Selection and Operational Considerations Harold Streicher, Vice President Sales Hansen Technologies, Inc.
Introduction to liquid refrigerant pumps Open Drive and Semi-Hermetic (sealless) centrifugal pumps Information required to make a pump selection Anatomy of a pump curve Examples selections Installation considerations Pump suction line, discharge line, Bypass/minimum flow lines, volute vent line what is it and why is it need Operational considerations of a pump Cavitation what is it and how to prevent it
Open drive Close-coupled or belt driven Single stage Semi-hermetic (Sealless) Canned pump Single or multi-staged
OPEN-DRIVE Mechanical shaft seal Requires oil reserve Separate air-cooled motor Oil lubricated bearings Impeller trimmed to match capacity requirement Ability to run dry for short period of time Nominal 1800 rpm Usually lower initial cost SEMI-HERMETIC Sealless design No oil reservoir or oil maintenance Integral motor Refrigerant cooled motor Hydrodynamic bearings Impeller matched to motor Frost and moisture tolerant Nominal 3600 rpm Usually higher initial cost
System Capacity (US gpm) Differential Pressure (feet of head or psid) Refrigerant and Temperature Net Positive Suction Head available Voltage and Hertz
Required US GPM =Tons of Refrigeration X Rate of Evaporation (GPM per Ton) X Recirculation Rate Multiply the system tonnage by the factor in GPM/Ton table at the required temperature. Multiply the resultant GPM by the system recirculation rate (i.e. overfeed rate 3:1, 4:1, etc. + 1) to determine the required GPM of the pump. Note: Recirculation Rate is not overfeed rate
GPM = System tons X GPM/ton X Recirc Rate Example: 300 system tons using R-717 at 0 F with 4:1 recirc rate GPM = 300 tons X 0.064 X 4 = 77 GPM
System Capacity Requirement Refrigerant Pump Bypass at Minimum Flow Per manufacturers guidelines Refrigerant Pump Motor Cooling (semi-hermetic) Total Pump Capacity = System + Bypass + Motor Additional Consideration Future Requirements
Static Losses Elevation to Highest Point Dynamic Losses Equipment Pressure Drop Valve Pressure Drop Pipe Pressure Drop Back Pressure Regulators
Pressure increase required by pump (inlet-to-outlet) PSID = Discharge Pressure - Suction Pressure Discharge Pressure = Static Head + Dynamic Losses Suction Pressure = Saturated Liquid Pressure PSID To Head [FT] Conversion Head[ft] = PSID x 2.31 SPGR Where SPGR = Specific Gravity
NPSHa is a function of installation 1. Pressure difference above the vapor pressure of the fluid 2. The static height of the fluid above the pump centerline 3. The pressure losses (frictional and form) due to fluid flowing through the suction piping, valves, and the pump s suction. 4. Heat gains in the piping to the pump suction. Low Level cut-out NPSHa Pump Suction inlet
NPSHr is a function of pump design at various conditions
Conditions: 488 Tons 4:1 RERC. Rate + 15 F ammonia 27 PSID CAPACITY 488 x.066 x 4 = 128 US GPM DIFFERENTIAL PRESSURE 27 PSID x 2.31/0.65 = 96 FT. TDH
+15 degree F Ammonia (SG 0.65) 128 GPM 96 Ft Head
Cavitation Due to inadequate NPSHa Vapor Entrainment Suction Recirculation
When static pressure of the flowing liquid falls below vapor pressure, bubbles occur, at areas of higher pressure vapor bubbles will suddenly implode.
at areas of higher pressure, bubbles will suddenly implode Normally at outer diameter / end of vanes
Sounds like gravel in pump Discharge pressure will fluctuate or drop Some evaporators may not be properly cooling Over-temperature thermistors cut-out Due to reduction of cooling of semi-hermetic pump motor material erosion break down of impeller
Temporarily close or partially close pump discharge line to see if issue goes away Increase of the static pressure on the suction side by increasing the liquid level Reduce flow requirment to system (HEV settings) Reduce of flow resistance in suction piping ( valves, filters, diameter of piping etc ) Prevention of turbulences at the inlet of the suction by a suitable construction ( special impeller design / Inducer ) Consider use of flow regulator (semi-hermetic only) picutre: inducer in front of impeller
Vapor entrainment Bubbles of refrigerant vapor migrating to pump suction Usually occurs due to system transients such as start-up conditions, when false loads are terminated (defrost, liquid make-up) Prevention: ensure proper recirculator design Suction recirculation Secondary reverse flow occurs within impeller due to insufficient flow through pump Ensure Q-Min line is open or set by-pass valve properly
recirculation line
(C) (C) (B) (A)
(A) (B) (C) (D)
Pump Suction Line Pump Vent/Bypass Line Pump Discharge Line Pump
Minimum pump suction line sizing from the accumulator vessel (pump recirculator).
Suction pipe downwards L = 5*DNs wrong Venting not possible Avoid any unnecessary pressure drop in the pump suction line from valves, strainers, and fittings. correct
Gas will collect in top portion of volute and must be vented-off Used during pump start-up and prior to servicing Needs to be separate from bypass or suction vent lines Volute Vent Valve
Safe guards pumps against insufficient flow Steps to set properly: 1. Open by-pass valve completely 2. Close discharge stop valve 3. Slowly close by-pass valve until discharge pressure unstable 4. Slowly open by-pass valve until pressure stabilizes
recirculation line