Centrifugal Pumps are the most common pumps used in HVAC application. Rotation of the pump impeller generates a centrifugal force that pushes fluid away from pump axis into the volute and flows outward through pump diffuser. Centrifugal pump mainly consists of:
1- Impeller or rotor: in centrifugal pumps, fluid enters the center of the rotating impeller and forced radially into the volute and flows outward through pump diffuser.
2- Pump case: is the body of the pump. Inline and end suction are the most commonly used in HVAC applications.
3- Shaft: Shaft is coupled to motor. For bigger pump sizes, motor is coupled to the shaft through flexible coupling.
4- Motor: usually electrical motors are used to drive centrifugal pump.
Centrifugal pump performance is evaluated by pump curve. Pump curve is a diagram shows total static head against pump flow rate. Also it shows shaft power, pump efficiency, impeller diameter, pump speed and the net positive suction head (NPSHA).
pumps shall be selected within range of operation, this range mostly between approximately 50% and 120% of Best Efficiency Point. For best pump performance and energy efficiency, pump may selected at the maximum efficiency.
Cavitations is a phenomenon occurs in centrifugal pumps when the static head of the fluid entering the pump approaches the vapour pressure of the liquid, in this circumstances fluid will boil at this temperature and vapour bubbles will form on the impeller passage, babbles may attack impeller body and make a damage to the pump. To avoid cavitation, it is required to increase the entering fluid head to exceed the vapour pressure of the fluid to certain value; this value is known as required net positive suction head (NPSHR).
Net positive suction head NPSH:
Each pump has its own NPSHR, also each system has its own available net positive suction head NPSHA, so NPSHA must be more than NPSHR to avoid cavitation. The difference between NPSHA and NPSHR is simply called net positive static head NPSH. NPSH is the head loss inside the pump itself, additional head at the pump inlet is required to compensate the head loss in the pump,
Whereas NSPHR is provided by pump manufacturers, NPSHA in the system can be calculated by:
NSPHA = Suction heat at pump inlet – vapour pressure of the liquid
Suction head is the static head hs plus dynamic head hv
NSPHA = hs + hv – Pv/ρg
hs : absolute static head at pump inlet, in m or ft.
hv:Velocity head at the pump suction portm, in m or ft. Often not included as it’s normally quite small.
hv = Vs/ 2g
hvp: Absolute vapour pressure of the liquid at the pumping temperature. hvp is important in heating system because as temperature goes up, so does vapour pressure. table 1 evaluates hvp for water at various temperatures.