Air and refrigeration compressors
Whilst stationary compressors for air are carried out in increasing numbers as srew compressors , air compressors for road vehicles are still being developed in the plunger type design. These have particularly small values of the general specific speed parameter.
The increasing need of energy for air brake systems and height adjustment generate the need for smaller and more efficient designs with higher delivery pressures. As delivery temperatures have to remain below certain limits, the compressors have to work in two stages.
Air brake compressors have to be installed on the side of the engine within limited space which therefore calls for small designs. Therefore mostly compressors with two or three cylinders in line are produced. They are fitted with reed valves, which allow high compressor speeds.
The cooling water of the engine also cools the compressor cylinder and the seat plate. With two stage designs an intercooler can be integrated within the cylinderhead. Because of the direct coupling with the engine an unloading equipment has to be fitted, which reduces the load in idling mode.
A large proportion of the number of piston compressors produced, both in numbers and in installed power, is taken up by refrigeration compressors.
For environmental reasons and due to the greenhouse effect of the refrigeration gases such compressors are carried out as hermetic design. By fitting both compressor and motor within one housing no shaft has to be led to the outside.
One differentiates between the fully hermetic design (with welded housing for example for household refrigerators) and the semi hermetic design (bolted up housings with static seals for larger compressors in the commercial field).
Due to the thermodynamic conditions with refrigeration compressors a multistage compression is in most cases not necessary.
High production quantities require designs with low production costs. As throughputs vary greatly with the application compressors are designed as hyghly standardized construction. (An example is the Octogon- series of Bitzer as shown below).
The housing is made of cast iron with covers on both ends , which are reinforced by ribs on the inside.The housing has ribs on the outside for cooling. The octagon shape of the housing in the area of the compressor permits the fitting of one to four double cylinders, which are bolted to the housing . (Fan type arrangement).
The common shaft of both motor and compressor has two bearings. Both slide bearings are relatively wide and are fitted close to each other. The crank pins are of such a large diameter, that the non split connecting rods can be slid onto the crank on assembly (excenter shaft). For the lubrication a centrifuge tansports the oil from the crank case into the enclosed main bearings. The oil backflow is so designed, that the entrainment of gas is kept to a minimum.
The gas enters into the crank case through a centrally located suction duct . Depending on the installation of the suction filter it is the led either via the motor or directly to the suction chambers of both double cylinders. The gas flow in and out of the cylinder is governed by several reed valves. (A delivery valve is shown). The large valve chambers also acts as internal pulsation dampeners.
In order to reduce the volume flow at part load of the installation, the volume flow to individual double cylinders can be closed via a solenoid valve.