Condensing units

The performance of the condensing-unit (compressor and condenser) subsystem could be developed as shown in Fig. 11-96 by superimposing two graphs, one for compressor performance and the other for condenser evaporating effecl . 

FIG. 11 96 Balance points of compressor and condenser determines performance of condensing unit for fixed temperature of condenser cooling fluid (flow rate and heat-transfer coefficient are constant). 

The filled appliance is designed to provide maximum heat transfer. The efficiency can be increased by the use of a condensing unit. 

The steam turbine is operated as a condensing unit when the exhaust steam is condensed as it leaves the turbine. This is usually below atmospheric pressure and often advisable in order to (1) maintain proper plant-wide steam and condensate balance and (2) use all reasonable energy in steam. Figure 14-18A. 

Some manufacturers of air-cooled packaged 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. 

Air-cooled condensers require a large air flow for a given heat rejection duty and the limitation on their use is reached on account of their size and the need to get enough air. Water or evaporative cooling should always be considered as a possibility, except for smaller sizes or where using packaged condensing units. 

Water chillers of these two types are not usually made as single packages with their condensing unit, owing to the bulk of the system and subsequent difficulty of installation. 

The basic condensing unit is a single package comprising the... 

Figure 13.1 Air-cooled condensing unit (Courtesy of Prestcold Ltd)...

Such assemhlies might have the compressor and drive only, for site connection to a remote air-cooled condenser. As such, they are correctly termed compressor units. Compressor and condensing units will he site connected to evaporators, and these components must he matched in capacity (see Chapter 10). 

Example 13.1 In the rating curves for an air-cooled condensing unit shown in Figure 13.3, what is the cooling capacity at an evaporating temperature of - 25°C and with air onto the condenser at 25°C By how much does this drop with condenser air at 35°C ... 

Example 13.2 In the rating table shown in Table 13.1 for a water-cooled condensing unit, what is the cooling duty at -20°C evaporation, with water onto the condenser at 25°C ... 

Since compressor and condensing units do not include an evaporator, they are not complete systems and will not be charged with refrigerant, but may have a holding charge of dry nitrogen, or a little of the... 

Figure 13.3 Capacity curves for AS75 condensing unit...

The size of a packaged unitwill be limited by installation restrictions, both in handling items into place and in the quantity of air required by the condenser. Units up to 50 kW cooling capacity are in common use. 

It is possible to run two or more indoor units from a single condensing unit, with economies in the number and costs of components. Such systems are referred to as multisplits, and several different types of circuit will be encountered. 

The usual split package air-conditioner comprises one condensing unit connected by pipes to one evaporator unit (Figure 13.4). Twin condensing units are made to save on outdoor casings and reduce the number of pieces on a roof or wall. Such twins will be connected in the usual way to two separate indoor units. 

Better balance between cooling capacity and load can be obtained by capacity control of the compressor (s). Targe systems will have a number of compressors, or built-in capacity control on the cylinders. A central condensing unit of this sort may be coupled to several fan-coils, each with its own thermostat and liquid solenoid value. The COP is good at all but the lowest load levels. 

The usual method for smaller units is thyristor speed control of the compressor, and such compressors can be slowed to 40% of full speed or less. Some can also be run at higher speeds than normal, to deal with peak loads, but the COP will fall off at this condition. As many as eight fan-coil units can be connected to one condensing unit, and the installed number can be nominally greater than the compressor capacity, on the basis that not all will be cooling at the same time. 

Rating curves for condensing units (see also Section 13.2) will be for stated entering temperatures of the condensing medium - air or water. These may not go as high as the particular application may demand, and figures must be extrapolated. 

Example 35.4 An air-cooled condensing unit is rated at 13.7 kW on R.22 when evaporating at 5°C and with ambient air at 43°C. Estimate the duty with ambient air at 52°C. 

Example 6. PA-6 from w-aminocaprolic acid in (3.32).59 To a wide-bore test tube (Fig. 3.18b) with a volume of 50 mL fitted with a nitrogen inlet and an outlet with a water-cooled condensing unit and distilled water, oj-aminocaprolic acid... 

To a 50-mL straight-wall three-necked flask (Fig. 3.18b) equipped with a magnetic stirrer, nitrogen inlet/outlet, and condenser unit in a heating block are added 11.29 g of nylon salt and 0.093 g of hexamethylene diamine (2 mol % excess). This mixture is reacted for 2 h at 210° C and 3 h at 270°C. The resultant polymer is colorless and transparent and has a r/rei of 2.54 (1% solution in m-cresol, 25°C). 

Example 12. PA-6,1 from diphenyl isophthalate.65 To a well-dried 50-mL straight-wall three-necked flask with nitrogen inlet/outlet, condenser unit, and magnetic stirrer in a heating block (Fig. 3.18b), 15.1 g of diphenyl isophthalate and 6.15 g of 1,6-hexamethylene diamine are added. The mixture is heated to 190° C over a 1-h period and to 253° C over a further 40 min, a vacuum is then... 

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