David Dunn, commercial director of Toshiba Air Conditioning (UK), discusses the advance of inverter technology in VRF and looks at the development of twin rotary compressors.
COMPRESSOR TECHNOLOGY is an area of continual development in the VRF (variable refrigerant flow) field of cooling/heating/heat recovery equipment, with emphasis on improving reliability, performance and control.
The twin rotary compressor has two fixed compression chambers and two off-centre rollers mounted on the same shaft.
Compression is achieved as the roller moves eccentrically in the housing/chamber and refrigerant gas is drawn in through a suction port in the housing and discharged after compression via a discharge port.
The rollers are offset from each other so that any out-of-balance forces are counterbalanced. This kind of compressor has been used for some time, but usually as a single roller.
The use of two rollers results in less wear on the bearings and chamber wall, meaning smaller bearings. The lubrication demand is also reduced, saving weight and making this kind of compressor suitable particularly to low-speed operation.
The working parts remain fully lubricated as a result.
A major development in VRF systems is the use of two inverter driven twin rotary compressors in every VRF outdoor unit. Both of the compressors are therefore variable speed.
Comparing this development of two inverter compressors with previous systems where the compressors used were either a) all fixed speed or b) a combination of fixed speed and variable speed in each unit delivers significant benefits. They are significantly lighter, utilise less footprint and deliver much greater energy efficiency.
Other key advantages of this type of compressor technology are :
· Either compressor can be the lead, enabling the load to be spread evenly between the two compressors, reducing wear and increasing their life-span
· Power surges at start up are greatly reduced, as each compressor can now be started at minimum load through the inverter drive. The starting current is as low as 1Amp. Mechanical and electrical stresses are also greatly reduced
· Where modular systems are used, the system demand is spread across all the inverter compressors in each outdoor module. This also results in the system being able to use all of the available condenser coils in all of the various modules which further serves to enhance the efficiency.
· In the unlikely event of a compressor breakdown, the second compressor can operate in isolation. Should both compressors in the outdoor unit fail, in a modular configuration (ie 56kW, 2 x 28kW), the second outdoor unit can continue the operation of the system.
Digital control of inverter drive
Toshiba is able to achieve extremely high energy efficiency through its innovative twin-rotary compressor and DC Hybrid Inverter Technology.
This, combined with the Pulse Amplitude Modulation (PAM), Pulse Width Modulation (PWM) and vector IPDU control, allows the compressor to run as low as 7.5r/s (450 rpm) and as high as 55r/s (3300 rpm). This enables closer control of the system demand at both partial and full load conditions.
With inverter compressors there is little lowering of oil surface before and after starting.
One of the problems with all oil-lubricated compressors is how to keep the oil in the compressor throughout operation.
Reduced pressure
Fixed-speed compressors reduce the pressure in the compressor housing as it starts-up.
The result is any refrigerant absorbed in the oil being released, which has the effect of lowering the oil level.
This can result in the bearings and wearing surfaces not being lubricated sufficiently. Another benefit is that twin rotary inverter compressors require less oil.
Other issues also need to be considered; any oil leaving the compressor will enter the system, which can result in reduced efficiency of heat exchangers in addition to insufficient lubrication in the compressor that can affect the life-span of the compressor.
Taking into account the fact that Toshiba's compressors have up to 90% less oil (compared with non rotary compressors) in the system in the first place, and the fact that the company uses exclusively inverter driven twin rotary compressors in all of its VRF range, Toshiba units have up to 90% less oil carry over into the refrigeration circuit than other units.
Toshiba units also utilise an advanced oil management system. This monitors the level of oil in each compressor and when an oil shortage is detected in either compressor, oil is automatically transferred from another compressor.
The advantages of utilising Toshiba's VRF technology ensures excellence in quality, the latest use of technology and the assurance that energy efficiency is maximised.
Contact Toshiba for free VRF selection software and demonstration.