Getting the Most, part 3 – Plate Heat Exchangers

Wed, June 06, 2018

Last time, we talked about pressure drop limitations and how they affect the sizing of a plate heat exchanger.  Today we will discuss a couple of other key considerations. First: approach temperatures. Approach is the difference between the outlet temp of one fluid to the inlet temp of the other.  Generally speaking for a given load, the larger the approach, the smaller the heat transfer surface required. Plate heat exchangers can be sized to approach temperatures as close as you wish, but there can be a huge cost penalty for smaller approaches.  A fraction of a degree can have a significant change to the number of plates required.  Again, your Bornquist sales engineer can show you the size and cost difference for any set of selections made. Because small approach temperatures can make such a large impact, independent certification is important.  For plate heat exchangers in HVAC application, where the fluids are both water, the total load is under 24,000 MBH and flow rates are under 2000 GPM, an AHRI 400-2001 certification is used.  AHRI is an independent agency responsible for certifying the performance of certain equipment, such as plate heat exchangers.  All Bell & Gossett plate units are sized for AHRI 400-2001 certification for the conditions mentioned here. A word about fouling.  One common mistake we see in plate heat exchanger specifications is the improper application of fouling allowances. Fouling allowances are very different for plate units versus shell & tube units. In a shell & tube heat exchanger, a fouling allowance adds heat transfer surface in the form of additional length.  That’s fine because the velocity though each tube is the same regardless of length. However, for plate heat exchangers additional surface means more plates, which reduces the velocity in each channel.  This then reduces the heat transfer rates, which means even more plates, etc.  A .0005 fouling allowance could result in over 25% more plates or more, depending on the approach temperatures and fluids!  Fouling allowances aren’t necessary in most applications, but if one is called for, we prefer a specific number of additional plates, such as 10%. Another option is to add port strainers to the specifications.  Port strainers are selected by the heat exchanger manufacturer with mesh sizes small enough to prevent the passage of particles large enough to become stuck between the corrugations of the plates. So how do you put all this together for the best selection?  Simple – use the B&G ESP-Thermal selection software, part of the B&G ESP-Systemwize software https://esp-systemwize.com  It’s a great tool for selecting pumps, trim, heat exchangers and much more.  It also is key to optimizing systems.  No product stands alone in a system, so why use software that only addresses one part of your system? Next time – Coiling tower optimization      

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Tags: engineering , heat exchangers , heat transfer , HVAC , plate heat exchangers