Towards Improving the Design of Cast Iron Components for Refrigeration Equipment - Part 3
The first two parts of this series dealt with the use of cast iron components in refrigeration equipment. This last part will focus on the construction of such components.
The optimal construction of molded cast iron pieces is established by choosing a functionally, technologically, and economically proper configuration, with suitable wall thickness. The wall of a cast iron piece must fulfill a multitude of conditions, such as wear resistance, being gas-proof, and light in weight (small thickness of the wall). This is why one can speak only about optimal wall thickness.
It is well known that the mechanical properties of cast iron are highly dependent on wall thickness. The variation of the mechanical properties with wall thickness is linear. The variation may be graphically represented in a system of coordinate axes as a straight line, inclined from the right up to the left down, which forms the angle α with the abscissa. The slope of this straight line, tg α = - a, always with negative sign, has been named the influence number, noted by "a". The influence number is determined with great accuracy for high quality cast irons (Fig. 1). It is apparent that the influence of the thickness decreases with increasing traction resistance, while the scattering range diminishes with the increasing quality of cast iron. Refrigeration equipment and mountings with small "a" values are preferred, with the object of higher security.
Since small wall thicknesses show high resistance values, one tries, in a sense, to make thick walls thinner, thus obtaining not only an enlarged resistance but also a decrease of weight. Efforts to reduce the influence coefficient are no good if the design engineer does not take into account this property of founded alloys and does not aim to design at the most advanced equalization of wall thickness. In the construction of cylinder blocks for compressors, the adjoining walls will be made with the object of malleability of the construction, with a view to reducing contraction and avoiding inner stresses.
In order to avoid blowholes and inner stresses it is unacceptable to change wall thickness. The differences in wall thickness ought not to exceed 30 percent. For this reason smooth passages must be created between walls of different thickness.
Configuration of cast iron pieces has a great influence upon removing casting strains, shrink holes, fissures, and especially upon the resistance of the piece to working strain. So the existence of strengthening ribs between the inner and outer walls of the compressor’s cylinder with cooling chamber increases the resistance, but leads to shrink hole formation.
Usually, the configuration of a piece is conditioned by its constructional aim. Its detailed form is given by the technical possibilities to be achieved and by the requirement for as large a configurational resistance as possible. This configurational resistance is obtained by an advantageous design of the piece, as far as uniform repartition of stresses is concerned, and by aiming at agreement with the technical properties of the material. The designer must take advantage especially of high stretching and compression resistance of cast iron in finishing off the form of the piece. Thus exposure of cast irons to flexure, even in their elastic range, is inefficient. Casting may allow the designer to subject as much material as possible to compression and traction, varying the proportion of elements strained in this way.
Besides the form of the cross section, at design, one must take into account the properties of the material and their dependence on solidification conditions. Thus, at the trunk of valves inner stresses will be diminished if the inner walls will have a smaller thickness as compared to the outer ones. But too large a difference of thickness between the walls of the molded pieces leads to the appearance of great differences of temperature during cooling; consequently, casting stresses can reach very high values (up to 30 percent of the fracture strength).
The transit angle from one wall to the other promotes the appearance of shrink holes, owing to the radial arrangement of the crystals. By rounding the edge sufficiently (Fig. 2) these defects do not appear.
Reduction of casting stresses may be performed by applying a heat treatment of annealing, by storage of the pieces for a long time with the object of ageing, or by their vibration. Stresses and failures formed during the casting of pieces are due also to uniform cooling of details. The danger of failures can be prevented by providing cooling flanges.
Cast iron, which is frequently used in refrigerating equipment, must fulfill certain security characteristics for this equipment. The use of cast iron, even of the nodular type, ought not to be extended to tanks and heat exchangers, which work in a flooding regime. At high temperatures (burning), or at low ones, cast iron pieces used in these components may fail.
We have seen that cast iron can be used in conditions such as temperature and pressure. Nevertheless, owing to these safety conditions, it remains best to use cast iron only in constructions such as cylinder blocks, with valves on the side of the intermediate agent of heat exchangers, etc.
How can we ascertain the respective security conditions of these pieces? The resistance of the piece can be verified by a pressure test with a liquid which does not oxidize the surface. If the piece to be controlled will be dried immediately after testing, this liquid can be water. Ensuring that apiece is gas-proof is done by testing the tightness of the material. The penetrating power of compressed air through permeabilities is 10 times larger than that of water, and three times larger than that of petroleum. Testing pressures are chosen according to recommended standards for refrigerating equipment.
Resiliency will be tested only when the behavior of the material at the minimum working temperature is not known. Testing of resiliency is made on Charpy test bars, in conditions identical with the working conditions of the piece. The position of the extraction of the test bar does not influence testing, but it is proper to know the place where the test bar has been extracted from in order to have a clear picture of material toughness.
In the case of air conditioning equipment, the noise level reduction is imposed (noise level under 35 db at a distance of l.5 m). The main source of noise is the motor-compressors, but the passing of gases with speed through differing fittings and valves, etc., are factors too. One of the methods of reduction of the noise level consists in the use of cast iron for these noise sources. This is because the damping capacity of cast iron is two to five times larger than that of steel. In this sense, configuration of the piece is important, too.
Utilization of cast iron in the construction of refrigeration components and equipment requires the investigation of some specific conditions of employing metals at low temperatures (Fig. 3).
Removal of porosities, which are so undesirable in molded pieces used in refrigeration equipment, may be achieved by alloying with nickel or chromium, while increasing of wear resistance is obtained by alloying with silicon. The optimum ratio Si/Ni or Si/Ni+Cr can be approximated quite well as a function of the wall thickness of the piece.
Determining the metal composition and producing molded pieces, as well as examining the working strains of the piece, along with the application of the desired configuration, are conditions to be fulfilled for a successful design.
Publication date: 11/5/2012