This Month’s Puzzler
Our makeup-air heat exchangers seem to have suffered ruptured tubes. On the shell side, we use 45-psig steam reduced from 200-psi boilers. We send 40% propylene glycol/water through the tubes; at the steam control valve inlet with a 50% load it’s 42 psig. The exchangers are horizontal U-tubes with ¾-in. × 0.049-in. (16 BWG) copper tubes. The tubes were rolled into a 316L stainless steel tube sheet and sealed with fluorocarbon gaskets. Both exchangers are 24-in. diameter and 6-ft long. The exchangers are about 50% oversized sometimes while running about 105% during the winter months. We run one at a time. Another concern is that the tube-side relief valve was sized using the old 77% rule — is that okay? I think a classic case of water hammer causes the crushing of a number of tubes at the top of the tube bundle that we see. When we operate at only 50%, there’s a thermal reservoir in the shell that pulls a vacuum; another engineer believes that’s the culprit. What do you think?
Check A Variety Of Items
Although data is sparse, there are several items you could look into.
Temperature gradient. Is the steam superheated or at saturation? Steam at 42 psig has a saturation temperature of ~289°F. Since you did not mention what the inlet temperature of the glycol-water mixture is, if you are using a heating media too hot and the temperature gradient (difference) is high, this may be causing additional stress on the tubes and causing premature failure. I suggest measuring the temperature difference between the glycol and steam and having a mechanical engineer determine if there is too much stress on the tubes, especially at the rolled connection at the tube sheet. Dissimilar metals — copper tubes and stainless tube sheet — could also be a concern when it comes to high temperature differences between the fluids. Check the mean metal temperature to be sure you have not exceeded the vendor’s recommendations.
If the steam is superheated, the thermal expansion stresses are more significant. You also mention you have significant over surface availability in the exchanger. Condensing steam has a high heat transfer coefficient, ~ 1,500 Btu/hr-ft2-°F. I would assume that the tube-side coefficient is controlling the overall heat transfer coefficient. You may want to consider using a different heating fluid that balances the resistances better. Is there a waste heat stream available that could be used instead of the steam? This could also help minimize stresses induced due to high temperature gradients.
The hammering may be a function of rapid condensation due to the excess surface area: condensate builds up in the shell too close to the inlet steam source.
Tube supports/baffles/tube thickness. If the tubes are not properly supported, large temperature gradients could cause failures along the outer surface of the tubes, especially at the weakest points, which may be the rolled connections. Have someone experienced in mechanical exchanger design count the tube supports and check baffle count and spacing to be sure they are adequate for the service. If you decide to change the heating medium, the baffle count and spacing may need to change to account for heat transfer and the potential resulting vibration. Your tubes are thin. Have you considered a thicker tube wall? Not knowing your flow, could a thicker tube provide the protection you need? Velocity and pressure drop are the keys to determining if you could afford a thicker tube.
Outlet glycol temperature. What is the outlet glycol/water temperature? How is it being controlled? Make sure you have not exceeded the boiling point temperature of the glycol mixture. (Propylene glycol, 40 wt-%, boils at ~219–220°F at 1 atm pressure.) You may want to check this because the steam condensing temperature is above the atmospheric boiling point of the glycol mixture. Vaporizing in the tubes will account for additional stresses that most likely will not have been addressed in the original design due to assuming no phase change on the glycol side.
Thermal reservoir. Are you condensing steam at 42 psig or higher? I’m not sure why you are pulling a vacuum if you are condensing at 42 psig. What is the chest pressure in the exchanger? If you are pulling a vacuum and the exchanger is not designed for vacuum or you do not have a vacuum breaker on the exchanger, this could be creating a problem. Perhaps you don’t need that much energy/heat transfer into the glycol stream.
Tube-side relief valve. I’m not familiar with the 77% rule. If the relief valve is not relieving, I’m not sure what part it will play in tube failure. You may want to check to see if it is leaking by. I’m not sure how much would have to be leaking by to cause a problem.
Propylene glycol degradation products. This may be a remote possibility, but propylene glycol, in the presence of O2 and metal, e.g., copper, can degrade at high temperatures into lactic acid. If the steam pressure is higher than the glycol pressure, un-deareated condensate might be entering the glycol mixture, degrading the glycol into lactic acid? I would check to be sure that you are not leaking condensate into the glycol mixture.
Eric M. Roy, principal engineer
Westlake Chemical, Houston
Add a Small Exchanger
There are several problems with this application: 1) poor selection of heat exchangers; 2) potential corrosion between the copper tubes and stainless steel tube sheet; 3) using the 77% rule for bypassing consideration of tube rupture in relief calculations.
Operating a heat exchanger at 50% of rate means that it is oversized. You don’t need a spare exchanger — you need a smaller one to meet demand when it is low. Operating a heat exchanger at a low rate affects the heat transfer coefficient: h50% ~ 0.57×hdesign based on Dittus-Boelter.