**2.3. Cylinder unloading capacity control**

**Figure 3.** Digital scroll piping.

126 Refrigeration

very close capacity modulation results [9].

For example, 20-second cycle and the solenoid are de-energized for 16 seconds and then energized for 4 seconds; the resulting capacity will be 80% [8, 9]. This method provides very wide capacity range with continuous modulation, high efficiency, and very tight temperature control. But higher initial cost is the disadvantage of these methods compared to hot gas bypass method. Also, variable speed and digital scroll capacity controls are compared, and they give

The application of digital scroll compressor in multi-type air-conditioning system was introduced by Hu and Yang [10]. The authors reported the results from the development and performance testing of a cost-effective, energy-efficient, multi-type air-conditioner. In this system, about 75% reduction in power consumption was observed, while a partial load of 17% is occurred. The capacity modulation range of the system is in the range of 17–100%; on the other hand, an AC variable frequency control system has a range between 48 and 104%. In this method, system cost is 20% cheaper than an AC inverter. There are many studies on digital scroll; here, I will give summary of them such as Jiang et al. [11] discussed the control of digital compressor applied in multi-type air-conditioning system. Kan et al. [12] analyzed the application of digital scroll compressor in cold storage container. Zhou and Zhang [13] and Ye and Chen [14] studied the energy saving of digital scroll compressor. Qiu and Qiu [15] Capacity control also can be occurred through reciprocating compressor's cylinder unloading. The cylinder unloading capacity control method (suction valve unloading method) works by lifting the suction valves of some cylinders to the open position. A thermostat (or pressure transducer) energizes a solenoid (or solenoids if there are multiple cylinders in the compressor) that forces the suction valve to stay open. The gas cannot be compressed within the open cylinders, which results a drop in refrigeration capacity. In order to prevent overheating of the compressor, a thermostatic expansion valve should be installed to provide cooling to the compressor suction gas. This capacity reduction is then followed by hot gas bypass. Their construction is relatively low in cost, but they usually require a multicylinder compressor. The achievable capacity graduations depend on the constructional design. With 4, 6, and 8 cylinder compressors, it is usual to operate two cylinders per load stage, which permits graduations of (25)–50–(75)–100% or 33–66–100% [7].

Cawley and Pfarrer [19] have realized a comparative study regarding the part-load efficiency of two-speed compressors using compressor unloading capacity modulation. They found that 49% higher energy efficiency ratio (EER) could be reached using a two-speed compressor instead of a cylinder unloaded compressor. Lower frictional losses at half speed of the twospeed compressor reduce power input. Wong and James [20] resulted that variable compressor speed control is more efficient compared to cylinder unloading control. By using variable speed operation, volumetric efficiency and isentropic efficiency as well as coefficient of performance (COP) are increased at lower compressor speed. On the other hand, cylinder unloading control reduces isentropic efficiency and COP. Wong and Legg [21] also studied the economic benefits of a variable speed compressor in another work. It was shown that variable speed control leads to reduced energy consumption, but for intermittent operation, it may not be economically viable due to the high capital cost of the inverter.

The later study related with cylinder unloading method is investigated by Yaqub and Zubair [22]. They studied cylinder unloading and suction gas throttling schemes to reduce the capacity of refrigeration and air-conditioning systems at reduced load. In the first scheme, an unloaded valve was used to unload one or more cylinders at part-load conditions. The mass flow rate of refrigerant reduces by unloading of cylinders; therefore, the system capacity reduces. A throttling valve is needed before the compressor to reduce the mass flow rate through the compressor. It was found that the cylinder-unloading method was the best and had the highest COP and minimum irreversible losses at any system capacity.

In another study, Yaqub and Zubair [23] investigated three different capacity control schemes. He compared cylinder unloading scheme with hot gas bypass and compressor suction throttling. These schemes are investigated for HFC-134a by considering finite size of the components that are used in the refrigeration systems. A comparative study was performed among these schemes in terms of the system coefficient of performance (COP), the operating temperatures, and percentage of refrigerant mass fraction as a function of the percentage of full-load system capacity. The models consider the finite-temperature difference in the heat exchangers, thus allowing the variations in the condenser and evaporator temperatures with respect to capacity and external fluid inlet temperatures.
