Sizing of a pump in a closed system

Jaberstein

Recently read an article in the May 2015 Consulting-Specifying Engineer magazine “Selecting an HVAC pump” (http://www.csemag.com/single-article/selecting-an-hvac-pump/6b970346df2dbfc4f12883a534782c55.html).

The one thing I did not understand was the reason to add system pressure for pressure loss in the selection of a HVAC pump in a closed system.

Table 1: Pressure losses
in closed-loop systems
Piping system 4
Fittings 1
Valves* 20
Coil 15
Chiller evaporator 25
System pressure 16
Total system pressure drop 81
Table 1: This shows a closed-loop
sample pressure drop calculation. Valves
are inclusive of shut-off valves, control
valves, strainers, and check valves.
Courtesy: JBA Consulting Engineers

Questioned JBA Consulting and received the following answer:
The system pressure associated with the closed loop system is the minimum pressure to be maintained in the system. In variable flow systems, most control valves require a minimum pressure differential to operate properly. This minimum pressure is treated as a fixed pressure loss for the purpose of calculating the pump head. This is explained further in the ASHRAE Handbook – HVAC Systems and Equipment.

Still doesn't make sense to me.

Jamie Hall

In a system such as your HVAC system -- or in many hydraulic systems -- there are two sources of head loss: friction losses, such as through pipes and elbows and many types of valves. These are dependent on flow. However, there are other valves -- some types of pressure control or pressure reducing valves, or flow rate control valves -- for which the head loss is not dependent on flow, but is a fixed amount. There are yet other valves -- such as the control valves mentioned above, but also including such common gadgets as spring checks or reduced pressure zone backflow preventors which are a combination; they won't open at all at less than a certain pressure differential across the valve, and when they do open, they maintain that pressure drop or something very close to it.

That pressure drop has to be added on to the head losses from friction to get the correct total head loss in a pumped circuit.