We are supplying this cheapish compressor to one of our customers. They are a small bakery and need a back up air system to run our liquid handling machinary when the compressor they share with the factory next door is swiched off at night!

We will plumb it in with a Non return valve and set the pressure slightly lower than the main compressor.

My problem is this, the compressor in the link comes with a manual drain tap / plug (underneath) which must be opened every 24hrs to drain condensate from the tank.

I dont think our customers will do this. They may start off with the best intentions, but my feelings are that after a month they will become a bit more carefree if you get my meaning.

Is there anyway I can fit some sort of autodrain to the compressor to eliminate the need for a manual daily draindown?

Apparantly, according to Machine mart, the compressor should be switched off when it is drained.

The compressor will be lightly used. Probably operating 20 litres of air every 15 mins or so at 5 bar.

The "shut off while draining" is nonsense.  I don't know where people come up with this kind of stuff, but it doesn't add a lot of value.  In Oil & Gas, we drain compressor vessels all the time with the driver running, and that gas can burn.  There is nothing about the fluid mechanics of the tank that would indicate that it can't be drained while running.

The simplest thing is to put a solenoid on the drain valve with a simple clock timer.  Every day at 2:53 am it can open for 5 minutes.  I'm not sure what the volume of the tank is, but you would want to size the solenoid to drain the volume likely to accumulate in a day within 5 minutes (or set the open time longer).

The next step in complexity (and cost) is to put a level switch in the tank that opens the drain valve when enough liquid has accumulated to lift the float.  These switches generally have a high contact and a low contact so that the drain will remain energised until the float reaches the low point.  Probably the most expensive part of this is putting a nozzle in the tank for the float.

First choice: For a cheap tank there is a number of tried and dependable suppliers of steam traps that would suit your purpose (Gestra, Spirax, Armstrong to mention a few) and drain the tank automatically when needed without external force (float operated)

I do not believe you would place your tank or drain at freezing conditions: condensate would tend to freeze inside the tank, and you would create indoor pipeline condensation problems caused by great temperature differences.

The second solution is either a pneumatic operated with spring return actuator, normally closed, small ball valve, y-type seatvalve or membrane valve. Operation via a solenoide valve and timer. (Manual operation in addition should be present for start up and control)

Do NOT use a solenoid valve directly, as small particles and dirt inside the tank, plus water can clog and corrode the solenoide valve.

Exception: some few solenoide valve constructions are built for the purpose, and can be safely installed.

The Armstrong Int'l Co. has a number of liquid drainers along with their steam trap models that drain condensate from steam piping and equipment.  Their Series 1-LDC is capable of your design pressure and probably has adequate capacity.  The plastic body might not be preferred, but there are other models of cast iron Series 1-LD, steel , and stainless steel Series 11-LD.  You could probably find some local steam trap suppliers with a similar float actuated mechanism to serve as automatic liquid drainer.

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