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100+ Free RSES CMS Dynamic Compression Practice Questions

Pass your RSES Certificate Member Specialist (CMS) — Dynamic Compression exam on the first try — instant access, no signup required.

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2026 Statistics

Key Facts: RSES CMS Dynamic Compression Exam

100

Practice Questions

OpenExamPrep

100

Exam Questions

RSES

80%

Passing Score

RSES

~$35

Member Exam Fee

RSES

CM

Required Prerequisite

RSES

Centrifugal

Dynamic Compression Focus

RSES

The RSES Certificate Member Specialist (CMS) — Dynamic Compression exam is a specialty credential from the Refrigeration Service Engineers Society (RSES) for HVACR technicians who work on centrifugal (dynamic) compressors and chillers. It consists of 100 multiple-choice questions and requires a score of 80% to pass, and it builds on the RSES Certificate Member (CM) credential, which is a prerequisite. The exam concentrates on dynamic compression rather than positive-displacement machines: centrifugal compressor theory (impeller, diffuser, tip speed, staging), surge and choke and capacity control with inlet guide vanes, lubrication and oil systems, shaft seals and bearings, purge units and non-condensables, and centrifugal chiller operation and troubleshooting. This free prep includes 100 research-based practice questions with explanations and an AI tutor.

Sample RSES CMS Dynamic Compression Practice Questions

Try these sample questions to test your RSES CMS Dynamic Compression exam readiness. Each question includes a detailed explanation. Start the interactive quiz above for the full 100+ question experience with AI tutoring.

1In a centrifugal refrigeration compressor, the rotating component that adds kinetic energy (velocity) to the refrigerant gas is called the:
A.Diffuser
B.Impeller
C.Volute
D.Inlet guide vane
Explanation: The impeller is the rotating wheel that spins the refrigerant outward, adding velocity (kinetic energy) to the gas. That velocity is later converted to pressure in the stationary diffuser. The impeller is the defining feature of dynamic compression.
2In a centrifugal compressor, the high-velocity gas leaving the impeller is converted into pressure primarily by the:
A.Impeller eye
B.Diffuser
C.Inducer
D.Shaft seal
Explanation: The diffuser is a stationary passage of increasing cross-sectional area that slows the gas down, converting kinetic energy (velocity) into static pressure per Bernoulli's principle. This velocity-to-pressure conversion is the second half of dynamic compression after the impeller imparts velocity.
3A centrifugal compressor is classified as which type of compressor?
A.Positive-displacement
B.Dynamic
C.Reciprocating
D.Rotary-vane
Explanation: Centrifugal compressors are dynamic compressors: they raise pressure by imparting velocity to a continuous stream of gas and then converting that velocity to pressure. They do not trap and squeeze a fixed volume the way positive-displacement machines do.
4Compared with a positive-displacement compressor, a centrifugal compressor generally:
A.Handles very low volumes at very high pressure ratios per stage
B.Handles large volumes of gas at relatively low pressure ratios per stage
C.Cannot be used with refrigerants
D.Produces a pulsating, intermittent discharge
Explanation: Centrifugal compressors excel at moving large volumes of low-density gas but produce a relatively modest pressure rise per impeller (stage). High overall lift is achieved by adding stages in series. Their discharge is smooth and continuous, not pulsating.
5Surge in a centrifugal compressor is best described as:
A.Excessive oil foaming in the sump
B.A momentary flow reversal where discharge gas flows backward through the impeller
C.Liquid refrigerant flooding the evaporator
D.The point of maximum flow where capacity stops increasing
Explanation: Surge occurs when flow drops too low for the head the compressor is developing; the impeller can no longer maintain forward flow, so high-pressure discharge gas momentarily reverses back through the wheel. This causes loud noise, vibration, and rapid temperature/pressure oscillation that can damage bearings and seals.
6The condition called 'choke' or 'stonewall' in a centrifugal compressor occurs when:
A.Flow is so low that the gas reverses direction
B.Gas velocity in a passage reaches sonic conditions, limiting maximum flow
C.The oil pump loses prime
D.The purge unit removes too much refrigerant
Explanation: Choke (stonewall) is the high-flow limit: as flow increases, velocity in the impeller or diffuser passages approaches the speed of sound (Mach 1), and the compressor cannot pass any more gas regardless of how much head is reduced. It is the opposite extreme from surge.
7The most common method of capacity control on a hermetic centrifugal chiller compressor is:
A.Cylinder unloaders
B.Variable inlet guide vanes (prerotation vanes)
C.Hot-gas bypass only
D.Suction service valve throttling
Explanation: Inlet guide vanes (prerotation vanes) at the impeller eye modulate capacity by pre-swirling and throttling the entering gas. Closing the vanes reduces flow and matches compressor output to the cooling load while staying out of surge. This is the primary capacity-control method on most centrifugal chillers.
8When the inlet guide vanes on a centrifugal chiller close to reduce capacity, they create pre-swirl that:
A.Increases the gas flow into the impeller
B.Reduces the gas flow and the work the impeller does
C.Raises the condenser pressure
D.Has no effect on compressor power
Explanation: Pre-swirl from partly closed inlet guide vanes spins the gas in the same direction as impeller rotation, reducing the relative velocity and the amount of work and flow the impeller produces. This unloads the compressor and lowers power draw as the load falls.
9A purge unit on a low-pressure centrifugal chiller is used to:
A.Add refrigerant to the system
B.Remove non-condensable gases (air and moisture) from the system
C.Lubricate the compressor bearings
D.Increase condenser water flow
Explanation: Low-pressure chillers (e.g., R-123, formerly R-11) operate below atmospheric pressure on the low side, so air and moisture leak in. The purge unit collects these non-condensables that accumulate in the condenser and expels them while returning refrigerant to the system, restoring efficiency.
10Non-condensable gases (air) tend to accumulate in which component of a low-pressure centrifugal chiller?
A.Evaporator
B.Condenser
C.Oil sump
D.Inlet guide vanes
Explanation: Non-condensables collect at the high point of the system where refrigerant condenses out and the inert gases do not — the condenser. They blanket the tubes, raise condensing pressure, and reduce heat transfer, which is why the purge draws its sample from the condenser.

About the RSES CMS Dynamic Compression Exam

The RSES CMS Dynamic Compression exam is a Certificate Member Specialist specialty test focused on centrifugal (dynamic) compressors and chillers. It has 100 multiple-choice questions and requires 80% to pass, and candidates must already hold the RSES Certificate Member (CM) credential.

Assessment

100 multiple-choice questions; 80% required to pass; a CMS specialty exam taken after the RSES Certificate Member (CM) credential. This practice bank is 100 selected-response items.

Time Limit

Timed multiple-choice exam (typically about 2 hours)

Passing Score

80%

Exam Fee

~$35 RSES member exam fee (CM membership required) (Refrigeration Service Engineers Society (RSES))

RSES CMS Dynamic Compression Exam Content Outline

22%

Centrifugal Compressor Theory

Impeller, diffuser, volute and inducer; velocity-to-pressure conversion; tip speed and developed head; single- vs multistage staging; construction and materials

10%

Dynamic vs Positive-Displacement Compression

Dynamic (centrifugal/axial) vs reciprocating, scroll and screw; continuous vs pulsating flow; pressure ratio per stage; gas-volume handling

18%

Surge, Choke & Capacity Control

Surge and choke (stonewall); the performance map and surge line; inlet guide vanes/prerotation vanes, variable-speed drives and hot-gas bypass

20%

Centrifugal Chiller Operation & Troubleshooting

Flooded evaporator/condenser operation; lift, approach and head pressure; low- vs positive-pressure refrigerants; surge, fouling and carryover diagnosis

13%

Lubrication & Oil Systems

Oil pump, prelube, oil heater and cooler; oil foaming from refrigerant migration; oil logging and eductor return; viscosity and bearing protection

12%

Purge Units & Non-Condensables

Purge operation on low-pressure chillers; air/moisture infiltration; condensing-pressure and efficiency effects; high-efficiency purges and EPA limits

5%

Shaft Seals & Bearings

Journal and thrust bearings; axial thrust and balance pistons; labyrinth and open-drive shaft seals; magnetic-bearing oil-free designs

How to Pass the RSES CMS Dynamic Compression Exam

What You Need to Know

  • Passing score: 80%
  • Assessment: 100 multiple-choice questions; 80% required to pass; a CMS specialty exam taken after the RSES Certificate Member (CM) credential. This practice bank is 100 selected-response items.
  • Time limit: Timed multiple-choice exam (typically about 2 hours)
  • Exam fee: ~$35 RSES member exam fee (CM membership required)

Keys to Passing

  • Complete 500+ practice questions
  • Score 80%+ consistently before scheduling
  • Focus on highest-weighted sections
  • Use our AI tutor for tough concepts

RSES CMS Dynamic Compression Study Tips from Top Performers

1Be able to explain the two-step dynamic compression sequence: the impeller adds velocity, the diffuser converts it to pressure
2Know surge vs choke cold — surge is low-flow flow reversal, choke (stonewall) is the sonic high-flow limit; control keeps the operating point between them
3Understand why low-pressure (R-123) chillers need purge units while positive-pressure (R-134a) machines generally do not
4Study the oil system carefully: prelube, oil heater (drives refrigerant out of the oil), oil cooler, and the low-oil-pressure cutout that protects the bearings
5Connect high lift to surge — fouled condensers, non-condensables, and high condenser-water temperature all raise lift and push the machine toward surge
6Complete all 100 practice questions and review every miss with the AI tutor before sitting the exam

Frequently Asked Questions

How many questions are on the RSES CMS Dynamic Compression exam and what score do I need?

The RSES Certificate Member Specialist (CMS) Dynamic Compression exam has 100 multiple-choice questions and requires a score of 80% or better to pass. It is a specialty exam covering centrifugal (dynamic) compressors and chillers.

Do I need the RSES CM credential before taking the Dynamic Compression CMS exam?

Yes. The Certificate Member Specialist (CMS) exams build on the RSES Certificate Member (CM) credential, so you must earn the CM first. The CMS then certifies advanced, specialty-level knowledge in dynamic compression.

What topics does the Dynamic Compression CMS exam cover?

It covers centrifugal compressor theory, dynamic versus positive-displacement compression, surge and choke with capacity control by inlet guide vanes, lubrication and oil systems, shaft seals and bearings, purge units and non-condensables, and centrifugal chiller operation and troubleshooting.

How is the Dynamic Compression CMS exam different from the RSES core CM exam?

The core CM exam covers broad HVACR fundamentals, while the CMS Dynamic Compression specialty drills into centrifugal/dynamic compressors and chillers — impellers, diffusers, surge and choke, purge units, and chiller troubleshooting that the general exam does not test in depth.

What does the RSES CMS Dynamic Compression exam cost?

RSES typically charges around $35 for a CMS specialty exam for members, and CM membership is required. Check rses.org for current member and non-member fees, which may change.

Is this free RSES Dynamic Compression practice as good as paid prep?

Our 100 practice questions cover the same dynamic-compression domains as the CMS specialty, with a teaching explanation for every answer plus free daily AI tutor interactions. All content is free forever and updated for 2026.