Unpressurized strainer/ HX dilemma

[pybyr]

I've got my boiler in- and my stainless panelized (unlined) storage tank partly assembled, but still need to design and put together my inlets and outlets and hook up the plate heat exchanger (a 5x12 70- plate FlatPlate) between the boiler and the storage.

I've got a Caleffi DirtCal dirt separator on the main primary/ secondary system that includes the boiler and the heat loads.

I also have a second Caleffi DirtCal dirt separator to install on the tank side of the plate HX. I've been planning to put the Caleffi DirtCal on the side of the HX that ties into the bottom of the tank, as I figure that crud will naturally be more abundant at the bottom of the tank than the top (flow will reverse on the tank side of the HX via a 4-way valve)

Various literature (such as the Flat Plate brochure, and general advice) says that I _should_ have a Y-strainer with at least a 20 mesh screen on each side of the plate HX.

BUT- as heaterman pointed out in the "squeaking circulator" discussions, if I put a Y- strainer on the suction side of the pump on the tank side of the plate HX (which has no pressure except the weight of the water)(and the HX and circulator will be near floor level to maximize the "head" on the circulator to minimize likelihood of cavitation), then any restriction on the flow created by the Y-strainer (especially if it begins to clog) equals flow restriction, which equals heightened risk of cavitation and circulator failure of the circulator on the un-pressurized side of the plate HX.

So- it seems if I am darned if I do include a Y-screen-strainer (because it introduces flow restrictions) and darned if I don't (because then I risk having particles clog the innards of the plate HX).

I am tentatively leaning to omitting the Y-strainer on the unpressurized side of the HX- but appreciate any thoughts that folks have on this.

Thanks

 

[sorethumbs]

If the screen starts to clog its going to add frictional loss (head) to your system. Your pumps preformace will start to fall down the pump curve. If the added frictional loss is on the suction side or the pump side - what does it matter. I'd opt for the WYE strainer and HX protection myself. Just clean the mesh if your pumps preformence suffers to greatly.

 

[steam man]

I find it interesting your working from the point of view about using a strainer for HX protection. I ran into a problem recently by overheating a tank with a long pipe run through a strainer. At a point where the tank level was decreasing and the temp was high enough the fluid in the pump would flash and nothing moved. The pump manufacturer was real specific on the temperature/head pressure/strainer mesh to use. It sounds like you won't have a lot of head on your pump. I would consider using a larger basket type strainer with a large flow capacity. It would give the least resistance. Just take it out and clean it once in awhile. Barring that, just put in isolation valves and clean the strainer frequently.

I thought you have that tank put together by now? LOL

Mike

 

[tom in maine]

Where is the dirt coming from?
If you are on the side of the tank, even a little off the bottom, I cannot imagine that you will see much dirt, unless you are expecting some corrosion, which seems unlikely.

I have never been in a tank with any appreciable "dirt" in the bottom. Of course, most of those tanks were plastic or EPDM lined, but they were pretty darn clean.

The only cruddy tank was a steel tank, and even there, one could draw off the tank,away from the bottom and not get into sludge or dirt.

I would lose the strainer.

 

[DaveBP]

Trevor,

even with perfect filtration mineral build up might be possible.

How about plumbing in a differential pressure gauge across your HX? Record or mark the gauge reading when the thing is pristine and watch for increased pressure over time. That way you can jump on it before it gets to be a problem no matter what sticks in there.