Mechanical energy is called foot-pounds, but in the case of energy the pounds are not mass (or weight) but force, sometimes abbreviated lbf. A pound of force is about 4.4 newtons, which is in units of Kg-m/sec^2. A pound of force is also about 32 lb-ft/sec^2. Multiplying by feet to get foot-pounds results in units of lb-ft^2/sec^2, and each foot-pound is about 32 lb-ft^2/sec^2. Now, to obtain a unit of energy per unit weight of water, divide by the weight of water in pounds to get 32 ft^2/sec^2, or simplifying slightly 32 (ft/sec)^2. Hence "head" is actually in units of velocity squared, not distance, or "feet". But apparently this is lost on most people in this industry.
Fundamentally, the term head goes back to Bernoulli’s equation, which is an energy conservation equation. Bernoulli’s equation can be written in several forms, one of which is: total head = P144/D + v^2/2g + Z With P expressed in lb(force)per inch^2, and D in lb(f)/volume (lbf/ft^3), all the terms simplify to unit of feet.
I keep thinking and i apologize ,yet , i have to ask this about pumped boiler and pumped system by-passes or circulators that see a different reference point to the point of no pressure change than system circulators in regards to the placement of the expansion tanks . Here is where this bit of a quandary occurs ,..surely upon start up or commissioning a system with large storage tanks , the pressure differential is much different from the operating conditions with smaller delta T in supply and return and that seems sufficient enough to give rise to how to proceed , should one have a circulator that is used with start up specifically and then changed out for another over the lifetime of the equipment ? or am i over thinking this? Forgive me for perhaps missing some of what i was listening to as i pondered this . *~//: )
Thank you for your comments, but we are not quite sure of your exact question. However, we recommend the use some other source of pressurized water to fill and purge the system during commissioning. The circulator in system should be sized based on the system normal operating conditions. Please let us know if you have additional questions.
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Mechanical energy is called foot-pounds, but in the case of energy the pounds are not mass (or weight) but force, sometimes abbreviated lbf. A pound of force is about 4.4 newtons, which is in units of Kg-m/sec^2. A pound of force is also about 32 lb-ft/sec^2. Multiplying by feet to get foot-pounds results in units of lb-ft^2/sec^2, and each foot-pound is about 32 lb-ft^2/sec^2. Now, to obtain a unit of energy per unit weight of water, divide by the weight of water in pounds to get 32 ft^2/sec^2, or simplifying slightly 32 (ft/sec)^2. Hence "head" is actually in units of velocity squared, not distance, or "feet". But apparently this is lost on most people in this industry.
Fundamentally, the term head goes back to Bernoulli’s equation, which is an energy conservation equation.
Bernoulli’s equation can be written in several forms, one of which is:
total head = P144/D + v^2/2g + Z
With P expressed in lb(force)per inch^2, and D in lb(f)/volume (lbf/ft^3), all the terms simplify to unit of feet.
I keep thinking and i apologize ,yet , i have to ask this about pumped boiler and pumped system by-passes or circulators that see a different reference point to the point of no pressure change than system circulators in regards to the placement of the expansion tanks . Here is where this bit of a quandary occurs ,..surely upon start up or commissioning a system with large storage tanks , the pressure differential is much different from the operating conditions with smaller delta T in supply and return and that seems sufficient enough to give rise to how to proceed , should one have a circulator that is used with start up specifically and then changed out for another over the lifetime of the equipment ? or am i over thinking this?
Forgive me for perhaps missing some of what i was listening to as i pondered this .
*~//: )
Thank you for your comments, but we are not quite sure of your exact question. However, we recommend the use some other source of pressurized water to fill and purge the system during commissioning. The circulator in system should be sized based on the system normal operating conditions. Please let us know if you have additional questions.