Bitzer UK’s engineering manager Kevin Glass looks at the application and technical aspects of small screw compressors
APPLICATIONS of small screw compressors in the food industry and supermarkets, processing technology as well as marine refrigeration and air conditioning markets are common. The latest designs of this style of compressor are also available with integral oil separators that are suitable for factory-produced chillers and cooling units. The main application of conventional compressors with an external oil circuit is in parallel systems in medium temperature and low temperature installations using any of the current range of refrigerants including ammonia.
Semi-hermetic screw compressors are designed for commercial and industrial installations and their modern rotary design suits universal applications. Efficient and robust, they have a compact design, low weight, offer high performance and low capital investment. Additional advantages include high cooling capacity and a high COP, as a result of economiser operation. Built-in capacity control and long-term durability, thanks to the patented bearing chamber lubrication system, are also key benefits. They are especially suited for parallel operation with up to six compressors using only one oil separator. Recently the capacity has been extended and is now from 18.5 to 105kW (nominal motor power).
For series production of chillers and air conditioning systems, semi-hermetic compact screw compressors are an ideal solution. The integrated oil reservoir and separator system reduces the installation time significantly and allows the use of the compressors in minimal space applications. Typically, Bitzer’s CSH range provides single compressors with motor powers up to 300HP and meets the demand of the air conditioning industry for larger units. With the industry demands for ever increasing capacity, it is now possible to supply a single compressor giving a capacity of nearly 1MW. All of the CSH range of compact-screws are supplied with “dual option” capacity regulation which can be regulated either stepless or with steps in the range of 25% to 100%.
Today’s hermetic compact screws are especially suited for application in railway air conditioning systems. They are customised for a typical rail coach and with speed control have a wide operating range. Their compact dimensions, low weight and very quiet running characteristics also meet the high demands of installation in roof spaces or under the floor. A number of models are available to suit most requirements within the railway industry.
Design of screw compressors
Screw compressors belong to the category of positive displacement machines as do reciprocating compressors. Designs historically resulted in a lower limit of displacement for screw compressors of 300m3/h. In the early 1980s, however, significant progress was made in screw rotor profile geometry as well as in production techniques, resulting in the mass production of much smaller, highly efficient screw compressors. This technology has advanced even further and today’s screw compressors have a lower limit of approximately 80m3/h displacement and the upper limit is being pushed higher all the time and currently stands at 910m3/h. Efficiency is also being increased.
Important design features of current small screw compressors include a twin-shaft design – with newly developed rotor profiles specially optimised for today’s design of screw compressors. Generously dimensioned moving parts and bearings including a robust tandem-thrust bearing arrangement with oil supplied by pressure differential, from an HP oil separator are key features. Open drive versions of the screw compressors are suitable for a wide range of industrial and marine applications where design criteria demand the use of ammonia, or in explosion-proof areas and where the electrical supply is unreliable.
Single or multi-stage capacity control is available with small screw compressors. This is achieved by hydraulically operated pistons, allowing automatic start unloading and as a result provides some protection against over-compression/liquid slugging.
Further advanced design features include an internal oil circulating system and a patented pressure reduction system for the high side bearing chamber which reduces the thrust load and provides the highest possible oil viscosity where it is needed most.
Parallel systems are simple to design when using small screw compressors as they have a common oil separator, common oil reservoir and a common oil return line leading to individual oil system components. Furthermore, compressors of differing capacities can be used easily to obtain the most suitable balance to the required capacity of the plant at both the highest and lowest load conditions.
Oil requirements
The main requirements of oil for screw compressors include the lubrication of HP and LP bearings, shaft seal and rotors. The sealing of gaps between rotors and rotors to rotor housing is essential. Oil will also cool the compressor and control the discharge gas temperature whilst also contributing to lower noise levels.
The three main types of oil cooling generally used are liquid injection (very limited application); external cooling which is the most simple and most commonly used; thermosyphon, which is very effective but requires exact calculation and installation. This last option normally needs the compressor manufacturer’s input.
Cost considerations
When it comes to initial investment costs, judgements are often made on the alleged higher costs when using screw compressors. These, however, only consider the cost of the components and true comparisons are not made. For example, the cost of pipe runs is reduced when using a lower number of larger compressors and economised screw systems have a lower mass flow. In low temperature applications, reductions of 40% are seen in mass flow due to the economiser system. This allows the reduction in pipe cross section and solenoid valve sizing which will give further cost savings.
The CSH semi hermetic screw compressor
Commissioning and installation
When commissioning screw compressors, a number of factors need to be considered. The installation of an oil flow switch, filter and solenoid valve is important although further design effort has removed some of these requirements on new designs by incorporating these devices inside the compressor. Oil charging considerations, direct charging into the oil separator and the oil volume also need to be considered. Only the manufacturer’s approved lubricants should be used. A leak test is essential and effective evacuation must be ensured by, for example, using generously sized additional connections.
When refrigerant charging, specific checks need to be made including the oil level and functioning of the oil heater and liquid charging into the receiver/condenser. Before starting, the oil level, oil temperature and safety devices must be checked together with the set point values of timing relays.
To ensure continued efficient performance, routine checking and servicing of screw compressors, including the checking/change of oil filters, should be undertaken with different regimes for HCFC/HFC refrigerants compared with ammonia. The oil solenoid valve should also be checked and replaced if the diaphragm is deformed which could allow oil to enter the compressor at standstill.
When it comes to repairing a screw compressor, repair works on site should only be carried out if the exact fault is known due to the very minimal number of serviceable parts inside. Possible repair works could include exchanging the check valve, exchanging the pressure relief valve, exchanging protection devices, exchanging the shaft seal (open tyres); checking the thrust bearing clearance along with slight adjustment. If in doubt, the compressor should be changed.
Bitzer UK
01908 642966