Service Hotline: July 2, 2001
Pump Flow
QUESTIONName withheld by request
I would like to know how a system is sized for pump flow and how the pump flow charts are used to size and determine the capacity in gal/min.
Answer:
By Dan Kramer P.E.
Specialist Grade Member of RSES
and Professional Engineer
To size a system for pump flow, the first thing you need to know or figure out is what you are pumping and how much (in gallons per minute, or gpm).
Let’s assume you are interested in pumping water. You have to determine the pipe size and material (such as copper, steel, etc.), then figure out the friction loss and maximum velocity. To this friction head you add the static head, if there is any. There might not be any significant static head if you are circulating water in a fully filled, closed system.
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If either the head or velocity is too high, you have to guess a new, larger pipe size and redo the friction loss calculation. However, retain the data in case you need to refer to it later.
Once you have the right pipe size with reasonable friction loss, pick a pump that will provide your desired flow under your friction (pressure) loss conditions. To do this, you need catalogs from one or more pump manufacturers. Also, some pump manufacturers offer free CD-Roms containing selection programs that will do most of the work for you.
If the liquid you are pumping is hot, you must also consider the suction head available (NPSH) to prevent cavitation.
For a start, I recommend that you buy (or have your boss buy) two ASHRAE handbooks. The 1997 Handbook — Fundamentals has a fine chapter on pipe sizing and data on many different fluids used in our industry. The 1996 Handbook of Systems and Equipment has a fine exposition on centrifugal pumps, together with many charts and curves to help your selection.
As you can see, pipe and pump selection is not a simple process. To save tears and heartache, I suggest you consult an engineer experienced in this process. Many pump manufacturers have such engineers willing to help you with a specific application.
Tons of Refrigeration
QUESTION:From Caosesvida Via E-Mail
What was the actual experiment that was used to find 1 ton of refrigeration?
ANSWER:
By Dan Kramer, P.E.
Specialist Grade Member of RSES and Professional Engineer
The concept of “tons of refrigeration” was developed from the cooling capacity of ice. Ice has a latent heat of melting of 144 Btu per pound. If we take a ton of ice, it has a total cooling effect of 144 x 2,000 Btu.
If we let the ice melt over a 24-hr period, the Btuh cooling effect that results is 144 x 2,000/24. If you divide that out, it comes to 12,000 Btuh. That’s a ton of refrigeration. The term “ton” doesn’t relate to any particular load or room size. It’s simply a handy way to specify a specific rate of cooling.
Nowadays, the SI people are designating cooling loads in watts. A watt is a rate of flow with 1,000 watts being a rate of heat flow of about 3,500 Btuh.