delta t across plate heat exchanger

[dman_110]

Whats a good temperature drop across a plate heat exchanger? im thinkin 15 - 20 degrees. ive got an OWB thats running hot, but the indoor boiler is firring every so often. im gonna go look at it again this weekend. I installed some boiler drains and back flushed the plates last weekend, but it didnt make much of a differance. the stoves been hooked up for two years and worked great till now. im open to any ideas.
Thanks. Dan

 

[jebatty]

A reasonable standard (approach temperature) would be 10F: Side A in 180, out 160; Side B in 150, out 170. This would apply at a particular flow rate. Many specs I have seen for plate HX's are at flow rates of 20 gpm and more; which equates to 200,000 btuh and more at delta-T=20F; and also requires 1-1/2" pipe or larger and an appropriate circulator. Most home applications require less flow, and less btuh, and this is where sizing a plate HX requires some real thinking and analysis of the performance specs to select the appropriate plate HX, circulator, and pipe size at the required flow rate.

A closer approach temperature can be achieved, and at a specific flow rate this requires generally a larger plate HX.

 

[dman_110]

Thanks Jim. Im going to look at it this afternoon. its a 60 plate HE, and hes running a 014 pump with about 100' of one inch underground, im sure were good there like i said it ran great for the past two years. I piped the PHE in series between the return header and the oil boiler. the oil boiler only comes on for a short few secounds then kicks off so im thinking its losing its temp on the off cycle , im gonna check the low limit temp setting also. its got to be something stupid.
thanks again. Dan

 

[dman_110]

well the low limit setting was 140, and the indoor boiler temp was 151, outdoor boiler temp 174, i didnt have my digital thermometer with me so ill have to go back one night this week, ill check gpm through the plate on the outdoor side, and delta t across the plates. just strikes me odd it ran great for two years now this. am i missin something?

 

[jebatty]

If it ran great for two years and nothing changed, the only thing that comes to my mind is a restriction somewhere in the plumbing or in the hx itself. It's difficult to determine actual flow rates unless the circ has tappings on the flanges so you can check the pressure differential, or unless you have a flow gauge. If you're sure the hx is clean and clear on both sides, I don't real know what to recommend except some way to check actual flow rates and narrow done the problem area. Good luck.