4 replies to this topic

[dchem]

Dear All,

In an exchanger, gas is being heated-up by steam as heating media on the other side. Gas outlet temperature has to be controlled at particular value. Steam is being fully condensed at the outlet, so steam trap has to be there on the outlet pipe. Then, what is the best way to control the outlet gas temperature? Normally, TCV is provided on heating media side, whether it should be on the steam inlet or condensate outlet line?

Consider following different options :
1). Inlet steam can be saturated or in another case it can be superheated.
2). Heat exchanger design options - vertical or horizontal
3). Consider steam on the shell side or can be considered on tube side due to some specific reasons.
4). Any other better method for temperature control?

Provide your valuable inputs, please.

Thanks in advance.

[TS1979]

1). For steam as a heating media, always use satuarated steam.
2). Use horizontal configuration for easier maintenance.
3). For steam condensing, steam should be at shell side for higher heat transfer coefficient.
4). TCV should be installed at steam inlet other than condensate outlet. Enough heat transfer area must be provided for the heat exchanger to reach the required temperature for the gas to be heated.

[Bobby Strain]

If you control condensate from the exchanger it is a much smaller valve (and lower cost). And, there is nothing wrong with a bit of superheat in the steam as long as the tube surface is wet or you provide sufficient surface.

Bobby

[dchem]

Thanks TS and Bobby for the replies.

Further to this, few more queries - if we want to install control valve on condensate line what will be the config of exchanger? Someone recommended me vertical exchanger with level control which ultimately will control temperature in cascade mode. Then, you need a trap or not? If trap is there, where will you install TCV, upstream or downstream? For strat-up we have superheated steam, will it make any difference from temperature control point of view?

[DB Shah]

Basically the difference between TIC on steam and condensate is -
On steam= the control is variable chest pressure
On condensate =the control is variable area

If you go for TIC on steam, the chest pressure will vary as per load and exchanger fouling. At lower load the chest pressure might go down drastically and condensate transfer to high pressure source will be a problem. This might lead to cyclic effect.

If you install TIC on condensate, the chest pressure will always be equal to steam pressure. Condensate transfer is always smooth. However if the condensate is flashed to atmospheric pressure for transfet to condensate polishing unit, you loose heat.