Science universe: Physics articles

  New and replacement refrigerants For the long term replacement of CFCs and HCFCs, various manufacturers have developed products designed to meet the requirements of the industry. In October 1990 ICI was the first company to introduce HFC 134a, an alternative to CFCs used in refrigeration and air conditioning. Since then other refrigerants have been introduced and marketed by ICI as Klea 134a. Klea 32, the second in the Klea range, went into production in 1992. As these refrigerants were successful a number of blends were introduced to the range including Klea Blend 60 and Klea Blend 66, ozone benign refrigerants based on Klea 32 and Klea 134a. These are the alternatives to HCFCs. Each of these is non-toxic, non-flammable and has a good heat transfer property. All have undergone extensive trials, both in laboratory and systems in the field. These refrigerants have a zero ODP and are currently not subject to regulation. European Union Regulation member states agreed to cea...

Ozone depletion The refrigeration industry has been aware of ozone depletion and global warming in so much as it has, during the late 1980s, paid particular attention to the ozone depletion. As a result atmospheric science and control regulations of undesirable substances have led to the introduction of a new range of refrigerants. The environmental issue of global warming became more important. Global warming results from atmospheric phenomena commonly described as the ‘greenhouse effect’. To simply describe global warming it has been deter- mined that infrared energy radiated from the earth’s surface is absorbed by certain atmospheric gases. These gases are classified as greenhouse gases or GHGs. The GHGs allow solar energy to penetrate the atmosphere but do not permit the resultant infrared radiation from the earth to be released. The long term effect of this is a gradual increase in the atmospheric and surface temperature of the earth. Numerous natural and man-made gases which ...

Commissioning of refrigerating systems Commissioning can be defined as the advancement of an installation from the stage of static completion to full working order to specific requirements. A number of different contractors may have been involved: refrigeration fitters, electricians, plumbers, builders, carpenters and shop fitters. Close cooperation among all concerned is essential. It is therefore desirable that the commissioning of a plant be carried out under the guidance of a single authority; that usually means the service or installation engineer. The customary safety procedures must be observed at all times. During commissioning the following will prevail: 1 Use only the specified refrigerant (manufacturer’s data). 2 Substitute refrigerants should only be used if approved by the equipment manufacturer. 3 Do not re-use refrigerant which might be contaminated. Contaminants During installation the system should be kept as clean and dry as possible, with the least e...

  Drive motors Before installing or replacing a drive motor, it must be established that the voltage and frequency of the motor (as stated on the nameplate) agree with those of the supply. A motor must not be connected to a supply outside the range stated on the nameplate. If connected to a low voltage supply it may not start. If it does start it may not run to its full speed; the windings will then overheat, and may burn out if continued starting attempts are made by the action of the overcurrent protector. It is therefore important that the overload protector is of the correct rating if a replacement is made. Those fitted to open drive motors should be rated as near as possible to 125 per cent of the nameplate current. Installation details Earthing The motor frame must always be connected to the local earth circuit. It is most important to ensure that the flexible earthing strip between the motor frame and the cradle is not broken and that the correct connection is ma...

Electrical circuit protection Every electrical installation must be protected from current overload. This is achieved by locating a protective device at the commencement of each circuit in the form of a fuse or circuit breaker. Their function is to protect the circuit conductors (cables) and not the appliance or user. Fuses Three types of fuse are used (Figure 100); the rewireable or semi-enclosed fuse; the cartridge fuse or fuse link; and the high rupturing capacity (HRC) fuse. Rewireable fuse This comprises a fuse holder, an element and a fuse carrier. The holder and carrier can be made of porcelain or bakelite. The fuse holder is colour coded as follows: 45 A green 30 A red 20 A yellow 15 A blue 5 A white This type of fuse is popular for domestic appliances and small commercial units because of cheapness and ease of replacement. It is not recommended for commercial refrigeration duty because of these disadvantages: 1 The fuse carrier can be loaded with the wr...

  Drive belts and couplings Drive belts Correct alignment of the flywheel and drive pulley, and correct tensioning of the drive belt(s), are most important during the installation or replacement of belt(s) or motor. Incorrect alignment and tensioning can result in excessive belt wear, loss of compressor efficiency, and motor failure due to faulty motor or compressor drive shaft bearings. Alignment This may be carried out with a piece of string or preferably a straight edge (AB in Figure 91), aligning the drive motor pulley to the compressor flywheel. It is essential that the areas X and Y are aligned accurately. Any variation in the pulley or flywheel thickness must be compensated for when aligning the flywheel and pulley faces (area Z). Selection All drive belts are subject to bending and compressing, which will inevitably result in stretching. Centrifugal force also adds to belt stress. The distance between the flywheel and the pulley plays an important part in the am...