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View Full Version : Compressor Demands of a Building Control System ?



EddyCurr
04-16-2012, 07:52 PM
How severe is the demand on a compressor when used for powering a boiler
control system in a multistory residential complex? I have some idea of what
the demands of sand blasting, painting, air tools and so on are, but no idea
of what loads a control system produce.

I have come across a domestic made industrial grade compressor that is
rated at a delivery of 19 cfm @ 175 psi and has a tank cert dated 2008.
This machine is offered by a building maintenance organization as surplus
to their needs.

The vendor says they took over a building one year ago and replaced this
single head compressor with a dual head machine as a Risk-Mgmnt measure
to ensure there would be back-up air for the boiler controls in the event of
a compressor failure.

I was told that the compressor was powered up 24/7 and free to cycle as
commanded by the tank pressure switch. I know that this machine is rated
for 175 psi but factory set to cycle at 150 psi. I was told that because the
control system operates on low pressure (20 psi), common practice is to
lower the pressure switch setting from 150 to 100 psi.

.

hardtail
04-16-2012, 08:34 PM
Most modern boiler controls are likely electic/electronic however much field hardware maybe pnuematic......the demands maybe high depending on the amount of equipment BUT it was likely sized right initially and the risk management stuff will probably provide you with your golden child. Our control air compressors are 4 pumps 44cfm each driven by 2 motors in an opposed arrangement, been running 24/7 since 1985, they've probably had 2 light freshenings.....I found a deal on a smaller Devilbiss 2 stage pump that was a control compressor for $40, they said it would never shutoff so bought a new one and when that wouldn't shutoff they started looking for the offending leaks......

EddyCurr
04-16-2012, 09:08 PM
Our control air compressors are 4 pumps 44cfm each driven by
2 motors in an opposed arrangement, ......Of course I have no sense of context, but that seems like a LOT of capacity
for what, in my mind's eye, consists of largely a closed system which consumes
only the air necessary to activate some diaphrams. Clearly, I was right to
suspect there is more to it than first comes to mind.

The issue here is that the compressor is no longer in situ and it isn't feasible
to power it up. I have been told that it received a cleaning and oil change
(possibly a filter) from the local brand rep/servco. I have asked for a contact
name there to discuss the machine but if the smoke escapes, repairs will be
on my dime.

.

Black_Moons
04-16-2012, 10:27 PM
I found a deal on a smaller Devilbiss 2 stage pump that was a control compressor for $40, they said it would never shutoff so bought a new one and when that wouldn't shutoff they started looking for the offending leaks......

Yea, I was thinking myself that it would be more or less an entirely 'closed' system, Except for... Thousands of tiny air leaks all adding up.

Come to think of it, the lines would likely be very long to some controls, if the valves are located at the point of use and the compresor/controller is central. they could contain a considerable volume that must be filled/vented for operation.

EddyCurr
04-16-2012, 11:30 PM
An excerpt from a case study


Hospital Air System Savings (http://www.airbestpractices.com/system-assessments/compressor-controls/hospital-air-system-savings)
By Ron Marshall CET, CIM, for the Compressed Air Challenge®.
Edited by Jan Zuercher and David Booth
Compressed Air Best Practices

"An energy audit of the centrifugal compressor showed while the
compressed air demand of the hospital was variable, the power
consumption of the compressor was very flat. Calculations showed
that the average load was slightly under 300 cfm yet the energy
consumption of the system was topping 1,400,000 kWh costing
$68,500 per year in operating costs. The specific power of the
compressor alone was averaging a staggering 49 kW per 100 cfm
(more than double what would be expected). The compressor also
used chilled water that required additional refrigeration energy to
produce and the system had non-cycling style refrigerated air
dryers installed, increasing the system specific power number
to 57 kW/100 cfm."

57 kW/100 cfm, that's some expensive air.

.

Black_Moons
04-16-2012, 11:34 PM
Lets see, 20A 240v = 4,800W for 20CFM (5hp home compressor, single stage)
So thats 24KW for 100CFM.

Sounds like a very poor efficiency compressor if it required 49kW without even the chillers.

EddyCurr
04-16-2012, 11:43 PM
Highly efficient, but of a essentially fixed supply design.

Losses arose due to excessive venting of surplus capacity during low points
in cyclical demand curve.

.

Black_Moons
04-16-2012, 11:50 PM
Highly efficient, but of a essentially fixed supply design.

Losses arose due to excessive venting of surplus capacity during low points
in cyclical demand curve.

.

Ah, I thought they often ran such large compressors on VFD's to vary the output as needed.

hardtail
04-17-2012, 10:23 AM
Well some of your control air system is closed but pneumatic thermostats when calling for heat are constantly bleeding a small volume of air and valves changing position have to vent off that excess, multiply that by hundreds and it adds up, thats a perfectly functioning system, then ad leaks over the years.........

If it has no power, I would merely pull the belts and rotate the flywheel by hand, you should get a good thump as the piston strokes, also look for oily leaks and discoloured paint around the valves externally.......

There are all sorts of things that happen at these sites, generally residential complexes don't want to pay for higher skilled work, the low $$$ income attracts less qualified people that may not be cogniscent of keeping the reciever free of moisture and dry, as it fills with water your reserve capacity of air can halve, making your duty cycle increase dramatically.......I've seen it personally. Take a little hammer and tap the bottom of the reciever, the sound should be similar with a nice ring as the sides if it's corrosion free......

hardtail
04-17-2012, 10:32 AM
How severe is the demand on a compressor when used for powering a boiler
control system in a multistory residential complex? I have some idea of what
the demands of sand blasting, painting, air tools and so on are, but no idea
of what loads a control system produce.

I have come across a domestic made industrial grade compressor that is
rated at a delivery of 19 cfm @ 175 psi and has a tank cert dated 2008.
This machine is offered by a building maintenance organization as surplus
to their needs.

The vendor says they took over a building one year ago and replaced this
single head compressor with a dual head machine as a Risk-Mgmnt measure
to ensure there would be back-up air for the boiler controls in the event of
a compressor failure.

I was told that the compressor was powered up 24/7 and free to cycle as
commanded by the tank pressure switch. I know that this machine is rated
for 175 psi but factory set to cycle at 150 psi. I was told that because the
control system operates on low pressure (20 psi), common practice is to
lower the pressure switch setting from 150 to 100 psi.

.

If the pump or package is also 2008, I would think it should have little wear, given it's commercial quality, cast iron pump, running at a fairly low rpm, I would want to see a 1740 motor on the drive..........I would think this latest risk management craze could work to your advantage.......that is about the right sized compressor almost most HSM would strive for unless your doing some special work often, the 2 stage will be better at suppling your air at a higher fixed pressure providing you stay below the cut in point of the pressure switch.

Duffy
04-17-2012, 11:08 AM
Eddie, I dont think that you can go wrong, if the price is right. I worked for Public Works Canada in Ottawa, and we had literally dozens of similar systems, serving buildings up to a million square feet. We did not change out very many for failure do to overwork. The equipment was well maintained for the simple reason that, if a thermostat failed somebody COMPLAINED, right now!
The commonest failure was on systems where some building operator had installed a "convenience outlet" in the mechanical room and used the airline for everything. This caused the compresswor(s) to short-cycle and carry over oil into the control systems. The results were spectacular! One building consumed BARRELS of R22 flushing out the control lines to EVERY thermostat. And the icing on the cake was this was all done on a weekend-overtime BIGTIME!

EddyCurr
04-17-2012, 04:35 PM
It is a nicely designed machine. 1725 RPM motor and 635 RPM pump.

While this one isn't powered, I recently looked at the same package mounted
on a vertical tank and conducted a comfortable conversation with the owner
at just 2 ft from the unit as it pumped up from 0 pressure.

That powered vertical unit has a 2005 tank cert and the asking $$ is MUCH
less. Unfortunately, for some reason the original 5 HP Baldor was replaced
with a 3 HP Baldor by the original owner, a national filling station chain.

The 3 HP powered up the pump easily and filled the receiver without getting
warm. I have spoken to a motor service techician about the longevity of
a 3 HP in a 5 HP application and he didn't seem unduly concerned, given
my intermittant duty cycle. Still, the correct 5 HP motor (Baldor L1410T)
is perhaps a $700 touch up here.

.

hardtail
04-17-2012, 10:12 PM
I'm not as much of a Baldor fan as many on here but I put a wanted ad on kijiji for a 5hp motor and along came a US motors 1750 rpm brand new for $150.....I'm not a huge US motors fan either but it is farm duty and does suffice for the compressor......Maybe they replaced a false 3450 5hp with a real 1740 3? Also see how the 3hp does wheh it kicks in at 100psi and is working against something vs pumping up from 0.........

Duffy
04-18-2012, 09:29 AM
The change from 5 HP to 3 HP was most likely because the "boilerplate" wiring diagram for their service stations did not have a circut with enough ampacity.
If you are doing a lot of sandblasting, then you probably need 5 HP, otherwise it will be fine.

Black_Moons
04-18-2012, 10:01 AM
Eddie, I dont think that you can go wrong, if the price is right. I worked for Public Works Canada in Ottawa, and we had literally dozens of similar systems, serving buildings up to a million square feet. We did not change out very many for failure do to overwork. The equipment was well maintained for the simple reason that, if a thermostat failed somebody COMPLAINED, right now!
The commonest failure was on systems where some building operator had installed a "convenience outlet" in the mechanical room and used the airline for everything. This caused the compresswor(s) to short-cycle and carry over oil into the control systems. The results were spectacular! One building consumed BARRELS of R22 flushing out the control lines to EVERY thermostat. And the icing on the cake was this was all done on a weekend-overtime BIGTIME!

What kind of air control systems can't withstand compressor oil?

R22 eh? I guess they just vented that out afterwards.. seems.. wasteful.

'Short-cycle'? As in the off period was too short for the water/oil to condense in the tank?

Blackadder
04-18-2012, 10:12 AM
depending on the controls used ie just dampers and valves it will not be to great

however for full control with air then the demand will increase


I used to work in a large computer ( data centre for Nat West ) and that was mainly controlled by air using Johnson Controls , they uses calibrated air leaks to measure air /water temps . along with hundreds of dampers and valve with positional feedback

we had four 500 ton chillers and 15 AHU from Baltimore co.

with this the air demands were huge two 15 hp reciprocating comps would not do , we had to install four large screw comps with two ten foot high by 3 foot diameter receivers to cope with the demand

this is before the solid sate controllers took over but we still had the dampers and valves to contend with

with the air control with fluidics we could keep the suite temp controlled to 0.5 deg C this did not get any better with the more modern controls again from JCS

Stuart

Duffy
04-18-2012, 06:18 PM
BM, the pneumatic systems used small diameter tube and, if a sag or dip occurred in a line, oil would accumulate and stop the thermostat from functioning. This gradually affected so many thermostats, the system was virtually non-functional.
I was told that the only EFFECTIVE treatment was flushing with R-22, venting from EVERY thermostat. This in a building of 900,000 sq ft. The really SAD thing about this exercise is that they did NOT replace the compressors with oil-free types. (After all the overtime, there was no money left!:D )
By the way, this was back in the 80s before R-22 got a bad name, (and it was still cheap!)
In case you were wondering, the building was National Defence Headquarters.