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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.