Keep Cool When Designing Heat Exchanger Piping

I misread the drawings about pipe installation but at least had some good excuses. It was 4 a.m. I had been sleeping in my car for five days between meetings and my nightshift duties as part of a reactor commissioning team. In addition, I’d recently assumed the role of instrument engineer as well as process engineer. While the design team should have clearly marked the orientation of the inlet and outlet connections on the drawings, it was stretched thin. This commissioning exercise was a catastrophe — but that’s another story.

[pullquote]

Pipe installation on jackets, coils, reactors and heat exchangers is tricky. If you get it wrong, you’ll have plenty of opportunity to kick yourself while you’re pulling things apart to re-do them right. Connections generally are flanged, not threaded, to avoid costly repairs when the threads are stripped; the exception is sanitary connections, which usually are the male end of a sanitary clamp.

Let’s start with the correct inlet and outlet orientation for jackets and coils. (Coils provide better heat transfer but are more difficult to vent, drain and clean because the flow often is down, around and then out the same direction.) In a jacket or coil using steam, the steam obviously goes in the top nozzle while the outlet (for condensate) goes at the bottom. Must-haves for a jacket include a vent at the top and a drain at the bottom. Extra nozzles help; split jackets may require multiple nozzles. It doesn’t matter if the unit sometimes uses steam and other times chilled water: the inlet is at the top and the outlet is at the bottom. However, using separate nozzles for steam and chilled water often makes sense. With steam condensate, it’s critical that the outlet is the lowest point — otherwise, expect problems from corrosion and fouling.

If the jacket serves exclusively for cooling, then opt for a bottom inlet and top outlet. Oil systems for jackets usually use a bottom inlet and top outlet, too. (The reactor I was commissioning used N2 and the correct orientation was bottom inlet/top outlet.)

With heat exchangers, there really aren’t set rules except for vertical evaporators and condensers, in which gravity rules: liquids drain at the bottom. Generally, for jackets, steam enters in the higher nozzle and leaves (presumably as condensate) from the lower nozzle.

Different guidelines apply to horizontal thermosiphons: recirculating thermosiphons — liquid (from the bottom of the tower) in bottom, vapor plus liquid out top; once-through thermosiphons — vapor plus liquid (from the side of the tower basin) in from bottom, liquid (at the side of the tower basin) out the top.

Locating relief valves also can be tricky. Position pressure safety valves (PSVs) and vacuum vents at the top of the jacket or coil — except when debris might collect at the mouth of the device. Put relief devices on heat exchangers as high as possible to avoid liquid partial flow, which could obstruct vapor flow. It’s generally assumed that if a tube ruptures, this involves a small hole causing flow into the shell; usually, only the area of the tubing head cover — not the cylinder — is considered in a fire.

Let’s now turn to pressure relief calculations. For fire, assume two phases for liquid in a jacket, coil or exchanger tubing. Thermal relief always is a concern with liquids. One relief valve may suffice but, if there’s a possibility of fouling or isolation of the PSV, then add as many relief devices as necessary.

Note that rupture discs are a better choice for high viscosity (>100 cP) oils. Some oils are so slippery that PSVs will require elastomer seals. It’s often sensible to upgrade pipe flanges to a higher pressure rating to add more bolts to the flange bolt circle. Give special consideration to nozzle reinforcement to account for the thrust: wound vessel sheets may be flimsy.

Mistakes in piping around heat exchangers and jackets can cause serious problems in operations and maintenance. Try to re-task nozzles as much as possible to avoid adding potential leak sources but make sure you plan wisely during design.