Plate Heat Exchanger - propylene glycol and water 3

[tronical]

Good Day to all,

I need a plate heat exchanger to heat water to 60 C from 49 C. The hot fluid is propylene glycol, 60 % by mass conc, with Temp of 90C at in and 70 C at out.

Flow rates for both are fixed: 6 m3/h for water, 4 m3/hr for pgly

2 bidders came back with different proposals. Both use 0.6mm thick stainless steel plates. However bidder A has pressure drops across the HX in the order of 30-40 kPa for both hot and cold circuits, which is reasonable. The other bidder B has pressure drops in the order of 0.3kPa, which is soo low.

Bidder A claims that his overall HTC ( aka U) is around 3.6 kW/(m2C), while bidder B has a U of 0.75 kW/m2C.

As you can imagine, this allowed bidder A to use less plates, offering a smaller and lighter HX.

I was thinking that both datas make sense. Large pressure drops indicates bidder A uses smaller plate gaps, i.e. faster velocities, turbulent flow, better U. I concluded the difference has to be because of the plate gaps, but i am not certain. I do not have plate gap information, but i did request it...pending still.

I tried to do a preliminary verification of bidder A's design to estimate a range of U by modelling the flow as a rectangular duct with a very large aspect ratio, and using his dimensions. It didnt work. The answer i got was in the order of 0.5 kW/m2C.

Moreover, it is hard to believe that a vendor has a HX four times better than the other, especially when the liquids are the same and the plate material and thickness is the same.
Can anyone tell me a rough idea of the U value of this application. Where can I see past examples of HX's used so that I can believe bidder A. I want to believe bidder A!

Thanks

 

[rmw]

If your bids contained the dimensional data for the Hx's, look at the dimension for the compressed plate pack. Take the plate count times the plate thickness and subtract that from the compressed plate pack dimension. Divide that by the number of plates, and you have a fairly close approximation of the plate gap.

What can have happened in the case of their being so different is that your duty might have fallen in a range where one mfg'r did not have a good selection due to limitations on their frame sizes, plate styles, etc.

Try getting 3-4 bids and see if more fall into a similar grouping.

I'll try to post more on this over the weekend, but I have an early flight back to the USA, and I am about to hit the sack for now.

rmw

 

[speco]

Tronical,

As you probably know by now, there are many variables in plate exchanger designs. These include the "hardness" of the plates, as well as the length and width. Hardness refers to the washboard or chevron pattern in the plate. The harder the plate, the higher the pressure drop and the better the heat transfer. Longer (or taller) plates result in generally higher pressure drop, since few are needed for the surface, and the velocities can be higher, increasing the U values.

You example appears to be a relatively small exchanger with the highest flow of 6 m^3 per hour (about 26 gpm for those of us who think in olde English units).

I did a very quick WAG of a plate exchanger coefficient with glycol on one side and water on the other, and got about 3.1 Kw/m^2 (550 BTU/(hr-ft^2-deg F)) which is on the order of your Bidder A's design.

I suggest going back to Bidder A and asking about the individual coefficients on the two sides of the exchanger. Also ask (if not shown on the data sheet) what is the safety factor in the design. I would want at least 15-20% margin. Plate exchanger manufacturers don't usually use fouling factors as are common in shell and tube exchangers. They usually say they don't apply to PHE's because the coefficients are much higher.

Additional bids may also be a good idea.

Regards,

Speco (

http://www.stoneprocess.com)

 

[zerok]

Hi tronical,

There was a similar discussion some month ago (see

http://www.eng-tips.com/viewthread.cfm?qid=118735&page=3).

I would suggest using a free software I've already provided a link to. It should give you a fair bit of insight.

here is the link:

http://www.sourceforge.net/projects/innohex

Good luck

zerok

 

[rmw]

Zerok did the two things I was planning to do, look up the earlier thread, and give the link to the online sizing program. I downloaded and used the program and found it to be quite satisfactory in analyzing some exchangers offered by a particular vendor.

rmw

 

[tronical]

I read your previous thread..what a long conversation. I can imagine that when you read my posting, you must have been thinking "NOT AGAIN!"

But you guys are really helpfull. I downloaded the software, and it does verify bidder A's range of U.

We are scheduled for a final clarification meeting before awarding the bid. Any suggestions to not look like a fool?

Thanks guys

Tronical

 

[rmw]

Play with the program that you downloaded a little bit and make some basic changes, chevron angles, pressure drops, flows, plate dimensions, etc, so that you understand all the interactions that are going on with changing parameters, and then, using what you learn, be prepared to ask the vendors to justify their offerings.

We don't mind helping. It is why we are members of this site. Remember, that we receive help just as well. The site has a nice ability to refer to previous threads, which helps us not have to repeat it.

Use the site tools to do searches for more information on the topic.

Good luck.

rmw