Plate heat exchanger versus shell tube heat exchanger

[Yobbo]

L.S.,

We are in the district heating business. The medium is conditioned water. We apply heat exchangers between the primary and the secondary circuit. Primary circuit contains pressurised water of maximal 150 Celcius; whereas the secondary circuit contains hot water of maximum 90 C. The pressure class is PN40 ( DIN system ).
The traditional setup was to utilise Shell Tube Heatexchangers. The transfer of plate heat exchangers is more efficient than that of a shell tube exchanger. We do have some negative experiences with plate heat exchangers due to problems with leaking gaskets. Is there any objective evaluation about where and when to apply a plate heat exchanger instead of a shell tube heat exchanger? We think that Shell Tube heat exchangers do have the important advantage of internal accessibility, so that cleaning and inspections can be carried out more easily. Even one leaking pipe can be closed and the process can carry on. A leaking plate heat exchanger needs to be opened completely in order to repair the leaking gasket. So we think that repair time and reliability are more favourable with a shell tube heat exchanger. On the other hand we don't have extended experience with plate heat exchangers in order to be able to make an objective comparison. Do you have a suggestion for me, as where to find a reliable comparison between the two types of heat exchangers?

Karel Postulart, The Netherlands
Nuon Power Generation

 

[SNORGY]

Based on my experience, you will have trouble with any reasonably priced and available plate pack gasket at 150 C. My opinion is that you want S&T in this service.

Regards,

SNORGY.

 

[TJOrlowski]

There are a multitude of articles on this comparison, do some searching around on the internet. It sounds like you've already made some valuations on your own.

Capital cost of plate heat exchangers is usually lower than shell&tube, but it's up to the owner (you) to do the cost-benefit with the continuing costs of owning a plate versus shell&tube.

If your heat exchanger is heating the secondary circuit to 90C (maybe entering the exchanger @ 70C), the efficiency of a shell&tube will be pretty good, depending on available flow of the primary circuit.

-TJ Orlowski

 

[StewPad]

On the thermodynamic side
The Plate Heat-exchange (PHE) have lower thermo-mass;
So if your process is fluctuate you won't get the dumping effect you get with a S&T
But since you already have one S&T HE you can use a second PHE (actually it will be better if the first one will be the PHE).

Regarding the Mechanical side I tend to disagree and think that PHE are easier for maintenance, and have a lower fouling factor due to higher velocities.
You just need to pick the right gasket.

 

[rmw]

PHE's take less space typically, and require less space for maintenance. With a S&T, you have to have room to pull the bundle.

Since your fluids are water/water, you don't have to worry so much about cleaning deep inside the tube bundle, but when the comparison is made with fluids that can get deep inside the tube bundle, PHE's take an advantage there.

S&T's have gaskets too, just not as many of them. Gasket installation and maintenance is very important with PHE's. Because of the potential for gasket leakage, PHE's are less forgiving of overhung pipe loads. I'd suspect that a lot of the leaking PHE's out there are due to deflection of the frame due to pipe loads.

If you replace gaskets, get it done by professionals. Gaskets aren't high value work and usually the lowest qualified guy in the shop gets stuck with the work. No wonder gaskets leak. Professional companies live and die by whether or not their workmanship is good.

Also, if you have to open the units, pay attention to the manufacturers closing dimensions. Those are set to insure proper gasket seating. Too often, that is ignored.

I'd rarely pick a S&T if a PHE would do, although as already pointed out, 150C is getting to the upper temperature limit of most common gaskets. You can't have high pressure at the time, so find out where the pressure limit is at that temperature and give it some margin. At high temps and pressures, S&T's take the advantage.

rmw