digitalscrolltechnology

8/7/2019 digitalscrolltechnology

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DIGITAL SCROLL TECHNOLOGY

Simon Wang Ph.D and Arup Majumdar

Copeland CorporationAsia Pacific Headquarters

10 th Fl., Pioneer Building, 213 Wai Yip Street, Kwun Tong, Kowloon, Hong Kong

ABSTRACT

This paper describes a new technology for capacity modulation Copeland DigitalScroll TM . This new technology isfundamentally different from the traditionalinverter technology. It is unique, reliable andvery simple to apply. The capacitymodulation in the Digital Scroll compressor is achieved by the time averaging of aloaded state and unloaded state. The

paper explains how these states are physically obtained in the Digital Scrollcompressor. The paper also describes theadvantages of using the Digital Scroll andthe future advances of this technology byusing R410A and vapor injection.

KEY WORDS

Copeland Digital Scroll, capacitymodulation, oil return, electromagneticinterference, R410A, vapor injection.

INTRODUCTION

Global warming and ozone depletion areserious issues facing the environment andthe HVAC industry has a challenging task tolimit this damage. For the air-conditioningindustry, the system efficiency is very

important because it determines the amountof energy that is being consumed for coolingor heating. Many countries are creatingminimum efficiency grades, to ensure thatthe industry continually strives towardsdeveloping more efficient systems andreducing the needs of energy. The increasing

buying power of consumers globally is alsogenerating a large demand for thedevelopment of air-conditioning systemsthat provide a higher level of comfort than

that provided by the standard fixed capacitysystems. These trend, economic andenvironment related, is placing a growingdemand for the development of variablecapacity systems. A variable capacitysystem offers unique benefits it has higher seasonal energy efficiency and is able tocontrol the room temperature to a muchtighter band, thus ensuring higher customer comfort.

Technologies that have been used to

achieve modulation so far have been thevariable speed compressor driven by theinverter, multiple compressors along with

bypass circuits (hot gas and liquid), 2 speedcompressors and also 2-step capacity controlcompressors. In this paper, we are going totalk about a new concept of achievingcapacity modulation that has been developed

by Copeland Corporation, after doing many


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years of research. This new technology,Copeland Digital Scroll TM, is unique, simple,and extremely reliable and has the potentialto make fundamental changes in the market.

DIGITAL SCROLL TECHNOLOGY

The beauty of this technology is its inherentsimplicity. The standard Copeland scroll hasa unique feature called axial compliance.This allows the fixed scroll to move in theaxial direction, by very small amounts, toensure that the fixed and orbiting scrolls arealways loaded together with the optimalforce. This optimal force holding the 2scrolls together at all operating conditionsensure the high efficiency of Copelandscrolls. The Digital Scroll operation buildson this principle. The physical hardware of the Digital Scroll is explained with the helpof Figure 1. A piston is fixed to the top

Figure1: Digital Scroll Compressor

SolenoidValve

ModulationChamber

Lift PistonAssembly

Bleed Hole

Spring

scroll to ensure that when the piston movesup, the top scroll also moves up. There is amodulation chamber at the top of the pistonthat is connected to the discharge pressurethrough a bleed hole of diameter 0.6 mm.An external solenoid valve connects themodulation chamber with the suction side

pressure. When the solenoid valve is in thenormally closed position, the pressure oneither side of the piston is discharge pressureand a spring force ensures that the twoscrolls are loaded together. When thesolenoid valve is energized, the dischargegas in the modulation chamber is relieved tothe low pressure. This causes the piston tomove up and consequently the top scrollalso moves up. This action separates thescrolls and results in no mass flow through

the scrolls. De-energizing the externalsolenoid valve again loads the compressor fully and the compression is resumed. Itshould be noted that the movement of thetop scroll is very small 1.0 mm andconsequently the amount of high-pressuregas that is bled from the high side to the lowside is very little.

The Digital Scroll operates in twostages the loaded state, when thesolenoid valve is normally closed andunloaded state, when the solenoid valve isopen. During the loaded state thecompressor operates like a standard scrolland delivers full capacity and mass flow.However, during the unloaded state, there isno capacity and no mass flow through thecompressor. The 2 states of the DigitalScroll are shown in Figure 2.

Figure2: Modulation Mechanism

Loaded Unloaded

At this stage, let us introduce the concept of a cycle time. A cycle time consists of aLoaded State time and Unloaded Statetime. The duration of these 2-time segmentsdetermine the capacity modulation of thecompressor. Example: In a 20 seconds cycletime, if the loaded state time is 10 seconds

and the unloaded state time is 10 seconds,the compressor modulation is (10 seconds x100% + 10 seconds x 0%)/ 20 = 50%(Figure 3). If for the same cycle time, theloaded state time is 15 seconds and theunloaded state time is 5 seconds, thecompressor modulation is 75%. Thecapacity is a time averaged summation of the loaded state and unloaded state. Byvarying the loaded state time and unloaded


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state time, any capacity (10%-100%) can bedelivered by the compressor.

Figure3: Concept Of Cycle Time

FixedCycle Time

VariableCycle Time

15secs

7.5 secs

7.5secs

15secs

7.5secs

7.5 secs 15 secs

15 secs

30secs

0% Cap.

100% Cap.

Pressure Traces

Due to the loading and unloading of thescrolls, the suction and discharge pressurefluctuates during any cycle. During theloaded state, the suction pressure startsdropping and the discharge pressure startsincreasing. During the unloaded cycle, thesuction pressure starts increasing and thedischarge pressure starts decreasing (Figure4). Figure 4 shows the suction and discharge

pressure with a 12 seconds cycle time and50% modulation, ie. 6 seconds loaded and 6seconds unloaded. In order to conserve the

thermal mass and to keep the flow of liquidto the evaporator, it is experimentallydetermined that a receiver helps. Ex.: A 5liters receiver for a 6 HP system is a goodchoice. This fluctuation in pressures has no

effect on the reliability of various systemcomponents like copper tubes, valves etc.

Power Consumption During the loaded state the compressor consumes full load power. But during theunloaded state, the motor runs freely withoutany load. The power consumption is verylittle, about 10% of the full load power.Fluctuation in power consumption ischallenging to measure. A power meter thatis able to integrate the power consumptionover a period of time is the ideal instrumentto record the total power drawn. This low

power consumption during the unloadedstate ensures a high efficiency for the DigitalScroll.

Cycle TimeCycle time is an important parameter in theDigital Scroll operation. The same capacitycan be obtained by using different cycletimes. For example, 50% capacity can beobtained by having a 7.5 seconds loadedtime and 7.5 seconds unloaded time.Alternately, 50% capacity can be achieved

by 15 seconds loaded time and 15 secondsunloaded time (Figure 3). Copeland hasexperimentally determined the ideal cycletime for each capacity modulation. Thecycle time and capacity modulation isinversely related. Lower the capacitymodulation, longer should be the cycle time(Figure 5) At those ideal cycle times; thesystem energy efficiency is the maximum.

4 .8

8 .3

11 .8

15 .3

0 2 4 6 8 1 0 12 14 16 1 8 2

T i m e ( S e c )

0

A m p s

D is chargePre s sure

R e c e i v e r

Evapora to r. In

E v a p o r a t o r O u t P r e s s u r e

( K g

/ C m

2 )

F i g u r e 4 : D is c h a r g e A n d S u c t io n P r e s s u r e T r a c e s

S u c t i o nP r e s s u r e


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Figure 5: Cycle Time

10

11

12

13

14

15

16

17

18

0% 10% 20% 30% 40% 50% 60% 70% 80% 90% 100%

Optimum Cycle Time (sec)

% Capacity

PERFORMANCE OF DIGITAL SCROLL

Wide Capacity Range

Capacity from 10%-100% is an unmatchedoutput from the Digital Scroll. This widecapacity output is continuous and seamless.This is an enhancement over the inverter technology, where capacity outputs can only

be achieved in steps. The seamless deliveryof capacity also ensures that there is a verytight control on room air temperature. Awide capacity output also contributes to ahigh seasonal energy efficiency of thesystem. Start stops of the compressor consumes more energy. The wider capacity

output of the Digital Scroll reduces thenumber of start- stops.

High Seasonal Energy EfficiencyFor modulated systems, single pointefficiency is not the right measure of theefficiency of the system. The seasonalenergy efficiency ratio (SEER) has to becalculated to get a good idea of the savingsfrom operating the system year- round. TheDigital Scroll performance has beenevaluated as per the JIS & ARI standard and

it shows excellent SEER. The SEER advantage becomes even greater for atandem configuration a Digital Scrollcompressor in tandem with a fixed speedcompressor. At full load capacity, when bothcompressors are operating, the system has ahigh EER and at 50% capacity, when onlyone compressor is operating at full load, thesystem operates at high EER also.

Oil ReturnOil return is a major issue in variablecapacity multiple evaporator systems.Current technologies use an oil separator and/or complicated oil return cycle to ensureoil return after some period of operation.The Digital Scroll is an unique compressor it does not require an oil separator or an oilreturn cycle. There are 2 factors that makethe oil return easy. Firstly, the oil leaves thecompressor only during the loaded cycle. Soat low capacities, very little oil leaves thecompressor. Secondly, as explained before,the compressor operates at full capacityduring the loaded cycle. The gas velocity inthe loaded cycle is sufficient to return oil

back to the compressor. Our testing hasshown that oil is able to return back to thecompressor in the worst operating condition

low modulation, 100 meter pipe length and30 meter elevation (with standard oil traps),

both straight and reverse elevation.

DehumidificationDehumidification is necessary to ensurecustomer comfort and this becomes moreimportant during low modulation operation.In the inverter system, at low modulation,the compressor operates at a lower frequency. This reduces the mass flow of refrigerant and results in a higher suction

pressure. This results in a higher SensibleHeat Factor (SHF). The Digital Scrollcompressor provides very gooddehumidification because it operates at alower suction pressure than the inverter. Asmentioned before, during any modulationoutput, the compressor operates at fullcapacity during the loaded part of the cycle.This full capacity operation results in alower average suction pressure that leads toa lower SHF.

Electromagnetic InterferenceElectromagnetic interference is a major issue in inverter driven systems. In manycountries, particularly Europe, there arestrict regulations on the amount of electromagnetic interference that any devicecan emit. The Digital Scroll systemgenerates negligible electromagneticinterference because the loading andunloading of the scrolls is a mechanical


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operation. This unique feature not onlyeliminates the need for expensiveelectromagnetic suppression electronics, italso adds to the reliability and simplicity of a Digital system.

Rapid pull downQuick pull down of room temperature andquick adjustment to demands are essentialfor customer comfort. Digital Scroll,

because it can transition from 100% capacityto 10% capacity or vice versainstantaneously by changing the loaded andunloaded cycle time, can react to thechanges in system demand much faster without having to pass through intermediatespeed changes as is necessary in the inverter systems.

ReliabilityReliability of system and electronics is anissue in developing markets in Asia. In aninverter system, the electronics is typicallycomplicated and exposing this complicatedelectronics to the uncertainties in installationand the extremes of weather, results inreliability issue. The situation is madefurther complex by the usage of various

bypasses hot gas bypass and liquid bypass.We will discuss about these bypass circuitsshortly, but the bottomline is that complexsystems have a higher probability of failure.The Digital Scroll system is fundamentallysimple. Figure 6 shows the typicalelectronics for the outdoor unit controller.

Figure 6: Typical Electronic Controller

Refrigerant Bypass CircuitsHot gas bypass and liquid bypass is used inmost of the present technology options.

These protection devices are required because the compressor is not able to godown to very low capacities. The Digitalscroll is able to go down to as low as 10%capacity and so these bypasses are notrequired, resulting in cost savings andsystem simplicity.

CompactnessA smaller footprint leads to lower materialcost, packaging, storage and shipping cost.A Digital Scroll system because of itssimplicity, can be designed to be morecompact and the savings in space can be asmuch as 30% over present technologyoptions.

APPLICATION FLEXIBILITY

The Digital scroll compressor can be used invarious applications single evaporator or multiple evaporators. A 6Hp Digital scrollcompressor can be used as the building

block to go to higher capacities tandemconfiguration. A fixed speed scrollcompressor of 6 Hp can be used in tandemwith a 6 Hp Digital scroll to get a tandemcapacity of 12 Hp. This concept can beextended to expand to much higher capacities in the commercial market.Designing of modular systems of higher capacities has been a challenging task withinverter technology, especially due to the oilreturn sensitivity. The Digital Scroll can beused easily to build the modular systems.

ALTERNATE REFRIGERANTS

Currently the Digital Scroll is available inR22 and R407C. Copeland Corporation isdeveloping this technology with R410A, aswe believe that R410A will be the futurerefrigerant of choice for the residential andlight commercial market. R410A offersmultiple advantages higher system energyefficiency, better Total Equivalent WarmingIndex, better dehumidification due to higher heat transfer coefficient, enhanced heat

pump heating performance, no fractionationor glide issues and cost optimization throughsmaller copper tubes and lesser refrigerant.


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FUTURE ADVANCES WITH DIGITALSCROLL TECHNOLOGY

An enhancement to the Digital Scrolltechnology is the Digital Scroll EVI(Enhanced Vapor Injection). The basicvapor injection cycle concept is the increasein evaporator capacity through enhancedsubcooling (higher delta H) instead of largedisplacement (higher mass flow). Theconcept is similar to a two stage cycle with asubcooler, but with the inter stage vapor injected back to the same compressor. For the scrolls, it is easy to inject the vapor during the staged compression process.There are several advantages of vapor injection higher efficiency than larger displacement at the same capacity (sinceaddition of the vapor injection power is donefrom the intermediate pressure and not thesuction pressure), better gain than using a

passive liquid suction heat exchanger (Figure 6 and 7). With HFCs like R410A,there is more favorable performance gain(Table 1).

0

10

20

30

40

50

60

-34.5 -28.9 -23.3 -17.7 -12.2 -6.6 -1.1 4.4 10

26.632.237.840.5

43.348.954.460.065.5

Tevap ( C)

Tcond( C)

Approx.% Capacity

Gain

Figure 6: Typical % Compressor Capacity Gain With Vapor Injection

0

2

4

6

8

10

12

14

16

18

-34.5 -28.9 -23.3 -17.7 -12.2 -6.6 -1.1 4.4 10

26.632.237.840.543.348.954.460.0

Tevap ( C)

Tcond ( C)

A p p r o x .

% E E R G a

i n

65.5

Figure 7: Typical % Compressor EER Gain With Vapor Injection

%Btuh Base Vapor Injection %BtuhGain Delta h Delta h / F SubCooling

R22 27% 77.9 98.9 0.3755

R134a 32% 73.8 97.3 0.444

R410A 33% 83.4 110.8 0.458

R404A 45% 58.4 85.0 0.625

Table 1: Refrigerant Effect On Vapor Injection

SUMMARY

Capacity modulated systems are a fastemerging need of the market and the Digitalscroll is a very good option for suchapplications. The Digital scroll system offersunique advantages an output capacity thatexactly meets the demand, better humiditycontrol at low capacity, a wide capacityrange, easy oil return even with long pipe

length and ease of application. There arefewer system components, no issue of electromagnetic interference and thus asimplified system architecture.

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