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

What is VRF? Variable Refrigerant Flow

Heat Pump VRF Systems COOLING HEATING

Heat Recovery VRF Systems Outdoor Unit or Water-source Unit

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

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

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

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

Accumulator

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

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.

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

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

Oil Separator

Condenser Air or Water Refrigerant changes from gas to liquid

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

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

Linear Expansion Valve (LEV) Controls refrigerant superheat or sub-cooling at evaporator Modulates by “pulsing” from 0-2000 Used on indoor units and in BC controller

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

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

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

Indoor Unit Cooling Coil Sensor (TH23) Heat Exchanger Airflow Sensor (TH21) Note: TH21 can also be located at the remote controller 41-500 Pulse Sensor(TH22) LEV 1 1 1 8 7 1 1

“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

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

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

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

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

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

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

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

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

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

Solenoid Valves Block of valves used to control refrigerant direction

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

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 - 131 ft (CU to Indoor) CU above Indoor units - 164 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

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

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

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

Sight Glass

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

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

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

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

Indoor Unit Control Auto Control

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

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