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Old September 24th, 2009, 10:42 PM   #1
theripley
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Air compressor switchover

Hi

Here's the situation:

1. We will install two air compressors, one will be the main supply while the other will be the back up. Both these compressors are connected to the same air tank, which supplies the whole production area.

2. A pressure sensor will be installed on the air tank, initially the compressor A will run till the required pressure is reached. Now, if the pressure has gone down to a minimum level due to peak production, compressor B will backup compressor A.

3. The above stated sequence will go on until sometime (let's say after 1 week). After 1 week, the compressor will switch over automatically, compressor B will be the main supply & compressor A will be the back up.

4. This switchover will go on constantly to protect the compressor from over heating.


Is this sequence possible? how?
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Old September 25th, 2009, 02:03 AM   #2
Kidblue
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Hi,

This is a very common scenario and one that has been covered many times on this forum. Do a search for any of the following 'DUTY/STANDBY', 'LEAD/LAG', 'AUTO CHANGEOVER' and you should find plenty of examples (and sample code) to start you off...

Hope this helps,

Rob
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Old September 26th, 2009, 03:54 AM   #3
shooter
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point 1: use a one way valve in each to depressurize the stopped compressor.
point 2 use a analog input to set the different values. if one compressor is running and the pressure is very low, however rising the second should not start.
point 3 week change over only when both compressors are stopped or when both are running.
start prevention is fine, however if every minute a compressor has to start, it should be running unloaded or use a bigger supply tank.
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Old September 26th, 2009, 04:36 AM   #4
theripley
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Hi Sir,

Quote:
Originally Posted by shooter View Post
point 1: use a one way valve in each to depressurize the stopped compressor.
Thank you for this information.

Quote:
Originally Posted by shooter View Post
point 2 use a analog input to set the different values. if one compressor is running and the pressure is very low, however rising the second should not start.
I will be using a pressure sensor with built-in contacts for high & low pressure PV. The low pressure contacts will be use to run the back-up compressor.

Quote:
Originally Posted by shooter View Post
point 3 week change over only when both compressors are stopped or when both are running.
I hope you don't mind, but how will the compressors switchover automatically when my low pressure contact is connected to the back-up compressor?
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Old September 26th, 2009, 12:59 PM   #5
leitmotif
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This is a very old setup and is done where automatic backup is essential - hydraulics, pneumatics circulation pumps etc.

In essance
Minimum pressure for plant is 70
LAG pump will start at 75 and shut down at say 90
If the LEAD pump can carry the load at over 75 it will.

So one pressure switch is set to turn on at 75 and off at 90
Other switch (lead) will start at say 5 or 10 and shutoff at 125 or whatever maximum ouptut pressure on compressor is.

Compressor A and B each have lead lag switches A is set to lead (connect to 5 to 125 switch) and B is set to lag (connect to 75 90 switch).
These switches should also have off position.

When you want to switch set the lag to lead and set the lead to lag.

IF mine I would use two completely separate units compressor controls tank etc etc. You can parallel the tanks with a valve. This enables taking one off line for service and repair and other can carry the load.

I see you are in Philipines and I will assume you are 100 miles away from a vendor. I would try to keep this as simple as possible and use very well proven parts. A small PLC will handle this plus it will let you do unload and automatic blowndown of water in air tanks. Probably the cost of the PLC will be at about (maybe less) the cost of relays switches hard wiring etc.

I would use regular adjutable pressure switches from Square D or Allen Bradley etc. These will be about 100 to 150 for two.

A pressure transducer would be nice but those start off at around 200 250. you will also pay more for the analog feature in PLC. However you can have one sensor control multiple functions off on blowdown unload etc.

If you want better service and less maintneance ie taking control valves apart all the time make sure
you install filters
for sure blow down your tanks
put in after cooler to remove water (Philipines as I recall had high humidity)
use heat scavanged off after cooler to heat hot water

Ingersoll Rand and several other compressor makers have good instructions on how to install compressed air distribution systems.

Dan Bentler
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Old September 27th, 2009, 07:54 PM   #6
Thermographer
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Our two plant compressors came with a communications module installed on each one and we just ran a cable from one to the other.
The lead and lag parameters are set up on the operator control panel.

They discharge to a common header going to the tank. Our lead operates at 120psi, and our lag compressor picks up at 105psi. To switch the lag and lead around, it's just a few buttons.

Last edited by Thermographer; September 27th, 2009 at 07:57 PM.
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