Vent Ratio for Deaerator 1

[newengr]

I was trying to determine the vent ratio for a deaerator vent ratio=(vent from deaerator/ (total steam to deaerator-vent from deaerator))....Since there isn't a indicator on the amt. of steam entering the deaerator, I am doing an energy Balance....which is
steam required= ((Qm * (T1-T2))/Hfg) T1= Steam temp.
T2= Water Temp. Qm=entering water Hfg=enthalpy evaporation

Is the method above correct? How do I determine the steam vent from the deaerator? Flash calculation? If so, how do I do a flash calculation..knowing only Temp, Press of steam and Inlet Water flow conditions and temperature.

 

[TD2K]

There isn't a ratio for the vent steam. The vent steam just carries away any stripped gases from the boiler feed water, you just need a small flow rate of steam to accomplish this.

The amount of steam required is just as you've indicated, the amount to bring the feed water up to saturation temperature at the deaerator operating pressure.

 

[newengr]

How do I determine the amount of vent steam?

 

[TD2K]

Drill a 1/4" hole or something like that in the valve in the vent stack off the dearetor

 

[dimitris]

I think that there is a small mistake in your energy balance calculation. T1 should be the temperature of the water leaving the deareator and T2 the water temperature entering the deareator. This amount of steam is the necessary steam for heating the water to its exit temperature (the saturation temperature for the pressure the deareator operates at). You then need to add the amount of vent steam. To calculate this you need to do a thorough stripping column calculation. In operating practice you adjust this amount to the minimum necessary, to keep the residual oxygen content in your BFW to a minimum.

 

[jproj]

Come on guys... don't we remember material & energy balances??? Newengr, your equation is flawed for the reasons dimitris stated, plus, it will not work (as written) if you have superheted steam. I suggest you use the following method:

Material Balance:
Mass(in) = Mass(out)
(M + S)in = (PS + V)out

Energy Balance:
Energy(in) = Energy(out)
(M*h + S*h)in = (PS*h + V*h)out

M = Makeup(in)
S = Steam(in)
PS = Pump Scution(out)
V = Vent(out)
h = Enthalpy

Either the makeup or the outlet flow and operating pressure (outlet temperature) must be specified. Steam, vent and either makeup or outlet flow are unknown. This gives three unknowns with two equations. In practice, the vent rate is specified to be a certain percentage of the steam required (usually 0.5-1%... sometimes more depending on the system), which yeilds three equations and three unknowns (solve away!).

jproj

 

[dimitris]

Dear jproj
You sound a bit tough saying that we forgot our basic chem eng knowledge. Anyway to be absolutely precise you have to add the sensitive heat effect of the superheated steam in the latent one, but usually this is negligble since in most cases the L.P. utility steam is superheated by only a few degrees.
I have the impression though that the amount of steam that you mention for venting is to small. By the way, do you operate a deareator in your plant? If so at what temperature does the BFW enter the dearetor??

Regards,
Dimitris...

 

[jproj]

Dimitris:

You are correct that for the most part DA's see sat. steam only, but there are cases where sat. steam is not available and superheated steam is used.

Also, (regarding newengr's equation), If you analyze the units, you find that Hfg must have units of temperature. I've never seen latent heat expressed as temperature (ºF).

Latent heat is, however, easily found in any steam table (with a bit of simple mathmatics) in the form of Energy/Mass (BTU/lbm). If you do some substitution & rearanging of the material & energy balance you can easily come up with an equation for the steam flow based on inlet flow rate (along with steam, inlet and outlet enthalpy)

The vent rate I mentioned above is an industry wide standard for pressurized deaerators. For certain applications, the vent may need to be larger (it really just depends on how much dissolved gas is coming into the unit).

Inlet water temperatures vary according to the process. If the DA is on the LP Drum, inlet temperatures can be pretty hot (250-350ºF) due to pre-heat. On the other hand, if the DA is on a condenser (and operating at the condenser pressure), the inlet water may be fairly cold (40-100ºF). If it's just a stand-alone unit, the inlet temperature varies quite a bit. Normal DA design uses 60-80ºF inlet water.

Regards,

jproj

 

[kvb]

The required vent steam amount can be found from the charts published in the book " Rules of Thumb for Chemical Engineers" - Carl Brannan.

Another practical way is to measure the dissolved oxygen content in deaerator exit water. Then throttle the deaerator steam vent valve and measure the dissolved oxygen level in exit water. Based on the oxygen level in water, the vent valve can be set at a position.

- KVB

 

Hmmmmmmm not sure about Jproj's material and energy balances there. Aren't they just the same equation!! i.e. you can't solve x = y and 2x = 2y simultaneously. Therefore your 3 equations 3 unknowns comment isn't entirely accurate.........

 

[jproj]

Guest9999999:

Actually, it is entirely accurate and exactly how you run a material and energy balance around a deaerator with the vent rate as a specified percentage of the steam usage.

Equation one is the material balance. Equation two is the energy balance. Equation three is as follows:

V = 0.005*S (with a 0.5% vent)

Giving you three equations for three unknowns (unknowns are S,V, & either M or PS).

jproj