Providing Solutions. Simplifying Regulation.

RETA Book 2 Chapter 9 – Two Stage Systems

Peter Thomas, P.E. | Resource Compliance
 Providing Solutions. Simplifying Regulation.

CIRO Exam Content

Fundamentals and Introductory Concepts 30 Questions
Safety Standards & Procedures 12 Questions
Electricity 12 Questions
Valves and Controls 12 Questions
Two-Stage Refrigeration System Operations 10 Questions
Other Refrigeration System Operations 13 Questions
Heat Flow of Refrigeration Systems 15 Questions
Troubleshooting 6 Questions
Measuring System Performance 14 Questions
 Providing Solutions. Simplifying Regulation.

Objectives of This Chapter

1. Discuss two stage systems
2. Understand the way intercoolers work
3. Understand why it costs less in energy to produce refrigeration with two
stages of compression instead of one
 Providing Solutions. Simplifying Regulation.

Low Evaporator Temperatures

• Lower evaporator temperature leads to lower evaporator pressure
• Greater distance between the suction and discharge pressure leads to
increased compression ratio
• Higher compression ratio leads to increased work (heat) that the
compressor must put into a pound of gas in order to compress it to the
required discharge pressure
 Condensation

Expansion
Evaporation

+20ºF Suction
90ºF Condensing
Single Stage
 Condensation

Expansion Evaporation

-40ºF Suction
90ºF Condensing
Single Stage
+20/-40ºF Suction
90ºF Condensing
Single Stage
 Providing Solutions. Simplifying Regulation.

Intro to Two Stage Compression

• Greater compression ratio = Higher discharge temperature
+20/-40ºF Suction
90ºF Condensing
Single Stage
 Providing Solutions. Simplifying Regulation.

Intro to Two Stage Compression

• Greater compression ratio = Higher discharge temperature
• Cannot cool to extremely low temperatures in a single stage because we
would break down the oil
 Providing Solutions. Simplifying Regulation.

Intro to Two Stage Compression

• Assume -40°F evaporator temperature, 95°F condensing
 Single stage → 20:1 Compression Ratio
 Huge economic penalties for single stage
 Large compression ratio → higher heat of compression
• White Board Comparison
Providing Solutions. Simplifying Regulation.
 Providing Solutions. Simplifying Regulation.

Booster vs High Stage Compressors

• Booster Compressor
 The first stage of a multi-stage system
compressor is a booster compressor.
 Receives vapor from the evaporator.
 Discharge from this compressor goes to the
intermediate pressure.
 Characterized as a high volume, low
compression ratio machine.
Providing Solutions. Simplifying Regulation.
 Providing Solutions. Simplifying Regulation.

Booster vs High Stage Compressors

• High Stage Compressor
 The second stage of a multi-stage system.
 Receives vapor that has been compressed by the
booster compressor.
 Discharge from this compressor goes to the
condenser.
 The rotary vane compressor cannot be used as a
high stage compressor, only booster.
 Providing Solutions. Simplifying Regulation.

Physical Characteristics of Low Temp Operation

• As evaporator temperature is lowered
 Discharge temperature increases
 Oil degrades faster
 Efficiency decreases
 Power consumption increases
 Oil return from evaporators becomes difficult
 Compression ratio increases
 Theoretical Suction Gas
Evaporator Discharge Compression Discharge Cubic Feet Hp / Ton
Psig/Temp °F Suction Psia Psia Ratio Temperature per Ton (est.)

45 / 30°F 59.74 195.8 3.28 190°F 2.04 0.96

33 / 20°F 48.21 195.8 4.06 204°F 2.52 1.16

24 / 10°F 38.51 195.8 5.08 228°F 3.14 1.37

16 / 0°F 30.42 195.8 6.44 251°F 3.95 1.75

9 / -10°F 23.74 195.8 8.25 275°F 5.01 1.89

4 / -20°F 18.30 195.8 10.70 295°F 6.47 2.38

2” hg / -30°F 13.90 195.8 14.09 325°F 8.44 2.84

9” hg / -40°F 10.35 195.8 18.92 355°F 11.13 3.43

 Providing Solutions. Simplifying Regulation.

Physical Characteristics of Low Temp Operation

• Screw compressor can operate up to 20:1 compression ratio because of
oil cooling
 3.1 Bhp/Tr at -40°F/95°F, Single Stage
 2.1 Bhp/Tr at -40°F/95°F, Two Stage
 Providing Solutions. Simplifying Regulation.

Compare Single-Stage and Two-Stage

• Given:
 Low stage evaporating temperature is -28°F with no superheat
 Condensing temperature is 95°F and there is no liquid subcooling at the
condenser
 Interstage pressure is 33 psig
• Single Stage: 284 Bhp
• 2-Stage: 97 Bhp & 130 Bhp = 227 Bhp
 Providing Solutions. Simplifying Regulation.

Two-Stage Compression Systems

Suction
Bypass Pressure = 181 psig
Valve Temperature = 95 0 F
Evaporator
Pressure = 0 psig Shell
110 0 F 210 0 F
Temperature = -28 0 F and Coil
Intercooler

Condenser

Receiver

High Temperature Second (high)

Low Temperature First Stage 95 0 F
Expansion Valve Stage Compressor
Expansion Valve (Booster)
Compressor
Pressure = 33 psig
Temperature = 20 0 F
 Providing Solutions. Simplifying Regulation.

Two Type of Intercoolers

1. Shell & Coil
 Providing Solutions. Simplifying Regulation.

Two Type of Intercoolers

2. Flash Type High Stage Suction

High Pressure
Liquid

Booster Discharge

HEV

FLASH
INTERCOOLER

Saturated Liquid (at

intermediate pressure)
to Low Temperature
Evaporator
 Providing Solutions. Simplifying Regulation.

Internally Compounded Compressors

• One compressor capable of doing
both stages of compression
 Providing Solutions. Simplifying Regulation.

Two Stage Systems and Non-Condensables

• Low stage almost always operates in a vacuum
• Leakage at joints, valve stems, and shaft seals is very common and hard
to detect
• Air accumulates in the condenser
• Water accumulates in the evaporator
• Solution…
 Seal cap valves
 Auto-purger
 Providing Solutions. Simplifying Regulation.

Cascade Systems
• Systems below -100°F
• Two different refrigerants
• Why cascade systems?
 -100°F means 27.4” Hg vacuum for ammonia
 Lubricants are stiff at -100°F
 -100°F/95°F would be a 159:1 compression ratio for single stage
 Providing Solutions. Simplifying Regulation.

Cascade Systems

Ethane Evaporator Heat Exchanger

Pressure = 9 psig (Ethane Condenser and
Temperature = -110 0 F Ammonia Evaporator)

Ammonia
Condenser

Nh3 Receiver

Ammonia System
Ethane Receiver High Temperature Side
Ammonia
Pressure = 16 psig
Ethane System Temperature = 0 0 F
Low Temperature Side
 Providing Solutions. Simplifying Regulation.

Cascade Systems
• One refrigerant will be at a higher pressure than commonly used
refrigerants (no vacuum)
• Low temp receiver typically requires a storage pressure of 650 psig
(ethane)
 Providing Solutions. Simplifying Regulation.

Summary
• Two stage systems require less Bhp/Tr than single stage systems
• Low temp systems require large overall compression ratios
• By using two stages of compression the function of de-superheating and
the function of medium temperature cooling units can be combined as
suction to the second stage of compression
 Providing Solutions. Simplifying Regulation.

Summary
• The overall compression ratio is not the sum of the two individual ratios,
but rather the product of the two ratios
• Discharge temp from each stage of compression must be suitable for the
oil used in the compressors
• Low stage compressors are high volume, low HP
• High stage compressors have lower volume, high HP
 Providing Solutions. Simplifying Regulation.

Summary
• The intercooler is used to desuperheat the discharge gas from the first
stage of compression
• More than one type of intercooler is available
• Purger is very important in low temp application
• A cascade system is not a 2-stage system. It is two single stage systems.
 Providing Solutions. Simplifying Regulation.

Questions?
pthomas@resourcecompliance.com