Compressor question

Hi all,

I recently got an old belt drive air compressor.

It woks well, reasonably quiet, but when it hits maximum pressure the motor does not shut off.

It sounds like the compressor stops pumping and the motor isn’t working as hard, but motor keeps running.

There’s no pressure switch to shut down, just a valve to adjust output pressure.

Is it just a matter of fitting a pressure switch in the line somewhere?

My knowledge of air compressors is very limited and I previously thought the motors always switched off when maximum pressure was reached.

Thanks

Sounds like it has some sort of unloader valve.

It looks like a capacitor start, capacitor run motor. Some of these type of motors have limited numbers of starts per hour or require some minimum time between starts so not shutting off the motor when at pressure may be due to this to ensure it does not overheat. The other possibility is that the motor will struggle to start if it is trying to work against existing pressure in the tank on restart. I have a compressor that bleeds off air until the motor is up to speed to allow it to start in a reasonably short time.

Martin C

Is it just a matter of fitting a pressure switch in the line somewhere?

Sadly no, it's not designed for it. You would need to insert a modern unloader in the large copper pipe and the pressure switch will need a new oulet from the tank + the old system may interfear.

Edited By Dave Halford on 03/09/2023 23:57:41

Whether it's worth 'doing' depends to a large extent on what you are going to be using it for and whether, once done, it will be the machine 'that you want it to be'...

What capacity is it, and what tasks do you hope it to perform with it?

..it looks like quite a small receiver for that type of pump, but it's hard to tell from a picture...

Edited By DiogenesII on 04/09/2023 06:54:11

Edited By DiogenesII on 04/09/2023 07:05:14

Hi Grotto, I agree with Bernard Towers about an unloader valve, these were often used in industry where they could/would be running 24 hours a day all week, where a plant would require a high demand of compressed air, the idea means that a fairly stable air pressure could be maintained, with the continuous time of use.

Regards Nick.

this looks like a commercial machine that hardley ever switches off....old style......

not sure about the electric side of things but the big fitting on the bottom of the copper pipe looks like a non return valve.......

a standard on/off pressure switch, the hiss u hear when it shuts the motor down is unloading the pressure from the comp down to the non return valve.....

that way the motor will start without pressure in the system......

personally I'd try a new type switch.....less tha £20.....

just have to get a couple of new fittings so that u can use the switch and the saftey valve from one tank fitting / port....

the small pipe are the unloader valves.....won't be needed with the new switch.......

nice old machine either way.....

Dunno what it cost, but presume the outlay, thus far, is minimal. That said, if the pump works to design (pressure and output) it will be worth modifying. That pump may well be rather superior to the current crop of cheap pumps on the market wrt to longevity. Likely claimed as less efficient than a modern pump, therefore old-fashioned, by those selling the modern ‘buzz-boxes’. My compressor is, by appearance, much older than that unit - and I have no intention of changing my old cast iron pump!

I see lots of dead compressors, or cheap unpopular machines. So I would obtain one of those, with the applicable parts, to convert the machine to the more usual operational specifications.

As stated earlier, if the compressor will only be needed to restart just a few times per hour, the motor would likely be OK. If one has space another possibility is to increase the reservoir capacity (another receiver in the system and just switch it off when fully charged.

Do carry out a hydraulic pressure test on the receiver. They are unlikely to fail, but they only fail once.

Just a guess but I notice the heads have an air intake at the front, and connect to a shared outlet manifold at the back, which then connects via a big pipe to what could be a non-return valve on the receiver. In addition both heads have thin pipes leading to the top: I suggest these move a valve that directs air back to atmosphere rather than compressing it into the tank. The higher the pressure in the tank, the more air is escapes back out the inlet, greatly reducing the load on the motor.

Not an efficient set-up, but it allows the motor to run continuously which is good for reliability because frequent restarts are bad for single-phase motors. I think the compressor is either designed for continuous operation or it predates electrical motor control. Another system uses a clutch to disconnect the pump from a continuously running motor.

I wouldn't attempt to modernise it: if it works leave it be.

My main concern is the receiver. Air tanks tend to collect water, which causes them to rust inside on the bottom, maybe cutting a groove into the metal along a water line. A tank that's perfect on the outside can be badly damaged inside. Poor maintenance or long-term storage with water inside are widow-makers.

Dave

Hi Dave (SOD), the unloader valves were not exclusive to single phase motors. In my old day job, all the plants had their own compressors, which were driven by three phase motors, and the idea of the unloader valves were to keep a reasonable constant air pressure in the systems while varying air volumes were needed during each batch of operations. Even three phase motors had a limited number of recommended stop/starts each day. All these individual compressors were eventually replaced with one very large multi-vane compressor with an air dryer and a very large air receiver and still had the unloader system so the three phase motor could run continuously.

An analogy to the unloader system, could be a voltage regulator in an electronics circuit.

Regards Nick.

Edited By Nicholas Farr on 04/09/2023 09:26:55

Small continuous run compressors fitted with unloader valves frequently use relatively small motors that don't have sufficient power to start against a pressurised tank. Motor is fine running up to pressure but doesn't have the oomph to start it under load.

To convert it to  motor start - stop operation you will need a modern pressure controlled starter switch with an outlet vent in the off position.

Easy way to set up is to add an auxiliary tank / manifold to the outlet side connected to the tank via a non return valve and vented via the pressure switch. When the tank reaches the set pressure the motor turns off and the manifold vents down to atmospheric pressure so there is no back pressure when the motor restarts. Auxiliary tank / manifold doesn't have to be very large. From memory the outlet collection manifold on my Atlas Copco KE1 16 cfm rated Vee twin was around 3" diameter and a bit over 1 ft long. Plenty big enough. Your compressor looks smaller so maybe your manifold is large enough as is.

Swanky way is to add an electrically controlled valve between the tank and the unloader valves in place of the existing pressure controlled valve. Use a time delay relay to open the valve for a few seconds when the pressure controlled motor switch kicks in.

I have a PDF of the KE1 manual with the basic details on how continuous operation and starter pressure switch unloading is set up on commercial machines. PM me if you'd like a copy.

I simply vented the manifold and the KE1 worked fine. Far as I know it's still going strong 3 owners and 20 years down the line. I'd have heard if it broke!

Clive

Edited By Clive Foster on 04/09/2023 10:09:45

What Clive says, but you really only need to "Tee" the normally open depressurising (unloading) valve into the large pipe between compressor and tank. The pressure switch can be connected to the tank port with the small pipe that currently feeds the pneumatic unloaders but you will have to remove the existing pressure regulator in that line.

Robert.

By the look of it it could be 50+ years old. Before you modify it I would do a hydraulic test ! IF it proves sound then consider modification. The motor may not be large enough in a stop/start set up and it would be a good idea to use a Dol starter to protect the motor (correctly rated ). Noel..

I initially got too clever with the KE1 and tried some over-engineered things to replicate the electrically controlled unloading valve system that didn't really work reliably. Probably 'cos I was too tight to buy proper parts and attempted to use "obtained" or "found" things that "ought have worked".

Ended up reverting all the unloading valve and pressure control switch connections to standard and fitting a £5 from Machine Mart non return valve directly to the tank inlet.

As bought the system was set-up for continuous run so had no non-return valve in the tank inlet.

Started out by simply fitting a small bleed hole to the manifold which worked fine taking about 3 minutes to bleed the manifold down. But the hiss was annoying. Then a a compressor savvy friend told me that the £15 from Machine Mart pressure controlled starter had a suitable bleed port built in so 2 ft of tube and couple of compression fittings later the hiss was history.

Clive

Edited By Clive Foster on 04/09/2023 12:10:04

Thanks everyone,

I think I’ll just leave it as is short term

I'll see how much use it gets before I make any changes, but lots of good inf.

As Noel said a hydralic test would be a very good idea. I have stopped using my compresser similar to this and resort to a lidl offering.

Old one came from a scapyard 50+ years go, lived though first 10 years and then got frit lol and had it tested but that was over 40 years ago. I don't use it anyone being wise "now" lol. Need to sell the new pump and motor, only forty years old. Don't need loads of compressed air these days.

Stay safe !

Pete

Edited By Pete White on 07/09/2023 09:41:17

If it were my compressor before I messed about with pressure switches etc I would get an air filter put on the left hand cylinder, as it is at the moment it is sucking in all the c**p in the workshop air. The right hand air filter doesn’t look too special either.

As I, too, said on the 4th “Do carry out a hydraulic pressure test on the receiver. They are unlikely to fail, but they only fail once.”.

Simple enough - fill the receiver with water and carefully increase the pressure from a pressure washer or (as I do it) with a grease gun. The grease gun method needs more grease than maybe expected as the receiver will expand as the pressure inceases.

If it's any help in sorting out what you need to do I used to have an almost identical compressor and it was badged as Sherry's of London. Mine had a Danfoss pressure switch that had a small pipe to the compressor to unload the compressor for starting and if memory serves a non return valve between the compressor and tank to allow the compressor to unload. Long gone as I needed something with greater capacity, but it was nice and quiet and would run a small spraygun. Search for Sherry's compressor may turn up some information, there again you may just get a cocktail recipe!

Just tried a google search for Sherry's air compressor and there are a few results, there's one for sale that comes up for a vintage and salvage company identical to mine and similar to yours,the one in red faded as mine was to a fetching pink, that shows the Danfoss switch. Yours may just have had the pressure switch and associated plumbing removed .

Edited By Dave Wootton on 07/09/2023 12:18:55

An unloader valve has a non return valve between it and the receiver and is only there to make the restarting of the motor easier by venting the cylinders as the motor stops when it reaches the preset pressure. The main concern would be the condition of the receiver as it is very old. Most failures are benign when a leak develops, as I was told by the man who recertifies the museum's compressors, but I wouldn't bet my life on it not exploding at any time.