June 7, 2010
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 generally consist of a compressor (or sometimes two compressors), a condenser with one or two fans, an electrical terminal box, an optional high-pressure and low-pressure switches, and a receiver. 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 because it is still possible to use a wide variety of evaporators and regulators.
FANSThe 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, though in actual practice most are being fitted with 115V or 230V, single-phase fan motors.
As single-phase compressors are used in relatively small condensing units, a corresponding type of fan motor is also used. However, a single-phase fan motor is often used even with three-phase compressors 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 airflow 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 three-phase fan motor by swapping two of the phases. If a single-phase fan motor has the wrong direction of rotation, contact the manufacturer for instructions.
RECEIVERA receiver should always be present in the condensing unit when the system uses an expansion valve as a throttling device. If a capillary tube is used for injection in the evaporator - which is usually the case with very simple and small systems - a receiver is not included in the condensing unit. Receivers in condensing units for expansion valve operation are usually mounted upright. This is better for ensuring a reliable liquid reserve, especially if the outlet to the liquid line is taken from the top of the receiver via a dip tube and only a small residual volume of liquid is left in the receiver as a reserve.
In condensing units with relatively large capacity, the receiver outlet can usually be shut off. This simplifies repairs to the refrigeration system that require opening the refrigeration circuit substantially. If the receiver outlet valve is closed, the refrigerant of the system can be pumped into the receiver if it has sufficient capacity. This makes it possible to carry out service work between the receiver outlet and closed suction side of the compressor without having to remove the refrigerant charge from the system.
PRESSURE SWITCHESWhether or not a condensing unit is fitted with high-pressure and low-pressure switches is primarily a question of price.
However, at least a high-pressure switch must always be installed. It is also not recommended to eliminate the low-pressure switch for cost reasons, because if it is not present - for example, in case of loss of refrigerant - the result can easily be a defective compressor. From a business perspective, this is more important than the price of a low-pressure switch.
The high-pressure side of a dual pressure switch has divided refrigeration system builders into two camps for many years already. One camp always uses a pressure switch with automatic reset after a cutout if pressure is returning to normal level. The other camp always uses a high-pressure switch with manual reset after a cutout for restarting the system to ensure that the high-pressure shutdown does not go unnoticed.
INSTALLATIONReliable performance of a refrigeration system with a condensing unit is particularly dependent on the installation. As a rule of thumb, the distance from the wall should be at least equal to the height of the condensing unit, with the direction of the emerging airstream facing away from the wall. This is necessary not only to ensure an adequate supply of cooling air, but also to allow regular cleaning of the coil with compressed air or nitrogen. Installing a condensing unit in a narrow airshaft creates the risk of a short circuit in the air circulation.
This means that the air that has just passed through the coil and been heated is sucked in and drawn through the coil again. This causes a strong increase in the condensing temperature resulting in higher energy consumption and risk of shutting down the unit at high ambient temperatures, when the cooling capacity is most needed. If the unit is installed outdoors, it is recommended to use an optional weatherproof housing.
A mistake that is often made in user-built constructions carried out afterwards by persons not trained in refrigeration systems is severe restriction of the airflow past the unit. Here, care must be taken to ensure that the condenser coil and the opposite side are left completely free. There is no objection to placing closed panels on the other sides or on the top.
COOLING CAPACITYThere is a special consideration with regard to specifying the cooling capacity of condensing units. The cooling capacity is always specified at a particular ambient temperature (usually 90°F). This may appear somewhat unusual, because a specific condensing temperature is normally taken as the basis. For example, the cooling capacity of a compressor is specified for a fixed condensing temperature of 120° or 130°, depending on the standard conditions.
However, if the same approach were used with condensing units, the size of the condenser would not be taken into account. For this reason, the capacity of a condensing unit is measured at a specific ambient temperature (under laboratory conditions). This means that a condensing unit with a larger condenser has a lower condensing temperature, which means it has a higher cooling capacity even though the same compressor is fitted. This also reduces the difference in cooling capacity resulting from using different refrigerants, as compared to considering only the compressor.
In other words, if (for example) you compare the capacity of a condensing unit with HFC-404A to the capacity of a condensing unit with HFC-134a, you will see a smaller difference than if you compared the compressors fitted in the units. As R-404A has a higher volumetric cooling capacity than R-134a, a compressor operating with R-404A has a higher capacity than the same model with R-134a at the same evaporating temperature. However, the condenser of a multi-refrigerant condensing unit will appear larger if R-134a is used instead of R-404A, so that the condensing temperature is lower than for operation with R-404A. This improves the cooling capacity of the condensing unit with R-134a.
In short, in case of doubt, it is a good idea to check the performance figures of the compressor when examining the performance specifications of condensing units.
MULTI-REFRIGERANT CAPABILITYThis 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 utilized commercially. The answer is yes.
If the condensing unit is designed to be suitable for use with various refrigerants, such as R-404A, HFC-507, HFC-407C and R-134a, 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. 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 R-134a. If the system is installed in a gas-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 R-404A without having to replace the condensing unit. This is by no means detrimental to the customer, since R-134a refrigerant has a very good energy rating and only rates second to R-404A (or R-507) in terms of general-purpose use.
FULLY EQUIPPED CONDENSING UNITSBeside 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 filter drier, and a sight glass are often only mounted as an option, whereas they are already included in some fully equipped outdoor condensing units. 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 of a refrigeration system, but the time savings with a ready-to-install unit are even greater. The growing popularity of these complete units has had a positive effect on pricing. As a result, these units have become a genuine alternative, especially for demanding end-users.
Publication date: 06/07/2010