HX pressure drop: can you explain it to a newbie?

28W

I'm just a homeowner looking to learn more about about my new mod-com system.  What does it mean when folks talk about a pressure drop across the heat exchanger?  And why does a pressure drop mean higher pumping loads (or does it?)  Thanks.

Ironman

Pressure Drop

Any component (heat ex., pipe, fitting, valve, etc.) through which water is circulated has a certain amount of resistance to the flow due to friction. This friction loss is typically expressed in feet of head per foot of pipe or per component. The higher the flow rate (gpm.), the more the resitance to flow, or the higher the "head". It can also be expressed as "pressure drop". 1 psi = 2.31 ft. of head.



The heat exchangers in many mod/cons have a substantial pressure drop due to their narrow water passageways and the volume of water going through them. This is especially true If the boiler incorporates a Gianonni heat exchanger. It transfers a tremendous amount of heat in a very compact area.



Most all mod/cons are "low mass". A conventional cast iron boiler is "high mass" and has relatively little resistance to flow.



Because of the mod/con's low mass and the low amount of water that the heat exchanger holds, it is critical that sufficient water is circulated through it any time the burner is firing. Without sufficient flow, the water in the heat ex. will flash to steam in a matter of seconds and the boiler will lock off. This will be accompanied by a loud "bang" when it flashes. With the burner applying heat and reduced water flow, the metal that the heat ex. is constructed from will began to suffer metal fatigue and eventually fail if the situation persists.



This is why most mod/cons require a separate circ. and primary/secodary piping or some other means of hydraulic separation from the system piping. Again, the conventional cast iron boiler doesn't have this issue though it may be piped pri/sec. for other reasons.



Some mod/cons (Triangle Tube, Knight WHN) incorporate a "fire tube" heat ex. that has a low head loss and may not always require a separate circ. or pri/sec. piping.

SpeyFitter

Friction

To create flow, pumps create a difference in pressure. So if the static fill pressure in a system is 12 PSI (what you fill your hydronic system up to pressure wise as dictated by your pressure reducing valve), your pump will create "pressure" downstream of it which causes the water to flow. So your circulator turns on and immediately, as an example, increases the pressure downstream of it to say 18 PSI (it has added 6 PSI to the static fill pressure). Since there is 18 PSI downstream of the pump, but still 12 PSI (static fill pressure) upstream of the pump, this 6 PSI difference causes the water to flow as pressure always tries to equalize in a system. The difference in pressure the circulator creates is the driving force towards the flow of water and subsequently the movement of heat from the heat source to the heat emitters and back.

Now the unfortunate thing is we know that we need a certain size boiler to heat a certain size house on design day, and we know that we need to flow a certain amount of water to work with our systems design. Lets say after sizing our system we determine we need 10 gallons per minute flow rate to move the appropriate amount of heat. Unfortunately we can't just go out and buy a 10 GPM pump. It doesn't work that way. Every system is different and has different numbers of fittings, longer or shoter runs of pipe, and different components that the water must move through. And each and everyone of these components and pipe/fittings offers resistance to the flow of water that the pump must overcome. This is called "pressure drop" or "head loss."  If we have a fixed run of piping that is say 100 feet long. that will produce a certain amount of pressure drop this will dissipate the pressure difference the pump has created as water flows. If we add piping or components to this system, the head loss will increase and subsequently the pumps flow rate will be reduced. And this works the other way - reduce head loss, and the flow rate increases.



Some heat exchanger designs, typically water tube style where the water is in tubes that surround a cylindrical burner, have high pressure drops, which means you need more pumping power to get the heat out out of the boiler. Some offer so much resistance that it would be prohibitive to flow above a certain amount of water as the head loss just increases so much at a certain flow rate that it's almost not worth it to pay more money for a larger pump, which in a lot of cases, is a LOT more money, and it uses more power to boot. Instead use certain tactics to work with these heat exchangers to extract that heat out of them to work with the system (such as higher delta T's).



If you are as interested as you say you are in learning about your system I highly recommend you buy and read some of Dan's books (off of this site). They will explain it in a much more reader friendly way than I have, plus he elaborates into other areas so you can help see what your system is doing.

I suggest "Pumping away" and "Primary-Secondary Piping made easy" to get started.

28W

Thanks, Scott and Ironman

Great explanations - very helpful. I will look into Dan's books.

icesailor

Third World Example:

Take a 25' garden hose and hook it to an outside sill cock. Turn it on full blast and run it into a 5 gallon plastic bucket. Time how long it takes to fill the bucket. Now, add a few more hoses to the 25' hose, like make it 100' or more. Now, turn on the water and again fill the bucket. The bucket will take longer to fill. Same size hose, slower fill rate with the longer hose. That is a third world demonstration of friction or head loss. To make the longer hose deliver the same amount of water that the shorter hose delivers, you need to increase the pressure in the system and that may not increase it to what the original hose delivered.

Consider the hose the HX.

In a nut shell.