Section 2 Design criteria

2.1.1 The proposed refrigerating plant, insulation and refrigerants are to be suitable for achieving the designed notation temperature. The refrigerating machinery and all components are to operate satisfactorily under the conditions listed in Table 1.3.1 Ambient operating conditionsin Pt 5, Ch 1 General Requirements for the Design and Construction of Machinery.

2.1.2 The properties of steel materials used in refrigerated holds are to be suitable for the proposed notation temperature.

2.2.1 These Rules are applicable to the primary refrigerants in Table 3.2.1 Primary refrigerants and their class of pipe.

2.2.2 Attention is to be given to any statutory requirements, regarding the use of refrigerants, of the National Authority of the country in which the ship is to be registered.

2.2.3 Within the parameters of pressures, temperatures, toxic nature and flammability, the class of pipe to be used with various refrigerants is shown in Table 3.2.1 Primary refrigerants and their class of pipe.

2.2.4 Design conditions as applicable to the classes of pipes are defined in Pt 5, Ch 12, 1.5 Design temperature.

2.2.5 The materials of Class I and Class II piping systems are to be manufactured at a works approved by LR and tested in accordance with the appropriate requirements of Rules for Materials. Particular attention is drawn to Ch 6, 4 Ferritic steel pressure pipes for low temperature service, of the Rules for Materials, where testing requirements for pipes used for low temperature service are given.

2.2.6 The materials of Class III piping system are to be manufactured and tested in accordance with the requirements of acceptable National Specifications. The manufacturer's test certificate will be acceptable and is to be provided for each consignment of materials.

2.2.7 Particulars of refrigerating systems using refrigerants other than those listed will be given special consideration.

2.3.1 A refrigerating unit is considered to comprise a compressor, its driving motor and one condenser. Where a secondary refrigerant, such as brine, is employed, the unit is also to include an evaporator (secondary refrigerant cooler) and a brine pump.

2.3.2 Two or more compressors driven by a single motor, or having only one condenser or evaporator (secondary refrigerant cooler) are to be regarded as one unit.

2.3.3 The refrigerating units of a classed cargo installation are to be completely independent of any refrigerating machinery associated with air-conditioning plant, or any domestic refrigerated installation, or any process plant, unless full details of any proposal have been submitted and approved.

2.4.1 The refrigeration capacity provided is to be sufficient to maintain the temperatures specified in the class notation when operating 24 hours per day with one unit on standby. The plant is to be able to cool down a complete cargo to its carrying temperature within the time specified by the manufacturer. The standby unit may be considered as an operating unit during the cooling down period of a non-precooled cargo. In order to compensate for deterioration of machinery and insulation over the life of the installation, the equipment is to be designed to have at least five per cent excess capacity over that required for maximum design output.

2.4.2 The proposals of both machinery and insulating contractors will be evaluated by LR in determining the theoretical capabilities of the equipment to maintain the duty temperatures. LR will advise the contractors after appraisal of the specification and plans if it is considered that additional refrigeration or insulating effect is required, but the temperature assigned on completion of the capacity heat balance test will be determined from the actual results of the test.

2.4.3 Where the units are not connected in common to all refrigerated chambers, the equipment serving each group of chambers is to comply with Pt 6, Ch 3, 2.4 Refrigeration capacity 2.4.1.

2.4.4 In the case of installations having a large number of small units arranged to serve individual chambers or groups of chambers, the question of standby capacity will be specially considered.

2.4.5 Where only two refrigerating units are fitted, the working parts are to be interchangeable.

2.4.6 Where a refrigerating plant is provided for sub-cooling the liquid refrigerant of other units, but is not arranged for cooling the cargo chambers independently, it will not be regarded as a unit.

2.5.1 The design pressure of the system will be regarded as equal to its maximum working pressure.

2.5.2 The maximum working pressure is the maximum permissible pressure within the system (or part system) in operation or at rest. No relief valve is to be set to a pressure higher than the maximum working pressure.

2.5.3 The design pressure of the low pressure side of the system is to be the saturated vapour pressure of the refrigerant at plus 46°C. Due regard is to be taken of defrosting arrangements which may cause a higher pressure to be imposed on the low pressure system.

2.5.4 The minimum design pressure of the high pressure side of the system (P _dh), is to be 1,11 × P _b, where P _b is an allowance for the compressor high pressure cut-out. P _b is to be at least equal to 1,11 × P _a, where P _a is the condenser working pressure, when operating in tropical zones and equates to the saturation pressure at 46°C.

2.5.5  Design pressures (MPa g) applicable to refrigerants are to be not less than the values given in Table 3.2.2 Pressure limits when condensers are sea-water cooled. The design pressure for other refrigerants and condensing arrangements is to be agreed with LR.

2.5.6 Due to the low critical temperature of carbon dioxide it is inappropriate to determine the design pressure in accordance with Pt 6, Ch 3, 2.5 Design pressures 2.5.3. The proposed design pressure for a carbon dioxide system is to be stated, taking account of the maximum working pressure and the maximum pressure at rest conditions. Where the maximum pressure at rest condition is maintained by the fitting of a supplementary refrigeration unit, condensing the vapour in a holding vessel, supporting calculation is to be provided to show that this can be undertaken with a local ambient temperature of 45°C. The holding vessel is to be thermally insulated to prevent the operation of the relief devices within a 24 hour period after stopping the supplementary refrigeration unit at an ambient temperature of 45°C and an initial pressure equal to the starting pressure of the refrigeration unit. Pt 6, Ch 3, 2.5 Design pressures 2.5.7

2.5.7 Where a carbon dioxide system is designed for hot gas defrosting, due regard is to be given to the possibility of a higher pressure being imposed on the low pressure system. The design pressure for this section of the system shall be 10 per cent above the maximum pressure experienced during defrosting.

2.6.1 Properties of materials used for thermal insulation are to be verified against known standards for the following parameters, as applicable, to ensure that they are adequate for the intended service. The test results are to be made available to LR for approval:

• Closed cell content.
• Density.
• Mechanical properties.
• Thermal expansion.
• Abrasion.
• Cohesion.
• Thermal conductivity.
• Resistance to fire and flame spread.
• Ageing.
• Bonding (adhesive and cohesive strength).

2.6.2 Where the in situ foam type of insulation is proposed, full details of the process are to be submitted for approval.

2.6.3 Where applicable, having regard to their location and environmental conditions, insulation materials are to be:

• suitably resistant to fire;
• suitably resistant to the spreading of flame;
• adequately protected against penetration of water vapour; and
• adequately protected against mechanical damage.