Pump Head Calculation In high rise 2

[HVACDomain]

hi guys,

Could you please help in calculating head of closed loop system in high rise building.
I am quite confused, as in closed loop piping we dont consider head loss due to elevation(static lift) , i have seen some article on internet in which they have considered some hydro static pressure.
In closed loop system, we consider head loss through piping friction,fittings and equipment like chiller,AHU etc.
but in the article they have considered the static lift also.



Please guide me on this.

I am attaching the file please see pg 81 chapter-7

 

[Drazen]

thermal lift is calculated, not hydrostatic pressure, as it is significant.

in today's circumstances that is more issue of balancing than pump selection as most of pumps would be variable speed which can cover range from zero thermal lift to nominal design conditions lift.

the rule of thumb is to take 50% of calculated thermal lift as nominal condition.

however, if i ever have enough influence on concept design, i would always tend to separate high building into few height zones.

 

[PEDARRIN2]

All the various pressures in Figure 7-3 are only looking at pressure changes from various different locations.

In a closed system, once the pipe is full of water - the static pressure on the suction side of the pump is the same as the discharge, no matter where you put the pump because. So when pumping starts, the only thing the pump has to overcome is friction due to pipe, fittings, etc.

So that is what the pump is sized for.

 

[HVACDomain]

Guys Thanks alot for your suggestion.

Drazon- If its closed loop system then why to consider building height in the head.

PEDARRIN2- Yup i agree with you, but they have shown the pressure at different level.


I m confused about the theory written in the document and the figure reflecting in the document.

 

[PEDARRIN2]

Taking a better look at the article and pictures, the authors are indicating the position of the equipment (not the pump) is what is in question for static pressures. Since the reference point appears to be the level of water in the expansion tank, the location of the chiller relative to that location will determine what pressure the chiller will experience. Solution A, with the chiller at the bottom, the chiller will experience the shut off head (140 ft) plus the static (900 ft). This is due to the static pressure exerted by the 900 feet of water in the pipe on the equipment. Putting the chiller at the top, it only has the shut off head (140 ft) and static (10 ft). So it would experience much less static pressure.

Static pressure at the bottom of a riser will always be greater than the static pressure at the top of a riser, so equipment that can only handle lower pressures should not be located on lower floors, unless some sort of pressure regulation is employed.

 

[HVACDomain]

pedarrin2 - agree. but why are we considering static as it is closed loop system.
what I know that, in close loop system we pipe is full of water and does not consider static lift.

in pump head calculation, we need to consider piping friction loss, fitting loss and loss in ahu, fcu and chiller for closed loop system.

correct me if I m wrong or not getting your point. I m jus beginner and will be thankfull to You if you guide me on this.

 

[PEDARRIN2]

Equipment and pipe and fittings typically are rated for maximum pressure.

If I use a pipe fitting or chiller that is only rated for 150 psig, I have to make sure that during no flow (static) and flow (dynamic) conditions - that fitting or chiller is not experiencing pressure exceeding its rating.

The article was talking about where to put the chiller.

From the article, "The decision about the level on which the refrigeration machines and the supporting chilled water and condenser water pumps are located in a building is a decision that can have a cost impact on the refrigeration equipment, the pumps, the piping, and the fittings and valves associated with the piping. The economic impact will be due to the change in the design working pressure to which the equipment, piping, fittings, and valves will be subjected by the system.... The working pressure on any equipment or the piping, valves, and fittings at any location in a building is the sum of the hydrostatic height of the water in the piping above the point being considered plus the dynamic pressure created by the pump at the point being analyzed. The hydrostatic and dynamic pressures are determined in feet of water. Their sum, when added together, is the total pressure or working pressure in feet at the referenced point. To determine the working pressure in PSIG, this total pressure in feet must be divided by 2.31. This is the conversion factor to convert pressure in feet of water to pressure in PSIG."

Using the example, the maximum pressure the chiller would see, if on the bottom floor, is approximately 900 ft (due to elevation) + 140 ft (due to pump shut off pressure, which is basically the TDH provided by the pump during operation and the shut off pressure of the pump) for a total static pressure of 1040 ft (450 psi) per Solution A. This would exceed the rated pressure of the described fitting/chiller.

If the chiller is on the top floor (Solution C), then the total pressure the chiller would see would be 65 psi. This would be within the pressure rating of the fitting/chiller. The pump is still contributing 140 ft to the pressure, but because of the difference in elevation, the static effect is much less.

In this case you would specify fittings/pipe with higher pressure ratings on the lower floors and decreasing ratings as you went up. You would locate the chiller at a level where the pressure does not exceed its maximum pressure.

I hope that helps.

 

[MTES93]

I didn't read in depth but I think they are showing (in figure 7-3)is the working pressure the chiller will see at different locations. While static pressure doesn't show up in the pump head calculation, it does show up in what the equipment/pipes/devices have to withstand. It also comes into play with expansion tank size (depending on where its located) and relief valve size on equipment as well as pressure ratings of equipment.

 

[ChasBean1]

Sooooo not rocket science. HVACDomain, you’re right in that the pump has to overcome system friction only. When you first fill the system in a high rise, you have to do it with a pump that can overcome the height head loss. If it’s a 300 ft tall building for example you need a pump with at least 300 ft head. The system’s pump that is designed for 600 gpm pump at 100 ft head won’t do much good until the loop is filled. After the loop is filled, the 600 gpm, 100 ft head pump will do fine to overcome the system friction.

 

[HVACDomain]

pedarrin2, chasbean1, 1124- thanks guys.

I agree the article is reflecting the effects of pressure on equipment according to the location of chiller and height of building. that's fine.

so, doesn't it effect the pump head calculation.?

1-if a building is 900 ft in height having close loop system, do we consider this 900 ft in pump head calculation? or we consider only head loss in piping and accessories?

2- does height of the building effect the static pressure of the equipment only?
it has nothing to do with pump head calculation?

 

[PEDARRIN2]

1 - no, unless you use the pump to fill the system, but then it would not be a closed system for that phase.

2 - yes.

 

[MTES93]

Just apply Bernouli's on a closed system: For every foot you 'rise' on the discharge, you have a foot of 'drop' on the suction.. The heights cancel one another.

 

[HVACDomain]

11241-thanks
yeah, agree.

but static pressure doesn't cancel. right.?
and we consider height to check the static pressure at the equipment.

 

[PEDARRIN2]

The static pressures at the suction and discharge of the pump in a closed system are equal - so they do cancel.

The static pressures at the suction and discharge of the pump in an open system are not equal, thus this pump has to be sized to compensate for this difference plus friction loss.

 

[Drazen]

hvac domain, as mentioned in high rise building thermal lift is significant and as it is proportional to circle height, height appears in the equation.

you have to calculate it in high rise, with about 30 floors it can already be on level of 15% or more of friction losses.

 

[Hatem2014]

There is a difference between the pressure required from pump which is generated by the pump itself (dP)and the hydrostatic pressure generated by elevation
the fist one, we considered it for moving the fluid but the second one we considered it for determining the max. working pressure inflicted upon pipes, pump's parts (casing , mechanical seal ,....)and all the elements in network
example:
If we calculated dP=5bar required from pump to move the water with the required flow rate.
these 5bar may be from 1bar to 6bar
and may be from 15bar to 20bar
The 2 cases have the same dP (friction losses and fittings ,....) but not the same max. working pressure

 

[Timothi]

For a closed loop system only friction loss is applicable. no static head because the pressure drop during lift is regained during down flow. No velocity head since there is no remarkable change in velocity.

 

[HVACDomain]

Timothy-Right, agree with you.

Hatem2014- Hello, Thanks alot for your comment, I just need more explanation and clarifications about it, I have started getting your pint now.
I request you to please describe it briefly , i will be highly thankful to you, also please elaborate how to CALCULATE static pressure to check the pressure in pipe.

 

[PEDARRIN2]

Static pressure is always calculated from a reference point, which is typically the lowest level. So if the lowest level is at 0' and the highest pipe is at 900'. Since the system is not open, you have to add the 140 ft of pressure from the pump shut off, so you have 1040 ft (450 psig) at the lowest point point. If you go up to mid height (450'), you would have 450 + 140 = 590 ft (255.4 psig). These are the pressures the pipe and equipment will experience so they have to be rated for those pressures.

 

[chicopee]

I have not read the book but I suspect that static head was considered since air will always get in somehow particularly thru shaft seals and ultimately fill the expansion tank if not released.