A condensing unit is a single package comprising the compressor, the condenser (either air — or water-cooled) mounted on a base plate or frame, and all connecting piping, together with the necessary wiring and controls to make the set functional (Figure 13.1). Condensing units generally include a liquid receiver and are ready for site connection to an evaporator. Accessories such as pressure cut-out switches, liquid line drier, sight glass and fan speed control may be included, and the specification should be carefully checked. They are built in sizes from less than a kilowatt to more than 100 kW cooling capacity and may incorporate more than one compressor. Air-cooled types may have two or more fans. Sometimes
Figure 13.1 Air-cooled condensing unit, 2-cyl semi-hermetic compressor, suitable for several refrigerants (Bitzer)
Figure 13.2 Outdoor air-cooled condensing unit, scroll compressor, suitable for several refrigerants (Emerson Climate Technologies)
The condenser is separate in which case the packaged part is termed a compressor unit. Units intended for outdoor application have weatherproof housings and may be suitable for either wall or floor mounting (Figure 13.2).
The condenser is matched to the compressor, and cooling capacity data refer to air or water temperature onto the condenser as detailed in Chapter 6. Since compressor and condensing units do not include an evaporator, they are not complete systems and are not charged with refrigerant, but have a holding charge of dry nitrogen, or a little of the refrigerant gas to maintain a slight positive pressure for transit. Suction and liquid interconnecting lines and electrical connection is installed on site.
Ratings for condensing units refer to entering temperatures of the condensing medium — air or water; traditionally presented as curves (Figure 13.3)
And now most commonly found from manufacturers’ computer selection programmes. The standard conditions for rating condensing units are given in European Standard EN13215, 2000. An estimate of condensing unit capacity at an ambient condition different to the one provided can be made graphically as indicated in Chapter 10.
Some manufacturers of air-cooled condensing units offer a range of condenser sizes for each compressor, and these should be closely compared in terms of higher duty and lower running costs. A small condenser will result in a higher — condensing temperature and therefore a drop in the refrigerating effect, since the liquid enters the expansion valve hotter, and a decrease in volume pumped due to the lower volumetric efficiency. There will also be an increase in the drive motor power.
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