Will Vacuum Lift Steam Condensate?

[BronYrAur]

I don't have much experience with vacuum return systems. I have always thought they were designed to remove air in the condensate line which will allow the condensate to return easier. I don't think they are intended to lift condensate, correct?

I have a 35 psig steam coil with a modualting control valve and an F&T trap. Condensate is under vacuum, but I don't know "Hg. There is a 4' lift out of the coil, and it occurred to me that this may be a problem when the steam valve is nearly closed. I have never witnessed it's operation, so maybe there is a low limit on the valve or something to insure enough pressure to lift the condensate.

My concern is that I want to lower the steam pressure to 15 psig. Condensate will still be under vacuum, but I will still have the 4' lift and less available steam pressure. Under low load, I forsee a hammering problem, unless the vacuum is emptying the line.

I could reroute the condensate down to another condensate pump and avoid the 4' lift, but it will be difficult. any thoughts on whether or not I will have a problem?

 

[quark]

I don't see any problem with lift of 4' when you have an upstream pressure as high as 34.6' gauge (15psig). What you should really worry about is the flashing of steam. Even if the condensate line is at atmospheric pressure rather than vacuum, you will have 3.9% flashing. the 4' lift will contribute additional 0.06%. You will have problems if the flash steam removal rate is not fast enough.

When your steam valve is fully closed, there will be no condensate formation either. Or I lost some point here.

 

[katmar]

There are two missing bits of information. These are the temperature of the fluid being heated and the pressure drop through the trap. If, for example, you are heating a fluid at 105 deg C then the steam in the coil must be at least this temperature and therefore at a pressure of at least 2.8 psi. The 4' of head is equal to 1.7 psi so you have 0.9 psi available for the trap (without relying on any "pull" from the vacuum). This is the limiting condition. Obviously the steam inside the coil needs to be hotter than 105 deg C so in practice you would have a bit more pressure drop available for the trap.

On the other hand, if you are heating a fluid at only 70 deg C the pressure in the steam coil may be less than atmospheric and then you would be relying on the vacuum in your condensate to "pull" the condensate out of the coil and through the trap.

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[quark]

Float type traps are not suitable for subcooling. So, I considered saturated conditions with respect to the given pressure.

 

[BronYrAur]

Quark,

I don't understand your last point. Should I remove the F&T traps and replace them with a bucket or something else? Regarding your first post, I am concerned with a low load condition when the valve is only slightly open.

 

[quark]

Bron,

Only thermostatic traps can hold condensate before it is subcooled to a few degrees (generally 10⁰C) before discharging. Otherwise, as soon as steam is condensed it is discharged. That means the condensate is at steam temperature. Further, the driving force for the condensate is steam pressure. So in normal case 4' lift is not a problem and, further, you have vacuum in the condensate line.

But, suppose, your heat exchanger is generously designed with a huge heat transfer area than required, at low loads, there are chances that the steam becomes condensate instantaneously and as the valve is partially closed, there may be chances of vacuum formation in the heat exchanger. This process is called as stalling of heat exchanger and this stops your condensate from flowing out.

You have to draw your process on a stall chart to check at what condition your HX stalls. This generally happens with back pressure acting on the trap. As you have vacuum in your case, there may not be any problem.

You should not have problems if you maintain same steam flowrates at 15psig (equal to that at 35psig)

The thermostatic trap in the F&T trap is only to vent air and non condensables and it should not be mistaken for a thermostatic steam trap.

What prompted you to go for lower pressures? Did you check the HX design parameters?

 

[BronYrAur]

Quark,

This is a duct reheat coil. The facility wants to lower their steam pressure to 15 psig for the summer months when the steam load is limited to domestic water and reheat. During the winter, this particular coil is at 35 psig, so I need to be able to easily switch between the two.

 

[quark]

If the lift is not a problem (generally there shouldn't be in your case), the problem may come from the other side.

The specific volume of steam at 50psia is 8.51 ft³/lb and at 30 psia is about 13.74 ft³/lb. So there is an increase of about 62%. I am not sure whether your pipeline and control valve can take care of this. If the piping and valves are designed for 35psig, then you may starve the coil and subsequently stall it.

It is not good to run the boiler at lower pressures than design. If you are reducing the pressure across a pressure reducing valve, then you may be loosing energy due to super heating of steam (about 17F).

Good luck,