New trends in packaged condensing units

Tuesday, 15 May 2012

Condensing units are an important basic product for commercial refrigeration engineering and in recent years there has been a clear trend towards additional features and add-ons. There has been a marked increase in the number of packaged condensing units currently being used by installers. These units come with additional preinstalled components such as dryers, sight glasses, fan speed controls, weatherproof housings and various electronic parts. So let us ask ourselves what such a packaged condensing units needs to be able to do and which innovations are going to be useful.

For any of the equipment used in commercial refrigeration plants, the most important criterion is always to be reliable. No installer wants to sacrifice time, money and customer goodwill with an endless round of warranty repairs – to say nothing of the secondary costs. You can deal with the reliability question early on, when selecting appropriate refrigeration components, by using only packaged condensing units with high-quality components. These guarantee many years of reliable operation and maximum seal integrity. Not only are the individual components quality assured, the packaged condensing units will also have been subjected to tests of its function and tightness. Thanks to these, plant engineers can be certain that this half of their refrigeration plant is leak proof for the foreseeable future and its components, including pipe diameters, are well coordinated.

Modern new-generation packaged condensing units such as the Optyma Plus™ already offer all of that and even take it a step further. An integrated electronic module monitors all the relevant pressures and temperatures inside the condensing unit, namely the suction pressure, condensing pressure, suction temperature, discharge temperature and the ambient temperature. Continually measuring all these parameters enables the regulating and monitoring module to reliably report that the system is functioning properly, or perhaps that there are early signs of a malfunction. An additional sensor can be connected if desired to measure, for instance, the room temperature. All alarm thresholds and delays can be flexibly configured. This means that it can be decided, in consultation with the operator, whether he or she would prefer to call out a service technician whenever there is any doubt – for instance, because the refrigeration plant is of critical importance – or whether this should be done only when it is obviously necessary. Alarms can be sent to an optional external display where the operator can see them, or via a floating output on the monitoring module. This floating output can be used, for example, to activate a red-light alarm on the switch cabinet or pass a general warning to the house utilities management system. For particularly demanding customers there is the possibility of integrating the monitoring module into an Adap Kool™ network based on an MOD or LON bus. This can be implemented locally or with a remote service option.

Additional functions such as the minimum operating time, minimum standstill time and minimum time between two compressor starts are also integrated into the regulation and monitoring module. Standard hermetic reciprocating and scroll compressors may not be started more than twelve times per hour. This means that they may not be activated more often than every five minutes. Once they have started up they should then remain in operation for a certain minimum time, and not simply switch off again half a minute later. These three parameters are important for long-term trouble-free compressor operation and are already integrated. An additional feature that helps avoid overloading the compressor is the discharge temperature protection function. This function is particularly important for scroll compressors and ensures that the compression does not reach an excessively high end temperature. If the maximum value is actually exceeded then the fan speed will automatically be increased to its maximum value. This normally has the effect of reducing the condensing temperature and hence the compression discharge temperature. If, after a configurable time, this measure has not been successful, then the compressor will be switched off and will not be switched back on until the discharge temperature at the compressor outlet has fallen by at least 10K.

Just as important as its reliability is the energy efficiency of the packaged condensing unit. Refrigeration units are always among the main energy consumers in any business enterprise, so they offer plenty of scope for making savings. In particular, intelligent regulation of the crankcase heater and condensing pressure enabled additional savings to be made for the new-generation Optyma Plus™. When coupled with the very latest compressor technology, flow-optimised fan blades and progressive microchannel heat transfer, these measures produced 20% savings in energy consumption compared to the class average.

Measurements taken in the field have shown that the crankcase heater is switched on for only half the time needed by conventional systems. The regulator module evaluates the ambient temperature and the suction gas temperature in order to determine whether the crankcase heater should be powered continually, intermittently or switched off altogether. This function is active only while the compressor is at a standstill. The greater the difference between the ambient temperature and the suction gas temperature, the less extra heat has to be supplied (or none at all), and the slope of this relationship can be modified at any time.

The regulation of the condensing pressure works by factoring the external temperature and the current target value into the continual recalculation of the current reference value for the condensing temperature and hence the fan speed. The resulting reference value will be appropriate to the configured target value only until the ambient temperature rises too high. When this happens the reference value is automatically decoupled from the current target value and adjusted upwards. It is also possible, for instance for night-time operation, to specify an increment to the target value so that the fans will run more slowly and therefore be even quieter. The current reference value for the condensing temperature is always a compromise between the lowest possible value for the condensation and moderate fan speeds. A low condensing temperature will generally achieve energy savings by reduced compressor power consumption, slower (or stopped) fans and less noise emitted by the condenser fan.

Most new-generation Optyma Plus™ condensing units are suitable for use with more than one refrigerant. One advantage of this is that making a single condensing unit suitable for a variety of refrigerants, such as R404A, R507 and R134a, ensures better availability from your refrigeration wholesaler. It greatly reduces the number of necessary types compared to what would be required if each unit were suitable for only one refrigerant. This advantage is particularly important for large condensing units, because the necessary warehouse space is that much greater and corresponding more costly. The second advantage is their increased flexibility. For example, if a new refrigeration plant is to be built and the customer is not yet sure whether or not it will have to be extended in a few years time, it will be advisable to install a condensing unit with multi-refrigerant capability and use R134a. If, in a few years, the service station shop needs to be expanded, it will then be possible to increase the cooling capacity just by changing to the refrigerant R404A, without having to replace the condensing unit. This is in no way contrary to the customer’s interests, because the refrigerant R134a has a very good energy profile and could be considered inferior to R404A (R507) only as regards its universal applicability.

After decades of using finned copper-piping condensers, heat exchanger technology is undergoing a revolution with air-cooled packaged condensing units. The trend now favours microchannel heat exchangers. Microchannel condensers are much lighter and more compact than the same capacity finned copper-piping condensers, which is extremely convenient for these packaged condensing units’ transportability and handling generally. A further contribution to the reduced weight of these heat exchangers is the smaller volume of refrigerant needed. The difference is about 30%, which is quite considerable and also offers immediate savings in material costs at commissioning. In countries that levy special taxes on classic refrigerants or impose strict limits on the volumes used this point may weigh more heavily than you at first think. A further advantage of microchannel heat exchangers is their excellent corrosion resistance. This is particularly due to their exclusive use of aluminium, which entirely prevents the corrosion caused by the formation of galvanic elements when two or more different metals are used. The necessary conditions for this were easily met by the aluminium/steel and copper in conventional finned copper-piping heat exchangers, and it greatly reduced these systems’ useful life, particularly under adverse environmental conditions.

Another advantageous point is the stronger fins of microchannel technology. In systems with air-cooled condensers, deformed fins are more the rule that the exception, but this doesn't have to be the case: microchannel heat exchangers are extremely resistant to dents. Their fins cannot simply be crushed by an incautious hand, and even a very incautious hand will not injure itself. This means that neither their appearance nor a loss of airflow volume will detract from the high quality of new-generation Optyma Plus™ condensing units – not even after years of operation.

Refrigeration technology would be inconceivable without condensing units with hermetic reciprocating compressors. Their wide range of possible applications makes them extremely versatile and they have demonstrated their reliability and long life millions of times over.

Reciprocating compressors are also practically unbeatable for small-capacity applications and deep-freezing. That is why new-generation Optyma Plus™ units (for normal refrigeration up to a piston displacement of 34 and deep-freezing models except of two biggest models in H4) are equipped with this compressor technology. However, the first choice for applications with medium to high evaporation temperatures and refrigeration capacity is modern scroll technology, and this has placed MLZ refrigeration scroll compressors at the core of these MBP HBP packaged condensing units. Their particular features include low-vibration operation and low noise emissions. To ensure optimum quietness in operation, every compressor used in new-generation Optyma Plus™ units – unlike those in standard condensing units – is housed in a separate, insulated compartment. This, together with the fan speed regulator and flow-optimised fan blades, means that wrangling over excessive noise emissions is a thing of the past.

Modern packaged condensing units also offer excellent serviceability. The new-generation Optyma Plus™ is designed with three service doors, plus full temperature and pressure information that is accessible without the need for external measuring devices, and also uses standard components such as flare dryers that can be replaced without soldering.

The three service doors offer rapid access to all the internal components of these packaged condensing units. They are also designed for frequent use and are completely free of noise from metal-on-metal vibrations. The additional access hatch for the fan compartment was integrated to facilitate checking the fan mechanism and cleaning the condenser from the inside during regular maintenance. In classic finned copper-piping condensers the coil is generally cleaned in the direction of flow; the recommendation for microchannels is to clean it in the opposite direction because dirt will tend to settle before the coil instead of penetrating the coil as in conventional condensers.

The service feature “full temperature and pressure information” makes use of the numerous sensors that are integrated into new-generation Optyma Plus™ units. Service technicians can easily read off all pressure and temperature values on the regulating and monitoring unit without having to fetch a manometer or temperature measuring kit from their vehicle. This feature is particularly useful when carrying out a brief system check. This type of check can also be carried out from an optional external display mounted somewhere other than on the condensing unit (e.g. for new-generation Optyma Plus™ units installed on roofs the external display can be mounted in a switch cabinet within the building). The regulator’s alarm memory can also be very useful for maintenance work or troubleshooting. For instance, if it shows a cluster of low pressure warnings the technician can straightaway look for a refrigerant leak or a problem with the defrost or evaporator fan. As well as when servicing, the regulation and monitoring module also assists with commissioning, because it has all the appropriate values already set up. You only need to preselect the refrigerant and the model is ready for use. You can modify some of the other parameters if you wish, but this is not absolutely necessary. This feature saves time when the system is commissioned.

To round off the optimised serviceability of these new-generation Optyma Plus™ units, they are constructed from standard components for which replacements can be obtained from any refrigeration wholesaler and are thus rapidly to hand when required. They also use flare dryers (with solder adapters) that can rapidly be replaced without the need for any soldering.

Modern complete condensing units are optimised as regards reliability, energy efficiency and serviceability. They offer high-quality components plus intensive monitoring to eliminate damage and downtime, intelligent regulation of the crankcase heater and condensing pressure, and they ensure that the service engineer always has all the information he or she needs.