Sizing pump head

[xpsboy]

What refernce are you using for fittings equivalent lenght?

Also do I consider each valves when calculating the total head?

Pipe lenght, fittings, coil press drop, valves (?) --- are this the information i need when do pump head?

Thank you in advance.

 

[Drazen]

I assume that your are student making your first steps in HVAC.

Yes, you need to calculate each and every pipeline, fitting, valve, heating coil, boiler etc. to get information about branch with highest pressure drop to be able to determine pump head.

In some projects you could add percentage for fittings instead of calculating each and every fitting, but you should have sufficient experience to get a feeling.

For other elements, especially valves and coils, such rules of thumb are even less desirable.

 

[DuctHunter]

In theory all those things you mention should be factored into calculating pump head, and every company I've worked with does it differently. Bell and Gossett (or any manufacturer) typically has charts you can use to approx. pressure drop across air seperators, Suction diffusers, etc. There are also many textbooks that you can get equivalent lengths for fittings as well. Very often the piping system is not installed the same way it is designed so safety factors need to be added in the calculations as well.

I use an excel spreadsheet. The main components, in my opinion that need to be accounted for are: Length and size of piping, chiller or boiler pressure drop, control valve, air separator, triple duty valve and suction diffuser and the coil pressure drop of the heat transfer device at the end of the run(reheat coil, Unit ventilator, fin tube, AHU coil, etc). These values are easily obtainable from the manufacturers of the equipment you design around. As for all the fittings, sometimes this is not as practicle to get so I typically take the pressure drop of the total piping run and add 50% for fittings (ie if the piping pressure drop is 22 feet then add another 11 feet for the fittings).

Like I said, Everyone has their own methods, but this method has never gotten me in trouble.

I hope this helps.

 

[Artisi]

One word of warning, do not over estimate any friction losses or add too many "just in case margins" as this can put you into bigger pumps / motors which are not required for the applications and result in numerous pump operating problems.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[Artisi]

Should have added, also don't under estimate as this will result in too small a pump / motor unit with won't achieve the duty.

It is a capital mistake to theorise before one has data. Insensibly one begins to twist facts to suit theories, instead of theories to suit facts. (Sherlock Holmes - A Scandal in Bohemia.)

 

[tys90]

Putting in too large of a motor isn't that much of a problem (less efficient) and you can usually trim impellers if needed as long as you don't select a pump that uses one of it's smallest size impellers at design conditions. Yes you want to avoid extremes but it's easier to reduce pumping capacity and to increase it.

 

[DubMac]

All discussion has been on "friction" head. When calculating total head make sure you include static head as well.

Properly, Total head is made up of:
1. net elevation change from supply point to discharge point
2. net external pressure (if any) applied to the liquid.
3. Frictional losses in piping, valves, fittings, etc.
4. Entrance and exit losses at supply and delivery point respectively.
5. difference in velocity heads at pump discharge and pump suction.

The first two terms together make up the static portion of total head and do not change with the flow through the system.

The remaining 3 terms are lumped together as friction head and vary with flow (pressure increases as the square of the flow increase). Most HVAC calculations can ignore the 4th and 5th terms as they are typically too small to make a difference.

 

[walkes]

Static head can be ignored in a closed loop (what goes up must come down and balances head due to the elevation).

 

[RRedmond]

Making a mistake in adding the components of head can be especially bad if you have a flat curve pump where GPM is affected by a rather small increase in head loss.