Instrument Air Dryers and the pressure dew point setpoint

[mpeck1982]

Can someone explain to me setpoints on a desiccant instrument air dryer. Why are setpoints shown with these units -40 F/C? I understand when you compress air above atmosperic pressure the dew point temp will change. What does -40 F/C actually mean for the dew point temp for the compressed air in the piping? Any info would help.

 

[Scipio]

Basically it's the temperature at which moisture starts to condense out of the gas in question. In this specific case, it means the dessicant in the air dryer is removing enough water vapour from the instrument air at the operating pressure that free water won't condense out (dew starts to form) until the ambient temperature drops below -40°C. Common for systems where the instrument air can reach low ambient temperatures so water won't condense out and potentially freeze in end devices.

Mike Henderson

 

[TheTick]

-40°C = -40°F. That is where the values for Fahrenheit and Celsius are coincidentally the same.

 

[mpeck1982]

The plant I work at uses 120-psig of instrument air for the site. The air temperature leaving the last stage of compression is from 90 to 75 deg F. Most of the instrument air loads consist of control valves for air handlers which the pressure is reduced to 30-psig. Other instrument air loads would be for expansion/compression tanks and 60-100-psig control valves for chillers, boilers, DA Tanks, and etc. Should there be a concern that the air dryer doesn't reach its dew point setpoint of -40 deg F? The air dryer can usually get down to -10 to -15 deg F, typically.

 

[Compositepro]

-40F is the dew point of the air when it is reduced to ambient pressure. This same air has a dew point of -3F at 100 psig. Dew point is only important in that it should be low enough to prevent condensation in the piping or when it is expanded in use. I've often seen water in compressed air systems, including instrument air that is supposed to be at -40F dew point. Air dryers are very sensitive to flow rate. They cannot dry the air during flow surges, which are very common in most compressed air systems. The dryer is usually located after the storage tank because the tank is very effective for cooling the air after the compressor and knocking out most of the water. But, as a consequence, it is exposed to the full flow rate of any surges.

When maintenance uses the air for air tools and other uses, that can create problems.

 

[Scipio]

Compositepro - yes and no. Just 'Dew Point' normally does refer to dew point at atmospheric pressure (also 'atmospheric dew point' is used sometimes) but there's also a 'pressure dew point', which is the dew point at some specific higher pressure. A lot of guys (like me) just get lazy when they're talking about plant instrument air systems and just shorten it to dew point, but it's implied that it's dew point at rated pressure for the I/A system. Gets taken in context with the application, I suppose. I'm used to Northern Alberta where the I/A systems are typically rated somewhere below -40C pressure dew point @ I/A package rated pressure to prevent water forming & freezing in the uninsulated outdoor headers around the plant.

Agreed about dryers being sensitive to surge, so in my experience the instrument air system, which needs to be dry, has a larger dry air receiver downstream of the dryers to stabilize the flow through the dryers (normally the dessicant type). If there is to be a utility air system for tools or things like that which don't require low dew point air they'll either use a separate compressor with the air supply coming directly off a wet air receiver, or they'll provide a separate takeoff upstream of the dryers on the main compressor package, or depending on the client they'll just heavily oversize the dry air receiver and run the whole plant off that, including utility air.

Regards,
Mike Henderson

 

[quark]

At normal ambient conditions, -40F/C is ADP for Instrumentation Air. If your inlet pressure to instruments is 100 psig and outlet is generally atmospheric (exhaust), the temperature at adiabatic expansion (worst case) is about -25F, starting with 70F. However, if you start at 150 psig then -40F is the worst case temperature. So, you need to maintain an ADP of -25 or -40F depending upon your inlet pressure. -40F is quite easy with desiccant dryers and you are probably using refrigerated type. You can easily achieve -40F/C with activated alumina and these dryers have good capacity control. Molecular sieve based dryers give you much lower ADP/PDP.

If you are lubricating instrument air then you have serious problems with moisture. As already mentioned by Mike, lower ambient temperatures requires lower ADP/PDPs.