Using an unusual compression technology gives Toshiba's latest range of VRF air conditioning systems a number of advantages, says Neil Hitching, UK projects manager Toshiba Air Conditioning
YOU may have wondered why Toshiba chose rotary compression over scroll and other types for its latest range of variable refrigerant flow (VRF) modular air conditioning systems?
The company says it did so because of the system's increasing popularity. With green considerations in mind, specifiers, contractors and end-users are paying increasing attention to the higher levels of energy-efficiency rotary compression can offer.
Toshiba VRF air conditioning systems use twin rotary compression with dual inverter control. The company says they also offer other advantages such as installation flexibility The layout is not limited in terms of which type of joint or header is allowed to follow which - giving a radius of operation of 65m per unit.
Twin rotary design
The range has two inverter-driven twin rotary compressors. A twin rotary compressor has two fixed compression chambers. An off-centre roller orbits each chamber to squeeze the refrigerant. The two rollers are both mounted on the same shaft but are offset to provide counter balance to each other.
The contact force required between the roller and chamber wall is lowered. This means smaller bearings can be used and lubrication demand is reduced, saving weight and making this type of compressor more suited to low speed operation.
A conventional 2-in-1 scroll compressor however comprises a fixed scroll (spiral) and an oscillating scroll. The oscillating scroll fits within the fixed scroll. Refrigerant is drawn from the outside of the meshing spirals and squeezed towards the centre of the scrolls, thereby pressurising the refrigerant.
Lubrication efficiency
To minimise leakage, the contact force required between the two scrolls is considerable and the scroll surfaces must be lubricated. At low compressor speeds lubrication efficiency is reduced, resulting in increased compressor wear. Systems that use this technology usually operate with one inverter-driven compressor and one fixed-speed compressor.
Each SMMS and SHRM outdoor unit uses two twin-rotary DC compressors. The Toshiba DC twin-rotary compressor is a so-called hybrid inverter-controlled system because it controls both pulse width modulation and pulse amplitude modulation. This enables the compressors to run optimally at all times rather than stopping and starting to match demand calls.
In addition, the company says a vector-controlled intelligent power drive unit brings a 15% efficiency rise in the use of power across the whole range of compressor speeds. It has inverter control on both compressors within each module.
This is unusual but means that where, in larger systems, more than a single pair of compressors is used, the Toshiba dual system can bring compressors in different units on at the same time. This lowers starting currents and reduces wear and maintenance. Dual control will run the fans on more than one unit without starting the compressors, to improve heat transfer efficiency and reduce noise.
The choice of compression type gives the inverter a range of 780-6,660rpm. A single rotary would be only 1,800rpm upwards, and a fixed-speed compressor would clearly give just the fixed 285rpm. This would make it continually switch itself on and off, reducing efficiency.
Oil management
The twin rotary compressor also has an oil management system a quarter the size of Toshiba's earlier twin scroll compressor. The whole body of the compressor acts as an oil separator, slowing the potential passage of any oil around the circuit.
This effectively eliminates the possibility of oil mixing with refrigerant because the suction pipes feed into the compression chambers. Traditionally, the suction pipe might be fed straight into the body of the compressor allowing any condensed refrigerant to fall and dilute the oil, reducing lubrication and lowering efficiency.
Even the potential condensation of refrigerant is reduced because Toshiba's design of inverter control means that at all compressor loads, even the lowest, there is still compressor activity.
The refrigerant charge itself can be 45% less than in R407C equipment to achieve the same output and the other design changes have enabled output to be further improved.
www.toshiba-aircon.co.uk