The choice of compressor type is a wide one, and at least two alternatives should be considered before making a final selection.
Compressor capacities may be published as tables or curves as in Figure 10.3 and will be for a given refrigerant and a range of condensing pressures. Similar curves for input power and current (amps) will usually be provided. Limitations
Evaporating temperature (°C) Figure 10.3 Compressor capacity ratings in graphical form
Of application range are also normally given. The compressor data should state the superheat and sub-cooling reference and this should correspond to a standard vapour compression cycle as defined in European Standard EN12900. The standard for the sub-cooling is zero. Several superheat options are given in the standard, and these are intended to represent conditions close to the normal running condition of the compressor. Smaller compressors can be rated at a constant suction gas return temperature which is applicable to remote installation where the gas temperature can be expected rise on its way to the compressor. Ammonia compressors are rated at 5 K superheat, corresponding to flooded evaporator applications.
The reason for these differences is to minimize any error arising from superheat corrections. The data will need to be corrected for the actual superheat, sub-cooling and any non-useful heat pick-up. Changing the superheat can affect the volumetric efficiency and for large changes a correction based solely on change in gas density may be inadequate. The correction for sub-cooling is a straightforward enthalpy ratio which can be made with the aid of a P-h chart or computer. If there are significant pressure drops between the compressor and either the condenser or the evaporator, these may also need to be accounted.
The rating conditions refer to the compressor refrigerant inlet and outlet (suction and discharge). There is no need for the user to consider change of conditions as the gas passes over the motor. All processes occurring within the machine are accounted in the published ratings. For enclosed types the input power refers to electrical input and for open compressors it refers to shaft power.
Semi-hermetic compressor is rated at 10 kW for R404A when evaporating at -35°C, condensing 40°C at standard conditions of 20°C suction gas temperature and zero subcooling. The application will be for 0°C suction gas temperature of which 5 K superheat is usefully obtained in the evaporator and the remainder is non-useful heat gain in the pipes. Sub-cooling is 5 K. Estimate the compressor capacity and the evaporator capacity.
(a) Mass flow correction
Compressor inlet temperature is changed from 20°C to 0°C. From R404A properties using P-h chart, tables or computer, the ratio of specific volume is 146.46/135.25 = 1.083. Assuming the compressor pumps the same volume flow rate:
(b) Correction for enthalpy at the compressor inlet
From R404A properties using p-H chart, tables or computer,
Suction gas enthalpy at 20°C and 0°C is 392.51 and 375.19 kJ/kg, respectively
Liquid enthalpy at the expansion valve inlet at 40° is 257.77 kJ/kg Evaporator enthalpy difference at rating condition = 392.51 — 257.77 = 134.74 Evaporator enthalpy difference with 0°C suction = 375.19 — 257.77 = 117.42 Enthalpy difference ratio = 117.42/134.74 = 0.871
Compressor capacity corrected for suction temperature change = 10 x 1.083 x 0.871 = 9.43 kW
(c) Correction for enthalpy at the compressor inlet and expansion valve inlet:
Liquid enthalpy at the expansion valve inlet at 35°C is 249.67 kJ/kg Evaporator enthalpy difference at application condition = 375.19 — 249.67 = 125.52
Enthalpy difference ratio = 125.52/134.74 = 0.932 Actual compressor capacity = 10 x 1.083 x 0.932 = 10.09kW
The reduction in suction gas temperature increases the mass flow rate, but because the enthalpy at the compressor inlet is reduced the effect of the change in suction gas temperature alone would be an overall reduction in compressor capacity. However this is more than offset by a decrease in liquid enthalpy due to sub-cooling.
(d) Useful enthalpy difference
Suction gas enthalpy at evaporator outlet, -30°C (5 K superheat) = 350.13 kJ/kg Useful evaporator enthalpy difference = 350.13 — 249.67 = 100.46 Enthalpy difference ratio = 100.46/134.74 = 0.7456 Actual evaporator capacity = 10 x 1.083 x 0.7456 = 8.07 kW
There is a loss of almost 20% capacity due to gas temperature rise in the suction line. Possible ways to reduce this loss include improved insulation (to reduce compressor inlet temperature), bring compressor closer to cooler, or use a suction-to-liquid heat exchanger (to raise suction temperature and thus reduce heat pick-up). The relationship between compressor and evaporator capacities is illustrated in Figure 10.4. (The condenser heat rejection in this diagram is for 100% efficient compression — the actual compressor power input should be used in calculations.)
As previously mentioned, the published compressor capacity refers to conditions at the compressor suction and discharge. If the pressure drop in the lines is significant, the evaporating and condensing temperatures at the compressor suction and discharge pressures must be taken, rather than those in the heat exchangers, when referring to compressor capacity data.
When selecting the compressor, a first guess must be taken for the condensing temperature, and this might be 15 K above the summer dry bulb for an air — cooled condenser or 12 K above the wet bulb temperature in the case of water or evaporative cooling. The balance condition between the evaporator and the compressor can be visualized in a graphic solution, superimposing the basic rating of the cooler on the compressor curves (see Figure 10.5).
ICO 150 3» ISO 3C0 3JC <0 — CO JOG
Figure 10.4 Compressor capacity and evaporator capacity
Figure 10.5 Balance condition between compressor and evaporator
While it is usual to consider only the balance at the maximum summer ambient, the application engineer should be aware of the running conditions in cooler weather. If this is not favourable to the product, some average choice may be made, or an evaporator pressure regulating valve is inserted to prevent
the evaporating temperature dropping too low (see Figure 9.2). A different set of conditions will occur if the compressor has capacity control. A compressor with 50% capacity control may be connected to two equal evaporators, and one of these shut off at half load.
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