Question re. pressures in a heating system. 1

[AngloSaxon]

I assume the answer to my following question will be fairly fundamental, but for some reason I can't seem to get my head around it...

What I'm confused with is the relationship between static head, dynamic pressure and total system pressure, with specific regard to a heating system.

Logic indicates that total system pressure is equal to the sum of static head + dynamic pressure. Now, is this correct?

If this IS the case, then if a circulation pump on a heating system is oversized, will it increase the total pressure system too high? For instance, in a LTHW system, operating pressure shouldn't exceed 3 bar. If static head is set to 1 bar, thermal expansion may increase total system pressure to 2 bar or more, then a circulation pump may increase it to 4 bar, which is over the limit of the boiler.

I think the point I'm basically asking is: does a circulation pump increase pressure in the system? Or, does it just increase the mass flow rate and velocity of the water? The pump is used to boost flow to the furthest point of the circuit due to resistance, but does that actually increase the pressure within the system? I know this is fundamental and I probably should already know the answer...

When I refer to static head I mean the pressure at the neutral point of the system, generally the boiler, due to the water content within the boiler (for a combi system), or F&E cistern (for a vented system), or as provided by the pressurisation unit (on a sealed system).

When I refer to dynamic pressure I mean the pressure created by the circulation pump.

When I refer to total system pressure I mean the sum of pressure within the heating system whilst it's running at full temperature.

Can someone please clear this up?

Thanks in advance.

 

[KiwiMace]

You might want to revise the dynamic pressure subject by taking a look at Pressure Gradient Diagrams. If you think of the pump as a neutral pressure point, you go high positive at the pump and gradually or abruptly reduce as the fluid moves through the piping and equipment until at some point you go negative as you make your way back to the pump. It's this negative draw, eating into your static head buffer, that means the pump isn't just adding 2 bar to your static.

I'm deliberately avoiding trying to properly explain dynamic head and getting it wrong, but hopefully giving you something that can be visualized.

 

[willard3]

Of course the pump increases both static and dynamic pressures.

Total pressure = static pressure + velocity pressure

 

[ChrisConley]

A hydronic system is at neutral (or fill pressure) at the expansion tank. The circulation pump then creates a differential pressure that creates system flow. The circulation pump can not increase the system fill pressure regardless of size because the system is closed. The discharge of the pump will be at the highest pressure in the system.

 

[Drazen]

To add to Chris points, normally circulation pump head would be in range of say 0,5-0,7 bar. If fill pressure in small system is 1 bar, this does add up to pump head, but is far below pressure rating of equipment/setting of safety valve.

In larger systems (spread over more storeys), static head (which imposes fill pressure) is higher, so influence of dynamic pressure on total pressure is smaller.

It means your concern is not an issue in practice, though most of your reasoning is correct.

 

[imok2]

TIP: Because head
calculations are more uncertain than flow calculations, and
because designers are likely to add safety factors, it makes sense to pick a constant-speed pump with the required duty point to the left of the BEP.( BEST EFFECIENCY POINT) By doing that, the pump will move toward higher
efficiency as it reacts to lower-system head loss. By selecting pumps that operate in the center or high efficiency part of the curve most of the time, we can lower operating costs and engthen the life of pump seals and bearings. This is because the
pump impeller generates increasingly high radial loads as its point of operation moves away from BEP toward lower
efficiency points. The pump net positive suction head required NPSHR) usually is low in the higher efficiency part of the curve.

 

[ChasBean1]

Anglo,

Think of a closed loop system with no pump running, with some building height of X. You want the whole system pressurized to some degree to keep from drawing in air. With the expansion tank set right, you might have 15 psig pressure at the highest point in the system and depending on building height, say 30 psig at the bottom (pressure = density of the water, times gravitational acceleration [9.8 m/s^2 or 32.2 ft/s^2], times height of the water column [X]).

Now we start the pump which is located in the bottom of the building. In the system, the pressure (which was 30 psig) now has the same net value but has 40 psig at the pump outlet and 20 psig at the pump inlet. This difference is what drives flow through the building.

The dynamic pressure is a function of velocity, which is a function of pipe size, which you should not be of concern when trying to evaluate whether a system can handle a certain pressure. Thermal expansion in a closed loop system should be alleviated by the expansion tank. You cannot design a closed loop system without providing expansion/contraction capability.

It is the pressure difference (similar to a battery and voltage) created by the pump that causes system flow. If the pump is oversized, you could have, say, 50 psig at the pump outlet and 10 psig at the inlet. The piping should be designed to accommodate orders of magnitude of pressure higher than those caused by pump differential.

Hopefully this is helpful! –CB

 

[AngloSaxon]

Thanks for all of the replies - most helpful!

This, in conjunction with a discussion with my college lecturer, I feel a lot more satisfied with my knowledge of hydraulics in a heating system now.

Cheers!