteristic are the flow rate, head (pressure), and efficiency of the pump. These factors are plotted on a graph to show the relationship between them and to determine the operating point of the pump.

## 3. How is a pump system characteristic determined?

A pump system characteristic is determined by conducting tests on the pump at various flow rates and measuring the corresponding head and efficiency. These values are then plotted on a graph to create the pump system characteristic curve.

## 4. What is the significance of a pump system characteristic?

The pump system characteristic is significant because it helps in selecting the appropriate pump for a given application and determining the operating conditions of the pump. It also helps in troubleshooting and optimizing the performance of the pump.

## 5. How can a pump system characteristic be used to improve pump efficiency?

A pump system characteristic can be used to improve pump efficiency by identifying the operating point of the pump on the characteristic curve and adjusting the pump speed or impeller size to achieve the desired flow rate and head. This helps in minimizing energy consumption and maximizing the efficiency of the pump.

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