3 replies to this topic

[NOEC]

Hi Everyone,

This is my first time posting. I am trying to calculate the current heat duty for a heat exchanger TEMA Type AJU. It uses 50 psig steam as the heat medium (tube side) which is being used to heat ethane/alkylate (shell side). These exchangers are known for having condensate build up. I believe that is what is occuring right now and causing the outlet condensate to subcool.

The steam tube side temperature readings are 310F (inlet) and 250F (outlet) with the outlet pressure being 35 psig. The shell side temperature readings are 169F (inlet) and 218 (outlet). The steam flow is 57,400 lb/hr, but I don't know the shell side flow or composition. So how would I go about determining the current heat duty for these exchangers? I'd like to use this to compare it to the heat exchanger design duty in order to calculate project economics. The spec sheet is attached. Any help would be greatly appreciated.

Thanks,

Eric

Attached Files

•  2011071112584809.pdf   66.17KB   68 downloads

[Art Montemayor]

Eric:

I am curious as to why you have opted to place the low pressure steam in the tube side – especially in a U-tube bundle. An AJU TEMA type exchanger is ideally suited as a steam condenser or – as in your case – as a steam heated exchanger with the process fluid inside the tube bundle.

I don’t understand why you make the statement that “These exchangers are known for having condensate build up”. Why that would be the case is something I frankly don’'t understand. Has that been your experience, or have you had that passed on to you by other engineers? I personally can't visualize any shell side draining problems with an AJU unit.

Why and how are you subcooling your steam condensate within the tube bundle? Your LP saturated steam feed should basically remain at a nominal 35 psig and 280 ^oF through the tube bundle. You haven’t said that you are feeding superheated steam, so I’m assuming it is saturated. At 50 psig, I would presume this is basic, LP (turbine exhaust) saturated steam. If it is superheated, you’ve got a problem and should be desuperheating it for heating purposes. Otherwise, I don’t believe your 310 ^oF steam temperature (saturated steam at 35 psig is 280 ^oF).

I once had a similar problem with 35 psig saturated steam in a U-tube bundle. The steam was in a BKU kettle reboiler and I would get sporadic heat transfer and couldn’t figure out why until I pulled the bundle and found that – like almost all U-tube bundles – this bundles sagged mid-way of its length. This was caused because during the fabrication of the kettle shell the support rails for the bundle were not installed correctly and the bundle was not 100% in the pure horizontal position. The bundle sagged downward and this created condensate “dams” within each of the hairpins, which prevented steam to continue flowing along the entire length of the hairpin – until the built up steam pressure pushed the condensate up and out of the bundle. I resolved this problem by having my spare tube bundle fabricated in a “chevron” pattern – rather than a pure “U” shape. Instead of a “U”, the chevron is in the shape of a “Vee” and this facilitates positive condensate drainage in the hairpins. I found that my reboiler started to transfer heat like never before when I employed my modified spare bundle and I could clearly see the constant draining effect through a sight glass that I installed in the bottom section of the bonnet on the tube bundle. Ever since I have always specified a chevron type of hairpin when bending my “U” tube bundles on reboilers. And in your case, I would do the same if I were forced to put the steam in the tube side.

Has this heater always exhibited this type of results in operation? Or is this a recent effect? Was this exchanger specifically designed for this service? If so, why was the LP steam put in the tube side?

The Spec Sheet shows saturated 50 psig, 298 ^oF steam in the tube side – both in AND out - so originally someone expected this unit to behave the way I have described (which is the standard, common way a steam heater works). Obviously something has happened to the performance –- or the unit never operated the way it is supposed to from the beginning. Please clarify with more basic data. Otherwise, we'’ll be here discussing this operation until hell freezes over.

[srfish]

Hi Whodat,

What am I missing?. There are all the ingredients to calculating the heat duty. There are the inlet and outlet temperatures and the flow of the steam.

[NOEC]

Art,

The heat exchangers are an existing design and were orginally designed for this service with the steam in the tube side. Operations has informed me that they are experiencing condensate build up in the exchangers. I verified that the condensate receiver level gauge was flooded. This isn't a recent problem because they've been experiencing this for over a year.

I apoligize for not clarifying but the steam in the tubeside is superheated. It passes through a flow control valve before entering the tube side so I would estimate that it's ~ 35-38 psig @ 310 F before entering the exchanger. The LP steam in the refinery (50 psig) is slightly superheated to begin with. Please see the attached P&ID for the temperature readings around the exchangers. The steam flow rate for both exchangers is ~60 mlb/hr.

I am trying to trouble shoot the condensate build up. Would the condensate build up cause the condensate to subcool and lower the heat exchanger efficiency? I've calculated the heat duty (using Hysys) for the tube side of each exchanger. Exchanger 8A has a duty of 59MMBTU/hr but has a much lower temperature difference on the shell side. Exchanger 8B has a duty of 56MMBTU/hr but has a higher temperature difference on the shell side. I would expect if the 8A exchanger had a higher duty then the shell DT would be higher. Am I missing something?

Attached Files

•  2011071215552243.pdf   473.67KB   24 downloads