Pump Types

Overview

HVAC pumps transfer fluid energy through mechanical work, converting motor shaft power into pressure head and kinetic energy. Pump selection depends on flow rate, total head, fluid properties, efficiency requirements, and system characteristics. The fundamental pump equation relates these parameters:

Pump Power:

BHP = (Q × H × SG) / (3960 × ηp)

Where:

• BHP = Brake horsepower (hp)
• Q = Flow rate (gpm)
• H = Total head (ft)
• SG = Specific gravity (dimensionless)
• ηp = Pump efficiency (decimal)

Affinity Laws govern pump performance when speed or impeller diameter changes:

Flow: Q₂/Q₁ = (N₂/N₁) or (D₂/D₁) Head: H₂/H₁ = (N₂/N₁)² or (D₂/D₁)² Power: P₂/P₁ = (N₂/N₁)³ or (D₂/D₁)³

Centrifugal Pumps

Centrifugal pumps dominate HVAC applications due to high efficiency, simple construction, low maintenance, and excellent reliability. These pumps add energy to fluid through centrifugal force generated by a rotating impeller.

Operating Principles

Fluid enters the impeller eye along the axis of rotation and is accelerated radially outward by impeller vanes. Kinetic energy imparted by the impeller converts to pressure energy in the volute or diffuser. The velocity head at impeller discharge transforms into static pressure through gradual area expansion.

Euler Turbomachinery Equation:

H = (U₂Vt₂ - U₁Vt₁) / g

Where:

• H = Theoretical head (ft)
• U = Impeller tip velocity (ft/s)
• Vt = Tangential component of absolute velocity (ft/s)
• g = Gravitational constant (32.2 ft/s²)
• Subscripts 1 and 2 denote inlet and outlet

End-Suction Pumps

End-suction pumps feature axial inlet and radial discharge, representing the most common HVAC pump configuration. The impeller mounts on a shaft supported by bearings external to the casing.

Characteristics:

• Flow range: 5-5000 gpm
• Head range: 10-400 ft
• Efficiency: 60-85%
• Temperature limits: -20°F to 250°F
• Pressure limits: 175-300 psi

Applications:

• Chilled water distribution
• Condenser water circulation
• Hot water heating systems
• Boiler feed service
• Glycol solution pumping

Design Considerations:

The shaft extends through the casing, requiring mechanical seals or packing to prevent leakage. Single mechanical seals suffice for most HVAC applications. Double seals with barrier fluid serve applications requiring zero leakage.

Radial thrust on the impeller requires adequate bearing capacity. Hydraulic unbalancing causes radial loads proportional to head and impeller diameter. Sealed bearings eliminate routine greasing in many designs.

NPSH requirements increase with flow rate and speed. Adequate submergence or suction head prevents cavitation, which causes noise, vibration, and impeller erosion.

Inline Pumps

Inline pumps mount directly in piping with suction and discharge flanges on the same centerline. The close-coupled motor mounts above the volute, creating a compact footprint.

Characteristics:

• Flow range: 5-1500 gpm
• Head range: 10-200 ft
• Efficiency: 55-80%
• Space-saving vertical configuration
• Direct pipe mounting without baseplate

Applications:

• Secondary chilled water loops
• Zone heating systems
• Small condenser water systems
• Pressure boosting
• Domestic water circulation

Design Considerations:

Inline pumps eliminate floor space requirements and simplify installation. The pump hangs from the piping, requiring adequate structural support for the combined weight of pump, motor, and fluid.

Pipe strain must not exceed manufacturer limits. Flexible connectors, spring hangers, or rigid supports prevent piping loads from distorting the casing. ASME B31.9 governs allowable pipe forces and moments.

Servicing requires in-place access or isolation valves and drains for cartridge removal. Some designs allow cartridge extraction without disturbing piping connections.

Horizontal Split-Case Pumps

Split-case pumps feature a casing divided along the shaft centerline, allowing upper half removal for maintenance without disturbing piping or driver alignment. Double-suction impellers balance axial thrust.

Characteristics:

• Flow range: 50-15,000 gpm
• Head range: 50-600 ft
• Efficiency: 75-88%
• Horizontally split casing
• Double-suction impeller
• Radially split design

Applications:

• Central chilled water plants
• Large condenser water systems
• District heating mains
• High-rise building distribution
• Campus loop circulation

Design Considerations:

Double-suction impellers eliminate axial thrust by balancing inlet pressures. This allows simple thrust bearings rather than angular contact bearings.

Radial bearings outside the casing simplify maintenance and allow heavy-duty construction. Oil-lubricated bearings provide long service life with periodic oil changes.

Mechanical seals mount in stuffing boxes accessible without impeller removal. Seal flush plans per API 682 ensure adequate cooling and lubrication.

Vertical Inline Pumps

Vertical inline pumps combine inline mounting with traditional bearing and seal arrangements. The motor couples to a vertically oriented pump with in-line suction and discharge.

Characteristics:

• Flow range: 10-3000 gpm
• Head range: 20-500 ft
• Efficiency: 65-82%
• Vertical motor mounting
• In-line flow path
• External bearing support

Applications:

• Space-constrained mechanical rooms
• Retrofit installations
• High-rise riser pumps
• Heat recovery loops
• Process cooling systems

Design Considerations:

Motor mounting requires structural support adequate for dynamic loads. Vibration isolation protects building structure from pump-generated forces.

Shaft length affects critical speed and deflection. Long shafts require intermediate bearings or rigid coupling to prevent excessive runout.

Drainage provisions at seal area prevent motor contamination from leakage. Seal leakoff piping routes fluid to drains or collection tanks.

Vertical Turbine Pumps

Vertical turbine pumps feature multi-stage construction with bowl assemblies stacked vertically. Long shafting extends from the driver to submerged impellers.

Characteristics:

• Flow range: 50-10,000 gpm
• Head range: 100-3000 ft per stage
• Efficiency: 70-88%
• Multi-stage capability
• Submerged impellers
• Line shaft or submersible motor

Applications:

• Cooling tower sumps
• Thermal storage tanks
• Deep well water sources
• Geothermal loops
• Underground storage retrieval

Design Considerations:

Bowl assemblies stack to achieve required head through multiple stages. Each stage adds approximately 20-100 ft of head depending on specific speed.

Shaft alignment requires precise column assembly. Rubber spider bearings in the column support the shaft and allow thermal expansion.

Submergence depth must exceed NPSH requirements plus safety margin. Insufficient submergence causes cavitation in the first stage impeller.

Positive Displacement Pumps

Positive displacement pumps deliver fixed volume per shaft revolution regardless of pressure. Flow rate varies directly with speed, making these pumps ideal for precise metering and high-pressure applications.

Rotary Gear Pumps

External gear pumps use meshing gears to trap fluid between teeth and casing. Internal gear pumps employ an internal rotor and external crescent.

Characteristics:

• Flow range: 0.1-1000 gpm
• Pressure range: 50-3000 psi
• Viscosity range: 1-500,000 SSU
• Efficiency: 75-90%
• Self-priming capability

Applications:

• Fuel oil transfer
• Glycol makeup
• Lubrication systems
• Chemical injection
• Heat transfer fluid circulation

Design Considerations:

Gear pumps require relief valves on discharge to prevent overpressure from deadheading. Relief valve setting should be 10-25% above maximum operating pressure.

Clearances between gears and casing control slip and efficiency. Tight clearances improve volumetric efficiency but reduce tolerance for contamination.

Viscosity affects performance significantly. Low viscosity fluids exhibit higher slip rates, reducing volumetric efficiency.

Rotary Screw Pumps

Screw pumps employ intermeshing helical rotors to move fluid axially through the pump. Twin-screw and triple-screw configurations serve different applications.

Characteristics:

• Flow range: 1-5000 gpm
• Pressure range: 50-1500 psi
• Viscosity range: 1-100,000 SSU
• Efficiency: 75-85%
• Smooth, pulsation-free flow

Applications:

• Heat transfer oil systems
• High-temperature thermal fluid
• Refrigeration oil pumping
• Boiler fuel supply
• Contaminated fluid handling

Design Considerations:

Timing gears maintain rotor clearances without metal contact in twin-screw pumps. Three-screw pumps drive the idler screws through fluid pressure.

Axial thrust requires thrust bearings adequate for pressure forces. Balanced designs minimize net thrust through opposed helix angles.

Clearance management becomes critical at elevated temperatures. Thermal expansion must not close running clearances excessively.

Rotary Vane Pumps

Vane pumps use spring-loaded or centrifugally actuated vanes sliding in a rotor. The eccentric rotor creates expanding and contracting chambers.

Characteristics:

• Flow range: 0.5-500 gpm
• Pressure range: 25-1000 psi
• Efficiency: 70-85%
• Compact design
• Moderate pressure capability

Applications:

• Light oil transfer
• Condensate removal
• Solvent circulation
• Waste oil pumping
• Transfer pumping

Design Considerations:

Vane wear affects volumetric efficiency and creates clearance leakage. Abrasive fluids accelerate wear rates significantly.

Vane loading depends on pressure differential and spring force. Excessive loading causes friction losses and heat generation.

Filtration protects vanes and cam ring from particulate damage. Strainers upstream prevent premature wear.

Progressive Cavity Pumps

Progressive cavity pumps employ a helical rotor turning inside an elastomeric stator with internal helical cavities. Interference fit creates sealed cavities that progress axially.

Characteristics:

• Flow range: 0.1-500 gpm
• Pressure range: 50-500 psi per stage
• Viscosity range: 1-1,000,000 SSU
• Efficiency: 75-90%
• Excellent solids handling

Applications:

• Sludge transfer
• High-viscosity fluids
• Shear-sensitive fluids
• Abrasive slurries
• Metering applications

Design Considerations:

Stator elastomer must resist chemical attack and temperature exposure. NBR, EPDM, and fluoroelastomers serve different fluid compatibilities.

Interference fit between rotor and stator creates sealing but limits temperature range. Thermal expansion can cause binding or excessive friction.

Abrasive content accelerates stator wear. Hard chrome-plated rotors extend life in abrasive service.

Specialty Pump Types

Axial Flow Pumps

Axial flow pumps move fluid parallel to the shaft axis through propeller action. These pumps deliver high flow at low head.

Characteristics:

• Flow range: 1000-100,000 gpm
• Head range: 5-50 ft
• Efficiency: 70-85%
• Propeller-type impeller
• High specific speed

Applications:

• Cooling tower circulation
• Large sump drainage
• Storm water pumping
• Process cooling loops
• Heat rejection systems

Mixed Flow Pumps

Mixed flow pumps combine radial and axial flow characteristics. The impeller imparts both centrifugal and axial forces to the fluid.

Characteristics:

• Flow range: 500-50,000 gpm
• Head range: 20-150 ft
• Efficiency: 72-86%
• Intermediate specific speed
• Diagonal impeller vanes

Applications:

• Medium-head cooling tower service
• Large HVAC distribution
• Process water circulation
• Irrigation pumping
• Flood control

Regenerative Turbine Pumps

Regenerative pumps use an impeller with radial vanes in a close-clearance casing. Fluid recirculates between impeller and casing, building pressure gradually.

Characteristics:

• Flow range: 1-150 gpm
• Head range: 50-500 ft
• Efficiency: 35-60%
• High head at low flow
• Compact size

Applications:

• Boiler feed makeup
• Pressure boosting
• Condensate transfer
• Small boiler feed
• Metering service

Canned Motor Pumps

Canned motor pumps integrate pump and motor in a sealed pressure boundary. The rotor runs in pumped fluid, eliminating shaft seals.

Characteristics:

• Flow range: 1-1000 gpm
• Zero external leakage
• Hermetically sealed
• Pumped fluid bearing lubrication
• High reliability

Applications:

• Refrigerant circulation
• Hazardous fluid pumping
• Zero-leakage requirements
• High-purity systems
• Thermal fluid loops

Pump Selection Criteria

Hydraulic Requirements

Pump selection begins with establishing flow rate and total head at design conditions. The system curve defines head versus flow relationship, while pump curves show pump performance.

Operating Point:

The pump operates where its characteristic curve intersects the system curve. This point determines actual flow, head, power, and efficiency.

Proper selection places the operating point near best efficiency point (BEP). Operation significantly left or right of BEP reduces efficiency and increases mechanical stress.

System Curve Equation:

Hs = Hstatic + K × Q²

Where:

• Hs = System head (ft)
• Hstatic = Static head (ft)
• K = System resistance coefficient
• Q = Flow rate (gpm)

Specific Speed

Specific speed characterizes pump geometry and performance characteristics:

Ns = (N × √Q) / H^0.75

Where:

• Ns = Specific speed (dimensionless)
• N = Rotational speed (rpm)
• Q = Flow rate at BEP (gpm)
• H = Head per stage (ft)

NPSH Requirements

Net Positive Suction Head Available (NPSHA) must exceed Net Positive Suction Head Required (NPSHR) by an adequate margin:

NPSHA = Patm/γ + Zs - Hvp - Hfs

Where:

• Patm = Atmospheric pressure head (ft)
• γ = Specific weight of liquid (lb/ft³)
• Zs = Static suction head (ft, negative if lift)
• Hvp = Vapor pressure head (ft)
• Hfs = Suction line friction loss (ft)

Margin Requirements:

ASHRAE recommends NPSHA exceed NPSHR by minimum margins:

• NPSHR < 10 ft: 4 ft margin minimum
• NPSHR 10-20 ft: 6 ft margin minimum
• NPSHR > 20 ft: 1.3 × NPSHR minimum

Insufficient NPSH margin causes cavitation, resulting in noise, vibration, reduced performance, and impeller damage.

Pump Efficiency

Pump efficiency varies with operating point. Wire-to-water efficiency accounts for both pump and motor losses:

ηw-w = ηp × ηm

Efficiency Requirements:

ASHRAE 90.1 establishes minimum motor efficiency based on horsepower and pole count. Pump efficiency standards apply to specific equipment categories.

Premium efficiency motors (NEMA Premium) provide 2-5 percentage points higher efficiency than standard motors. Variable frequency drives add 3-5% loss at full speed.

Performance Characteristics

Pump Curves

Manufacturers provide characteristic curves showing head, power, efficiency, and NPSHR versus flow rate at constant speed.

Curve Shape:

Centrifugal pump curves slope downward from shutoff head to runout flow. Steep curves indicate high resistance to flow variations. Flat curves show sensitivity to system changes.

Unstable curves exhibit rising head with increasing flow in certain regions. This creates potential for surge and hunting in the system.

Power Characteristics:

Non-overloading power curves peak before maximum flow, ensuring motor capacity suffices throughout the operating range. Overloading curves require motor sizing for runout conditions.

Variable Speed Operation

Variable frequency drives (VFDs) enable pump speed modulation to match varying loads. Energy savings result from reduced speed at part load conditions.

Affinity Law Application:

Reducing speed 20% decreases flow 20%, head 36%, and power 49%. This cubic relationship with power produces substantial energy savings.

System Curve Interaction:

Systems with high static head exhibit less savings potential than friction-dominated systems. The static head component remains constant regardless of flow.

Savings Potential:

Power Ratio = (Q₂/Q₁)³ × [(H₂ + Hstatic)/(H₁ + Hstatic)]

Where Hstatic represents the non-variable head component.

Parallel and Series Operation

Parallel Operation:

Multiple pumps in parallel combine flow rates at the same head. The combined curve adds individual pump flows horizontally at each head value.

Parallel pumps serve redundancy, capacity expansion, and part-load efficiency optimization. Individual pump operation at higher efficiency points can reduce total power.

Series Operation:

Pumps in series combine heads at the same flow rate. The combined curve adds individual pump heads vertically at each flow value.

Series operation achieves higher heads than single pumps. Multistage pumps internally employ series staging.

Comparison of Pump Types

Installation Considerations

Foundation and Mounting

Pump foundations must provide rigid, vibration-free support. Concrete bases should be 1.5-2 times the baseplate mass and isolated from building structure.

Grouting:

Non-shrink grout fills the space between baseplate and foundation. Grout thickness should be 1-2 inches minimum. Baseplate should contact grout across entire area.

Alignment:

Shaft alignment between pump and driver prevents premature bearing failure and seal damage. Alignment tolerances per manufacturer specifications, typically 0.002-0.005 inches.

Piping Arrangement

Suction Piping:

Suction piping should be short, direct, and free from air pockets. Pipe size equals or exceeds pump inlet size. Velocity limits: 5-7 ft/s for suction service.

Eccentric reducers prevent air accumulation at pump inlet. Flat side up for suction lift, flat side down for flooded suction.

Straight pipe runs of 5-10 diameters upstream minimize inlet flow disturbances. Avoid elbows immediately upstream of pump.

Discharge Piping:

Check valves prevent reverse flow and water hammer. Swing check valves require horizontal installation. Spring-loaded checks work in any orientation.

Gate valves provide pump isolation for maintenance. Ball valves serve smaller sizes. Butterfly valves reduce space and cost for large sizes.

Flexible connectors absorb thermal expansion and reduce vibration transmission. Install per manufacturer orientation requirements.

Design Best Practices

Pump Sizing

Select pumps for 10-15% margin above design flow to accommodate future modifications and fouling factors. Avoid excessive oversizing which reduces efficiency.

Design point should fall at or slightly right of BEP. Operation at 70-120% of BEP flow ensures acceptable efficiency and mechanical reliability.

Redundancy Strategy

Critical systems require redundant pump capacity. Configurations include:

100% Standby: Two pumps each sized for 100% capacity. One pump operates while one remains on standby.

Parallel Operation: Multiple smaller pumps combine to meet total flow. Allows staging for part-load efficiency and provides partial redundancy.

Lead-Lag Control: Alternates pump operation to equalize runtime and wear. Automatic switchover upon failure detection.

Energy Efficiency

Variable speed drives provide energy savings in variable flow systems. Economic payback typically 2-5 years depending on load profile.

Properly sized pumps operating near BEP maximize efficiency. Trimming impellers or changing sheaves optimizes performance for actual conditions.

Primary-secondary pumping decouples production and distribution, allowing central plant pumps to operate at constant flow and high efficiency.

Control Integration

Pump control integrates with building automation through:

• Start/stop commands
• Speed control (VFD systems)
• Status monitoring
• Alarm indication
• Performance trending

Differential pressure sensors provide feedback for variable speed control. Pressure setpoint reset based on demand optimizes energy consumption.

Flow measurement enables performance monitoring and energy analysis. Ultrasonic or magnetic flowmeters avoid pressure drop penalties.

Code and Standard References

• ASHRAE 90.1: Energy Standard for Buildings Except Low-Rise Residential Buildings
• ASME B73.1: Specification for Horizontal End Suction Centrifugal Pumps
• HI 1.1-1.5: Centrifugal Pump Standards (Hydraulic Institute)
• HI 3.1-3.5: Rotary Pump Standards
• API 610: Centrifugal Pumps for Petroleum, Petrochemical and Natural Gas Industries
• NFPA 20: Standard for the Installation of Stationary Pumps for Fire Protection

Maintenance Requirements

Preventive maintenance intervals:

Monthly:

• Visual inspection for leaks
• Bearing temperature monitoring
• Vibration checks
• Seal condition assessment

Quarterly:

• Coupling alignment verification
• Bearing lubrication (if required)
• Mechanical seal inspection
• Performance testing

Annual:

• Complete performance curve verification
• Bearing inspection/replacement
• Impeller clearance measurement
• Seal replacement (as needed)

Predictive maintenance using vibration analysis, thermography, and motor current signature analysis extends mean time between failures and prevents unplanned outages.