1. Pressure-Enthalpy and the Variable Refrigerant Cycle
Joe Cefaly
OEM Applications Manager - CEM, LEED AP
Mitsubishi Electric US
Cooling & Heating

2. What is VRF?
Variable
Refrigerant
Flow

3. Heat Pump VRF Systems
COOLING
HEATING

4. Heat Recovery VRF Systems
Outdoor Unit or
Water-source Unit

5. Basic Refrigeration Cycle
RED = Higher Temp/Pressure
BLUE = Lower Temp/Pressure

6. Basic Refrigeration Cycle
Evaporator
High-Pressure
Vapor
High-Pressure
Liquid
Condenser
Low-Pressure
Liquid
Low-Pressure
Vapor

7. Basic Refrigeration Cycle
RED = Higher Temp/Pressure
Subcooled Liquid
Condensing
Liquid
Gas
Expansion Cycle
Liquid and Gas
Compression Cycle
Pressure
Evaporating
Superheated Gas
BLUE = Lower Temp/Pressure
Enthalpy

8. Accumulator
Provide compressor protection from liquid slugging during startup
Stores refrigerant not required in low load conditions
Assures adequate vapor and oil return

9. Accumulator

10. Scroll Compressor Intake Compression Discharge Orbiting Scroll
Fixed Scroll
Orbiting Scroll
Assembled
Scrolls
Discharge

11. Inverter-Driven Scroll Compressors
What is an Inverter?
A variable speed drive that changes the voltage and frequency being fed to the motor
Think of the inverter as a throttle control
Changes electrical frequency from 60 Hz to a varying range of 15 Hz to 125 Hz
Frequency is affected by:
Number of indoor units operating
Outdoor unit model
Outdoor unit target temps/pressures
Greatly reduces energy usage
To convert AC to DC is to rectify, a device that converts DC to AC is an inverter.

12. Compressor Evaporator High-Pressure Vapor High-Pressure Liquid
Condenser
Low-Pressure
Liquid
Low-Pressure
Vapor

13. Oil Separator Allows long piping runs Eliminates need to trap
Increases operating range

14. Oil Separator

15. Condenser
Air or Water
Refrigerant changes from gas to liquid

16. Segmented Heat Exchangers
Segments use valves
Capacity control
Improves efficiency
Simultaneous heat/cool/heat recovery

17. Condenser Evaporator High-Pressure Vapor High-Pressure Liquid
Low-Pressure
Liquid
Low-Pressure
Vapor

18. Linear Expansion Valve (LEV)
Controls refrigerant superheat or sub-cooling at evaporator
Modulates by “pulsing” from
Used on indoor units and in BC controller

19. Linear Expansion Valve (LEV)
Evaporator
High-Pressure
Vapor
High-Pressure
Liquid
Condenser
Low-Pressure
Liquid
Low-Pressure
Vapor

20. Evaporator Fins Suction Header Feeder Tubes Refrigerant Distributor
Heat and moisture is removed from the air stream
Refrigerant evaporates
Fins
Suction Header
Feeder Tubes
Refrigerant Distributor
Evaporator Tubes

21. Evaporator Evaporator High-Pressure Vapor High-Pressure Liquid
Condenser
Low-Pressure
Liquid
Low-Pressure
Vapor

22. Indoor Unit Cooling Coil
Sensor (TH23)
Heat
Exchanger
Airflow
Sensor (TH21)
Note: TH21 can also be located at the remote controller
Pulse
Sensor(TH22)
LEV 1 1 1 8 7 1 1

23. “Smart Coil” Operating Parameters
Sub-cooled
refrigerant
Super-heated
refrigerant
Condensing
High psi
Pressure
Expansion Cycle
Refrigerant in
liquid and gaseous states
Compression Cycle
Low psi
Evaporating
TH 22
TH 23
Superheat Differential
Enthalpy

24. Cooling Coil – Max Capacity
High Fan Speed
High Fan Speed
Numbers are approximate for explanation purposes

25. Cooling Coil – Reduced Capacity
High Fan Speed
Numbers are approximate for explanation purposes

26. Cooling Coil – Low Capacity
High Fan Speed
Numbers are approximate for explanation purposes

27. Low or Extra-Low Fan Speed
Cooling Coil – Dry Mode
Low or Extra-Low Fan Speed
Low or Extra-Low Fan Speed
Numbers are approximate for explanation purposes

28. Cooling Coil – Dry Mode Fan runs continuously Indoor LEV THERMOSTAT
ROOM TEMP ON
OFF
CALLING
82° or more
9 Minutes
3 Minutes
79° - 82°
7 Minutes
75° - 79°
5 Minutes
64° - 75°
less than 64° --
Always
SATISFIED
64° or more
10 Minutes
We don’t sense humidity. You must either schedule dry mode or manual turn it on from the R/C. Only way to do it is to use a DIDO board to sense humidity and send signal to MNET.
Fan runs continuously

29. Reversing Valve
Reverses refrigerant flow when systems changes between heating and cooling

30. Branch Circuit Controller
Refrigerant “traffic cop”
Gas/liquid separator
Tube in tube heat exchangers
LEVs
Solenoid valves

31. Gas/Liquid Separator Hot gas is used for heating
Liquid is used for cooling

32. Tube-in-Tube Heat Exchangers
Provides sub-cooling in heating mode
Provides super-heating in cooling mode

33. Solenoid Valves
Block of valves used to control refrigerant direction

34. Full Port Valves Recommended at BC controller
Helps with evacuation during install
Isolates branches for maintenance
Allows additional branches for future use

35. VRF Piping Considerations
Keep pipe as straight as possible to avoid pressure drops
Long bends instead of elbows when possible (90° ~ 1ft length)
No refrigerant specialties/accessories/traps (minimize restrictions)
Oil separator and accumulator in outdoor unit
Selection program sizes piping for you
Pipe length = pressure changes = capacity/performance
Vertical difference between units:
CU below Indoor units ft (CU to Indoor)
CU above Indoor units ft (CU to Indoor)
131/196 ft length from BC controller to furthest indoor unit
49 ft height difference between indoor units and/or BC controller

36. Filter Drier Installed in liquid line
Filter foreign material from the system
Desiccant core removes moisture and small dirt collected in refrigerant
Internal screens filter any solid material
NOT Required or recommended on new Mitsubishi systems when installation procedures are followed
Used in replacements

37. Filter Drier Liquid Line Evaporator High-Pressure Vapor High-Pressure
This is an example of where the filter drier needs to be installed in refrigeration system.
Condenser
Low-Pressure
Liquid
Low-Pressure
Vapor

38. Sight Glass In Liquid line
Visual indication whether system is contaminated with moisture
Green is dry, Yellow is wet
Shows bubbles which could indicate problem & assists in charging system
NOT used in VRF equipment
Refrigerant state is constantly changing

39. Sight Glass

40. Piping Considerations
Suction line velocity ~ 1200 ft/min
Sized for minimum capacity
Oil return is #1 concern
Pitch piping towards compressor
Trap with condenser above evaporator
Help with oil return especially during off cycle
Inverted trap with evaporator above condenser
Helps to prevent slugging liquid
Not required with pump down

41. Oil Traps Assist in oil return to compressor
Prevent oil accumulation in evaporator
Installed in suction line when compressor is above evaporator
Not required or recommended in VRF systems

42. Compressor Control Maintains evaporation temperature in cooling
Maintains condensing temperature in heating

43. Heat Exchanger Control
Heat exchanger is separated for capacity control
Sections of heat exchanger are enabled/disabled as required
Condenser fan modulates on air cooled units

44. Indoor Unit Control
Auto Control

45. Indoor Unit Control Modulates LEV and fan speed based on setpoint
Open valve = more flow/capacity
Close valve = less flow/capacity

46. Thank you for your time