Subcooling region at a vertical BEU HEX

[pastyl]

Dear all

I have proposed the installation of a vertical BEU HEX were in tube side flows natural gas and in shell side steam is condensates.
Natural gas outlet temperature is measured with a temperature transmitter which gives the order to a steam control valve opening, adjusting thus steam flow to required duty. Very simple and accurate method.
As a result I have no consider any condensate pool to be created inside HEX and it will be so. In other words steam will condensate on the tubes and condensate will flow from condensate nozzle. As a result condensate temperature will be ever near saturated (am I right ?).
With this method you can handle from a minimum up to maximum load . In minimum load steam will condensate higher on the tubes and in maximum lower. In any case condensate will be saturated. There is no matter of over sizing in this case(am I right ?).
Our supplier consultancy firm advice for use of control valve at a condensate region and I am starting to thinking if they want condensate level control ??
At my opinion this is completely wrong since there is no any pump to protect downstream and condensate level control cannot be accurate and cannot be calculated very easy.
Apart from that my HEX cannot be handle minimum up to maximum flow with condensate only control. As a result I cannot guarantee my equipment and I think that it is logic that I cannot confirm my equipment performance with a method I have never propose.
Any comment will be very appreciated

 

[Sharik]

The condensate will not always be at saturation temperature, especially at low loads. At low loads the condensate level in the exchanger will be higher but the gas flowing through the tubeside will still be absorbing heat from the condensate, assuming the gas temperature is lower than the steam condensate saturation temperature.

How is the condensate recovered? It is unlikely there isn't a condensate pump. For an installation such as you describe, we would put a condensate pot at an elevation lower then the bottom of the exchanger. The condensate pot provides suction to the condensate pump which is on a pot level control. This would either start/stop the pump or control a minimum flow return valve depending on the size of the pump.

 

[pastyl]

No pump is installed dns HEX so no reason for subcooling. In fact dns HEX there is a flash tank
I am asking if with one control valve upstream HEX you will have or not level. I think that you will have not because the mass of steam entering HEX will be the same with the condenser leaving the HEX. (I am not speaking for condensate back-up case but for continuous level)
In cases were subcooling is necessary for providing NPSH there are 2 control valves one ups and one dns and by adjusting their openings you can have condensate pool.
Another thing I am asking is since I think you will not have level then you will not have subooling condensate.
In other words condensation will be done with physical way. At low loads will end higher in tubes and then liquid droples will be extracted from condensate nozzle. At normal and max. load condensation will stop lower in tubes and the same thing will happen.

 

[Sharik]

The mass of steam entering and the condensate leaving will be the same, but the volumes will be different. As long as the condensate nozzle and piping are adequately sized and the piping is adequately graded to the Flash Tank (condensate pot), there will be no condensate level in the HEX. I don't know if anything else feeds the Flash Tank but if the Flash Tank is open to atmosphere then with just the system as you are describing, there will be problems with the NPSH on the Flash Tank pumps.

If there are other systems feeding the Flash Tank or if the tank is vented to a low pressure steam system, the NPSH problems will be reduced.

 

[pastyl]

I am going to provide to the plant only the HEX skid for heating natural gas.
As I have informed no pumps will be installed between HEX condensate outlet and flash tank (maybe it should be).
Client consultancy firm wants the condensate flow to be done by the pressure condensate has at HEX outlet i.e. condensation pressure.
But at low flow, and because the HEX is sized to handle a larger load, condensation will be done at near 0.6barg and there is no pressure gradient to transfer the condensate to the tank .
Acc. my opinion cond. pumps shall be installed.
But I think is not my problem to find a way for condensate to be transmitted at higher pressure to flash tank.
Normally if you create a level to the HEX condensation will become at higher pressures but:
1) I cannot take the risk of create level to the HEX
and acc. my calculation at very low load, even if HEX
is full with water, natural gas will be transmitted at
higher than required temperature
2) I do not think that you can adjust the level to the
duty and I cannot guarantee accurate control of natural
gas outlet temperature if level is created in HEX

 

[Sharik]

This is up to you but it would seem to me, put a condensate pot, with a pump, below the HEX. The pump controls would maintain a condensate level in the condensate pot. The HEX would not have a maintained condensate level. Control the temperature of the natural gas buy varying the steam flow to the HEX.

 

[pastyl]

Due to the fact that HEX is beu,vertical type I cannot think of any pot to be added to it.I think pumps will be suitable for create pressure grandient
Another thing I am afraid is that as natural gas is transmitted to the sub cooling region and for the second tube pass ( as it pass from saturation area and is is hot)there is the possibility to heat the subcooling.
I am afraid of possibility of boiling the subcooling because the heat added to it at second pass can be higher than the heat is absorbed from first pass, were gas is cold.
For this reason I do not think that a BEU vertical HEX is suitable for create to it a controllable or not level.