Icing up of valve on blow down
Icing up of valve on blow down
(OP)
Hi,
I have a 50 cubic foot air reciever which is kept at 10 barg.
Occaisionally I need to blow down the vessel quickly and have noticed that ice forms on the valve and sometimes in the pipework after the valve, when I do this. There are restrictions in the pipework after my valve which resist the flow as well as locally increase the flow velocity, and these suffer from icing.
I realise that ice formation does depend on the moisture content in the vessel, but is this the only factor, if the air was "dry" would it still happen ? also as the output of the compressor does not go through a drier is there another way to remove moisture at the output of the compressor before it gets to the vessel to reduce the incidence of ice ?
Any pointers to redude the ice would be appreciated.
I have a 50 cubic foot air reciever which is kept at 10 barg.
Occaisionally I need to blow down the vessel quickly and have noticed that ice forms on the valve and sometimes in the pipework after the valve, when I do this. There are restrictions in the pipework after my valve which resist the flow as well as locally increase the flow velocity, and these suffer from icing.
I realise that ice formation does depend on the moisture content in the vessel, but is this the only factor, if the air was "dry" would it still happen ? also as the output of the compressor does not go through a drier is there another way to remove moisture at the output of the compressor before it gets to the vessel to reduce the incidence of ice ?
Any pointers to redude the ice would be appreciated.





RE: Icing up of valve on blow down
http://
Good luck,
Latexman
RE: Icing up of valve on blow down
#1 - yes: no water = no ice. More succinctly, if the dewpoint of the air is less than the temperature reached in blowdown (adiabatic expansion or JT expansion), then no ice.
#2 - removing the water from the output air constitutes drying the air, so "is there another way to remove moisture" would have to be answered "no". There are a variety of ways to dry the air, from refrigerant consensate removal systems (aka cold traps), to dessicant wheels, to simple condensate traps at points of use. The latter are the most useless. You could theoretically heat the tank (being careful not to exceed safe operating pressure/temperature limits of course), raising the total temperature of the air, so that the subsequent expansion cooling during blowdown would be above the dewpoint. In practice, doing so would vaporize condensate that is likely puddled in your receiver, and raise the dewpoint of the air as well, negating any benefit.
RE: Icing up of valve on blow down
RE: Icing up of valve on blow down
RE: Icing up of valve on blow down
Specifically:
"There are restrictions in the pipework after my valve which resist the flow as well as locally increase the flow velocity, and these suffer from icing. "
If ice on the o.d. surface of pipe or valves is a concern for some reason, the usual corrective action is insulation on the pipe o.d.
RE: Icing up of valve on blow down
I agree that trying to dry the air as its drawn in is a ridiculously expensive and/or impractical solution
My suggestion would be to either heat the piping with an external heat source, depending on the pipe material you could use IR, an electrical blanket, induction or direct flame; OR you could inject hot air into the blowoff stream with a venturi system
RE: Icing up of valve on blow down
Check page 5 in the link below for a possible arrangement.
http:/
RE: Icing up of valve on blow down
Reduce moisture inside and outside tank; raise air temperature of the room to reduce RH; insulate valve and dicharge piping to eliminate icing on the outside surfaces of valve and discharge piping.
RE: Icing up of valve on blow down
If ice is forming inside the pipe and restricting flow (pretty rare in air, less rare in gases that form hydrates) then you might look at getting rid of downstream restrictions that allow the ice to accumulate and moving your isolation valve to closer to the end of the pipe.
David
RE: Icing up of valve on blow down
I have asked about drilling out the holes, but am told that this will reduce the back pressure the silencer puts on the system which in turn will increase the mass flow through the unit, which in turn will compromise the warranty on the silencer.
When we have taken it apart (while still iced up) there is ice both on the inside and outside of the pipe leading to the silencer.
RE: Icing up of valve on blow down
RE: Icing up of valve on blow down
RE: Icing up of valve on blow down
When the lowest operating temperature is below water freezing point (water tripple point), how can we predict the pure water vapor pressure? Is Antoine equation still valid?
RE: Icing up of valve on blow down
Is the source of the blowdown your "reciver"?
If so, during blowdown, how is the 10barg maintained and is icing noticed on it (the reciever)?
RE: Icing up of valve on blow down
The vessel is blown down from 10 BarG to atmospheric pressure, the icing is only noticed on the outlet pipe from the valve between the silencer and the valve.
This bit of pipe is about 2 feet long, interestingly the icing is first noticeable about 8 inches after the valve outlet, there is some ice build up on the outside of the pipe, due I think to condensation which freezes.
There seems to be lots of filters on the market with auto-drains on the bottom, so I guess they cause some of the moisture in the compressor discharge to drop out, even though they don't appear to dry the air I am going to try one of these, they seem quite cheap.
RE: Icing up of valve on blow down
You should be draing the water off the bottom of the tank that holds the air before blowing down.
RE: Icing up of valve on blow down
Conditions in the receiver would be:
T/Ti= (P/Pi)^[(gamma-1)/gamma]
where T is absolute temperature
P is absolute pressure
gamma is the ratio of the specific heats, Cp/Cv
subscripts i represents initial conditions in the receiver.
Clearly the final air temperature would be quite low compared to initial conditions.
Further considering that flow velocity would lead to lower temperatures in the piping, it is reasonable to assume icing is possible within and outside the piping and fittings.
RE: Icing up of valve on blow down
Alternately, heat the pipe/valve.
RE: Icing up of valve on blow down
Tank - adjustable valve- pressure gage - cyclone separator- blowdown valve- silencer
When the blowdown opens, the air takes an expansion to a dewpoint state, say 5C through the partially open valve, goes through a cyclone, and takes the final expansion dry at the blowdown valve.
Push the expansion math around:
T/Ti = (P/Pi)^(gamma-1/gamma)
P=Pi (T/Ti)(gamma/gamma-1)
Set T=278K
Gamma air = 1.4.
solve
P=8.6 barg
Check the math, it's been a while.
For the first trial, set your partially closed valve so that you have 8.6 bar at the gage after the valve when the blowdown opens.
RE: Icing up of valve on blow down
http://ww