Condensing Pressure Increasing on shell side of heat exchanger

[gte682n]

I have a heat exchanger with Dowtherm A vapor on the shell side in condensing service. This is part of a closed heat transfer system with a fired heater maintaining the supply temperature and pressure. The outlet pressure increases over time on the return side of the Dow system. Is this any indication of reduced heat transfer? There is a control valve on the inlet of the Dow side. We maintain a constant flow, but the valve has to open up more over time. The tube side fluid is an oil that partially vaporizes. Am I getting inerts in the system? What else could cause the pressure increase? The Dow system should be a clean fluid.

 

[don1980]

If you are changing the heat demand, or the exchanger is fouling, then pressure variations are expected. That's how a condensing exchanger works. The pressure on the condensing side rises and falls with changes in heat duty (demand). When the heat load changes, the exchanger dT has to change, and to do that the condensing side pressure must change. These pressure variations occur naturally, in response to changes in the control valve position. In addition to the the pressure changing in response to heat demand, it also changes (increases) if fouling is occurring on either side of the tube. The increased heat transfer resistance caused by fouling reduces the U value. And in order to maintain the same heat transfer, the dT must rise (pressure must rise) to offset that reduction in U.

 

[georgeverghese]

It could either be fouling on the shellside or thermal degradation of the Dowtherm that has resulted in the buildup of non condensibles on the shellside.

The Dowtherm A fired heater may have been poorly designed resulting in high tubeside film temps or maybe hot spots due to poor burner selection / location leading to this thermal degradation ?

 

[gte682n]

don1980,

If I am maintaining a constant flow via flow control, how is my heat demand changing? We do not temperature control anything on this exchanger. The Dow valve is opening up more overtime, but for the same flow.

Can this pressure increase on the shell side be attributed to fouling? Or is it external to the exchanger, for example condensate return system.

Also, I do not see any changes on the process side dT. However, at some point in the future I will be limited in how much Dowtherm I can pass through the exchanger because of the rising pressure on the outlet side of the exchanger.

 

[jari001]

The inlet control valve is maintaining a certain vapor flow rate through the HX as measured by the vapor flowrate on the exit of the HX?
Thus, the more you condense, the less vapor you have, and therefore the control valve has to open to increase the vapor flow rate. The increased condensation is a symptom of varying heat demand as don1980 pointed out.

You don't see changes in the process side dT because the utility fluid side is still able to provide the needed heating but you are condensing the Dowtherm at a faster rate. Perhaps you can look at the energy consumption of your heater and notice an increase in fuel usage because more liquid is being returned?

I would try to check there is no fouling on both the utility and process side.

Another thought is perhaps more oil than expected is vaporizing and now more heat than intended is being absorbed by a vapor phase?

 

[georgeverghese]

The explanation for this increased shellside pressure will be found in the pressure vs. enthalpy charts for Dowtherm A.

Have a read of Perry Chem Engg handbook also for Dowtherm A thermal degradation mechanism etc.