- Digital scroll compressor technology
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Digital scroll compressor technology improves the performance of a scroll compressor. Inside a scroll compressor, there are 2 components called scroll sets. The 2 scrolls mesh against each other in the compressor. There are several pockets that are formed when the scrolls mesh with each other. The pocket of compression on the outside (which is the longest and traps the most refrigerant) moves inwards as the scrolls orbit. As the pocket of gas moves progressively towards the center, it gets smaller in size and gets compressed more and more. Finally, at the center, where the size of the pocket is the smallest, the gas is discharged at a high pressure.
There is a unique feature in the Copeland Scroll called axial compliance that is the basis of Digital Scroll. When the 2 scrolls are meshed with each other, they have to be held in the vertical direction with an optimal force. If the scrolls are held too tightly, then the frictional losses during scroll orbiting will be high, resulting in lower COP. But if the scrolls are held together lightly, then there will be leakage of refrigerant from the high pressure side to low pressure side through the scroll tips and will lead to lower COP. This means that there should be an optimal force that has to be used to hold the 2 scrolls together to ensure the highest EER from the compressor. The axial compliance feature of the Copeland scroll allows the fixed scroll to move in the axial direction, by very small amounts, to ensure that the fixed and orbiting scrolls are always loaded together with the optimal force. The Digital Scroll operation builds on this principle.
Contents
Digital scroll
The beauty of this technology is its inherent simplicity. During the normal compression process, the 2 scrolls are always held together with the optimal force in the vertical direction. The floating seal maintains a constant even pressure on the scroll tips. Now if the fixed scroll, through some mechanism, is made “unfixed” and is lifted by only 1 mm, there would be very little gas compression even though the motor and the orbiting scrolls are moving. This is the simple mechanism of the Digital Scroll.
The fixed scroll on the top is moved up by 1 mm (in order to get no compression) and pushed down and meshed with the orbiting scroll (in order to get compression). The up and down movement of the fixed scroll is achieved through a pressure differential/spring arrangement inside the compressor and is actuated by an external solenoid valve.
The Digital Scroll operates in two stages – the “loaded state”, when the solenoid valve is normally closed (0 V to solenoid coil) and “unloaded state”, when the solenoid valve is open (220 V to solenoid coil). During the loaded state the compressor operates like a standard scroll and delivers full capacity and mass flow. However, during the unloaded state, there is no capacity and no mass flow through the compressor and during the unloaded stage the compressor consumes only a small percentage of the full load power. The amount of power consumed at this time can be calculated the same way a fully loaded compressor is based on the amount of torque requirements from mass and the speed of the compressor. The net result of kWh usage therefore becomes a summation of the time spent in the loaded and unloaded state. Typically as a comparison to an inverter controlled compressor the difference is less than 2% more kWh usage. So in a 10 seconds cycle time, if the loaded state time is 2 seconds and the unloaded state time is 8 seconds, the compressor modulation is (2 seconds x 100% + 8 seconds x 0%)/ 10 = 20%. If for the same cycle time, the loaded state time is 5.0 seconds and the unloaded state time is 5.0 seconds, the compressor modulation is 50%. The capacity is a time averaged summation of the loaded state and unloaded state. By varying the loaded state time and unloaded state time, any capacity (10%-100%) can be delivered by the compressor.
So the simple formula to calculate cooling capacity: Cooling Capacity = ((loaded time x 100%) + (unloaded time x 0%))/total time. So if you need 10% capacity, the loaded time is 1 second and unloaded time is 9 seconds. If this loading and unloading is repeated, the average capacity is 10%. If 15% capacity is required, we need to repeat a 1.5 seconds loading and 8,5 seconds unloading cycle. So, all it needs is a time signal to the solenoid valve on top of the compressor, to make the compressor provide variable capacity seamlessly from 10 – 100%.
Advantages
Wide capacity range
Capacity from 10%-100% results in an unmatched output from the Digital Scroll. This wide capacity output is continuous and seamless and that ensures that there is a very tight control on room air temperature.
A wide capacity output also contributes to high seasonal energy efficiency of the system. Frequent start-stops of the compressor consume more energy. Wider capacity output of the Digital Scroll reduces the number of start-stops.
High seasonal energy efficiency
For modulated systems, single point efficiency is not the right metrics when measuring the efficiency of the system. The Integrated Part Load Value (IPLV) has to be calculated to get a good idea of the savings from operating the system year-round.
The wide capacity range which results in lesser start stops provides a high IPLV for Digital Scroll based systems. The IPLV advantage becomes even greater for a tandem configuration – a Digital Scroll compressor in tandem with a fixed speed compressor. At full load capacity, when both compressors are operating, the system has a high EER and at 50% capacity, when only one compressor is operating at full load, the system operates at high EER also.
Oil return
Oil return is a major issue in variable capacity multiple evaporator systems. Current technologies use an oil separator and/or complicated oil return cycle to ensure oil return after some period of operation. The Digital Scroll is a unique compressor – it does not require an oil separator or an oil return cycle. There are 2 factors that make the oil return easy. First, the oil leaves the compressor only during the loaded cycle. So at low capacities, very little oil leaves the compressor. Second, as explained before, the compressor operates at full capacity during the loaded cycle. The gas velocity in the loaded cycle is sufficient to return oil to the compressor.
Dehumidification
Dehumidification is necessary to ensure indoor room comfort and this becomes more important 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 Sensible Heat Factor (SHF) and creates issues with dehumidification.
The Digital Scroll compressor provides very good dehumidification because it operates at a lower suction pressure than the inverter. As mentioned before, given any modulation output, the compressor operates at full capacity during the loaded part of the cycle. This full capacity operation results in a lower average suction pressure that leads to a lower SHF.
Negligible electromagnetic interference
In many countries, particularly Europe, there are strict regulations on the amount of electromagnetic interference that any device can emit. The Digital Scroll system generates negligible electromagnetic interference because the loading and unloading of the Scrolls are mechanical operations.. This unique feature not only eliminates the need for expensive electromagnetic suppression electronics, but also adds to the reliability and simplicity of a digital system.
Rapid pull down
Quick pull down of room temperature and quick adjustment to demands are essential for customer comfort. Because Digital Scroll can provide transition from 100% capacity to 10% capacity or vice versa instantaneously by changing the loaded and unloaded cycle time, it can react much faster to the changes in system demand without having to pass through intermediate speed changes as required in the inverter systems.
Reliability
Reliability of system and electronics is an issue in developing markets in Asia. In an inverter system, the electronics is typically complicated. Exposing this complex electronic system to uncertainties of installation and extreme weather conditions results in reliability issues.
The situation is made more complicated by using various bypasses – hot gas bypass and liquid bypass. Adding hardware to the circuit makes the systems more complex and susceptible to reliability issues.
Compactness
Smaller footprint leads to lower material cost, packaging, storage and shipping costs. Because of its simplicity, a Digital Scroll system can be designed in such a way that it can be more compact and the savings in space can be as much as 30% over traditional methods of air conditioning.
References
- Development and testing of a multi-type air conditioner without using AC inverters Shih-Cheng Hu *, Rong-Hwa Yang http://www.ntut.edu.tw/~f10870/myweb12/journal_paper/p20.pdf
- Experimental study of performance of digital variable multiple air conditioning system under part load conditions by Dongliang Zhang, Xu Zhang, Jun Liu, The College of Mechanical Engineering, Tongji University, Shanghai 200092, China http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V2V-50P4N4W-3&_user=10&_coverDate=08%2F03%2F2010&_rdoc=1&_fmt=high&_orig=search&_sort=d&_docanchor=&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=b83ea89c9acedaa338a166465f41f0a6
- The greening of IT: how companies can make a difference for the environment By John Lamb page 141
- Basic Refrigeration and Air Conditioning By Ananthanarayanan, McGraw-Hill Education
External links
Categories:- Compressors
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