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Chapter 1. Condensing Units
The compressor is often described as the heart of a refrigeration system. However, for systems up to a certain capacity, refrigeration specialists often like to use a compressor integrated into a condensing unit. Air-cooled condensing units - which are the exclusive subject of this article - generally consist of a compressor (or sometimes two compressors), a condenser with one or two fans, an electrical terminal box, and optionally high-pressure and low-pressure switches and a receiver.
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Preassembled Modules
There are also water-cooled condensing units, in which the air-cooled condenser with fans is replaced by a water-cooled condenser. Coaxial condensers are the preferred choice for relatively small condensing units, but they gradually give way to shell-and-tube condensers and plate heat exchangers as the capacity increases. The entire module is pre-configured with the tubing necessary for the refrigeration circuit, and the electrical connections are fed to a separate terminal box. This means that part of the refrigeration system is pre-assembled and does not have to be put together on site.
This yields distinct time and cost savings without reducing flexibility, since it is still possible to use a wide variety of evaporators and regulators.
Fans
The size of the condenser depends primarily on whether it is fitted with one or two fans. If there are two fans, which are normally arranged next to each other, the condensing unit is somewhat wider. If there is one fan, the base plate of the condensing unit is usually very compact. As a result, the complete unit is somewhat higher. Fan motors are available in single-phase and three-phase versions, with the vast majority of devices being fitted with 230-V, single-phase fan motors in actual practice.
There are two main reasons for this. As single-phase compressors are used in relatively small condensing units, a corresponding type of fan motor is also used. However, a 230-V fan motor is often used even with three-phase compressors operating from a 400-V supply voltage, since variable-speed control can be implemented less expensively for a single-phase motor. Pressure-led variable speed controls with phase-angle control are very widely used in refrigeration systems, and they can usually be retrofitted in standard condensing units as an option. However, it is necessary to pay attention to suitability for phase-angle control operation (or the insulation class of the fan motor), since fan motors become warmer with phase-angle control than without, which can lead to problems - especially during summer operation. The direction of air flow in a condensing unit is always suction through the condenser coil, and thus from the fan toward the compressor.
This ensures better air contact with the air-cooled condenser. If the fan does not rotate in the desired direction, this can be corrected with a 400-V fan motor by swapping two of the phases. With a single-phase fan motor with a capacitor, the direction of rotation can also be changed by swapping those two leads from the motor where also the two leads of the capacitor are connected. In case of doubt, consult the wiring diagram in the terminal box of the condensing unit.
Chapter 4. Multi-refrigerant Capability
This leads to the question of whether these differences in the refrigerating capacity of a condensing unit due to the refrigerant that is used can be utilised commercially. The answer is 'yes'. The first benefit is obvious: if the condensing unit is designed to be suitable for use with various refrigerants, such as R404A, R507, R407C and R134a, availability from the refrigeration wholesaler is better than if a different unit is used for each type of refrigerant. This benefit is especially important with relatively large condensing units, since they take up more space in the warehouse of the wholesaler (and space is expensive). The second benefit is increased customer flexibility. For example, if a new system is being built and the customer is not yet sure whether an expansion will be carried out in the coming years, it is advisable to install a condensing unit with multi-refrigerant capability and operate it with R134a. If the system is installed in a petrol-station shop, for example, and the shop is enlarged five years later, it is possible to extract more cooling capacity by simply changing the refrigerant to R404A without having to replace the condensing unit.
This is by no means detrimental to the customer, since R134a refrigerant has a very good energy rating and only rates second to R404A (or R507) in terms of general-purpose use.
Fully Equipped Condensing Units
Beside conventional condensing units with a basic configuration, fully equipped condensing units are presently very popular. With conventional units, features such as fan speed control, a weatherproof housing, compressor protection, an emergency stop switch, a dryer and a sight glass can only be retrofitted as options, they are already included in a fully equipped conden-sing unit outdoor module.
These units are usually more compact than standard condensing units in terms of the required installation area. A standard unit provides time savings during installation, and the time savings with such a ready-to-install unit are of course even greater. The growing popularity of these complete units also has a positive effect on pricing. As a result, these units have become a genuine alternative, especially for demanding end customers. System builders espe-cially appreciate features such as integrated, pressure-controlled fan speed regulation and a built-in electrical cabinet with an emergency stop switch.