9 replies to this topic

[zahraniazi]

Dear All

 

In our project, there is a flash drum for separating of the phases of hot condensate suspected of steam traps. feed of this flash drum has 127 degree Celsius and 2.5 bara, and flash drum works in 107.1 degree Celsius and 1.3 bara. outlet temperature of flash drum (107.1 degree Celsius) is too high for us, because this water will be used as feed of polishing unit and as you know the high temperature for polishing units is too risky.

 

for cooling of this water, our owner suggested to use an internal coil for flash drum which is cooled with product water of polishing water (temperature=52 degree Celsius).

 

but, is it possible for us to use this water as cooling water and drop the temperature of flash drum under 60 degree Celsius?

 

the other information, we may need, is:

flow rate of feed water of flash drum: 20m3/hr

diameter and height of flash drum: 1m X 5m

 

Best Regards

 

[fallah]

Hi,

 

Theoretically appears to be possible, but practically:

 

- To prevent negative effects of possible temperature cross, the coil arrangement inside the drum should be such that the complete counter flow with respect to condensate flow direction be provided

 

- Adequate cooling water flowrate (coil diameter) and heat transfer area (coil length) in such low diameter drum to be provided

 

- Deposit precipitation due to high temperatire water inside the coil should be taken into account

Edited by fallah, 14 April 2014 - 04:25 AM.

[zahraniazi]

Dear Fallah

 

Thank you so much for your response.

 

have you ever seen similar case for cooling of flash drum in other projects?

[fallah]

Dear Zahra,

 

For your special case no, but cooling by internal coil with cooling water inside is a common cooling method of the fluid inside the drums, reactors and storage tanks...

[zahraniazi]

dear Fallah

 

I mean, i usually heard for cooling the product of flash drum, it is entered to a plate heat ex changer for cooling. is it right?

[breizh]

Zahraniazi,

 

Consider to drop the temperature of this condensate  by heating a process stream and use it as make up water for your cooling water system ! PHE  can be used for this purpose . Heat integration and water recycling , special care is needed  to ensure the quality of the water , in line monitoring .

 

Hope this helps

 

Breizh

[fallah]

 

I mean, i usually heard for cooling the product of flash drum, it is entered to a plate heat ex changer for cooling. is it right?

 

Dear Zahra,

 

The PHE can be used for such cooling but the performance of these exchangers is highly affected by fouling; then the fluid is to be cooled by PHE should be clean enough...

[xavio]

Hello zahraniazi,

 

I think you have sufficient upstream pressure to install an external heat heat exchanger (PHE or STHE) at the inlet of the drum.

I can't do calculation right know because of insufficient data, but I don't think the required coil surface area will be small.

Also as fallah said, small delta T or even cross might happen if your cooling water flow is inadequate, requiring more area.

I am afraid that the coil would be too big to be inserted into the drum.

 

Is the drum an existing equipment?

Modifying existing equipment would be troublesome, and you might need to check the foundation due to change in equipment weight.

 

By installing a external exchanger, you can have more generous delta T, thus smaller equipment.

For example, you can lower down the feed temp to 80degC, and check if you can get <60degC after flashing.

 

Good luck

 

xavio

[zahraniazi]

Hello zahraniazi,

 

I think you have sufficient upstream pressure to install an external heat heat exchanger (PHE or STHE) at the inlet of the drum.

I can't do calculation right know because of insufficient data, but I don't think the required coil surface area will be small.

Also as fallah said, small delta T or even cross might happen if your cooling water flow is inadequate, requiring more area.

I am afraid that the coil would be too big to be inserted into the drum.

 

Is the drum an existing equipment?

Modifying existing equipment would be troublesome, and you might need to check the foundation due to change in equipment weight.

 

By installing a external exchanger, you can have more generous delta T, thus smaller equipment.

For example, you can lower down the feed temp to 80degC, and check if you can get <60degC after flashing.

 

Good luck

 

xavio

dear Xavio

 

based on our calculation, the surface of coil will be about 50 m2 and as you said for the flash drum such dimensions is too high for installing internal of flash drum.

But can we install a heat exchanger  at the inlet of flash drum? or we must install it at the liquid outlet of flash drum?

also, i studied another forum about understand of throttle/flash drum and now, i'm confused about is it necessary to install a throttle valve at the inlet of flash drum? or fluid with high temperature and pressure enters to flash drum without any throttling valve and it flashes? 

 

Thank you so much for answering my questions...

[xavio]

hello zahraniazi,

 

Sorry, I missed the big picture here.

You're flashing water-rich liquid; VLE dictates that equilibrium temperature of is slightly above 100C when pressure is 1.3 bara.

If we put a cooler at inlet, you might end up with no flash vapor.

Coil inside a flash drum (or any external HE) is installed for heating purpose, not cooling.

You mention nothing about the flash vapor stream, where do you want to route it?

I am not sure how your process is designed, can you provide a sketch?

 

If you don't need any vapor to be flashed, just add the cooler on the line and remove the drum.

If you need the flash vapor, you can add the cooler at liquid outlet of the drum, and subcool to your desired temp.

 

Throttle valve acts as pressure reducer as well as boundary between high pressure side and low pressure side.

You mentioned that you have steam trap upstream; the trap will do the function of throttle valve in your case.

 

Your drum should have pressure control, usually it is done via control valve at vapor outlet.

Therefore, your drum pressure is fixed at the desired pressure of 1.3bara, which is lower than the 2.5 bara upstream of steam trap.

Immediately after passing the trap, the condensate pressure starts to drop progressively, until it reaches 1.3bara when entering the drum.

Pressure at trap outlet equals to drum pressure + line pressure drop.

It is possible that the water begins to flash at trap outlet, that's why condensate line should be somewhat oversized.

 

Good luck!

 

xavio

Edited by xavio, 24 April 2014 - 02:06 AM.