Pressure-Enthalpy and the Variable Refrigerant Cycle

Joe Cefaly

OEM Applications Manager - CEM, LEED AP

Mitsubishi Electric USCooling & Heating

What is VRF?

Variable

Refrigerant

Flow

Heat Pump VRF Systems

COOLINGHEATING

Heat Recovery VRF SystemsOutdoor Unit or Water-source Unit

Basic Refrigeration Cycle

RED = Higher Temp/Pressure

BLUE = Lower Temp/Pressure

Basic Refrigeration Cycle

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

Basic Refrigeration Cycle

RED = Higher Temp/Pressure

BLUE = Lower Temp/Pressure

Expa

nsio

n Cy

cle

Condensing

Evaporating

Com

pres

sion

Cycle

Liquid and Gas

Liquid Gas

Subcooled Liquid

Superheated Gas

Pres

sure

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 CompressorIntake

Compression

Discharge

Fixed Scroll Orbiting Scroll

AssembledScrolls

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

Compressor

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

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

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

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)

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

Evaporator

Refrigerant Distributor

Suction Header

Feeder Tubes

Fins

Evaporator Tubes

Heat and moisture is removed from the air streamRefrigerant evaporates

Evaporator

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

HeatExchanger

LEV

Sensor(TH22)

Sensor (TH23)

41-500 Pulse

Sensor (TH21)

Note: TH21 can also be located at the remote controller

Airflow

Indoor Unit Cooling Coil

Expa

nsio

n Cy

cle

Low psi

High psiCondensing

EvaporatingTH 22

Com

pres

sion

Cyc

le

Refrigerant in liquid and gaseous states

Sub-cooled refrigerant

Super-heated refrigerant

Superheat Differential

TH 23

Pres

sure

“Smart Coil” Operating Parameters

Enthalpy

Cooling Coil – Max Capacity

Numbers are approximate for explanation purposes

High Fan Speed

Full capacity operationSaturated Suction temperature 45FSuperheat 10FEntering air temp 80FDB / 67FWBLeaving air temp 55FDBAir flows over approximately 3 rows of active coil surface for high humidity removal and sensible temperature drop.Humidity is continuously being removed

LEV

Entering Air

High Fan Speed

Cooling Coil – Reduced Capacity

Numbers are approximate for explanation purposes

Reduced Capacity (66% of maximum)Saturated Suction temperature 45FSuperheat 15FEntering air temp 77FDB / 64FWBLeaving air temp 60FDBAir flows over approximately 2 rows of active coil surface. This reduces humidity removal but all air still flows over active coil surface continously.Humidity is continuously being removed

LEV

Entering Air

High Fan Speed

Cooling Coil – Low Capacity

Numbers are approximate for explanation purposes

Reduced Capacity (33% of maximum)Saturated Suction temperature 45FSuperheat 20FEntering air temp 75FDB / 63FWBLeaving air temp 65FDBAir flows over approximately 1 row of active coil surface. This reduces humidity removal but all air still flows over active coil surface continously.Humidity is continuously being removed

LEV

Entering Air

High Fan Speed

Dry Mode Saturated Suction temperature 45FSuperheat 10F or lessEntering air temp 80FDB / 67FWBLeaving air temp 50FDBAir flows over approximately 3 rows of active coil surface for high humidity removal and sensible temperature drop.High amounts of Humidity are being removed

Entering Air

LEV

Cooling Coil – Dry Mode

Numbers are approximate for explanation purposes

Low or Extra-Low Fan Speed

Low or Extra-Low Fan Speed

Cooling Coil – Dry Mode

Indoor LEV

THERMOSTAT ROOM TEMP ON OFF

CALLING 82° or more 9 Minutes 3 Minutes

CALLING 79° - 82° 7 Minutes 3 Minutes

CALLING 75° - 79° 5 Minutes 3 Minutes

CALLING 64° - 75° 3 Minutes 3 Minutes

CALLING less than 64° -- Always

     

SATISFIED 64° or more 3 Minutes 10 Minutes

SATISFIED less than 64° -- Always

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

High-Pressure

Vapor

High-Pressure

Liquid

Low-Pressure

Liquid

Low-Pressure

Vapor

Condenser

Evaporator

Filter Drier

Liquid Line

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

Thank you for your time