**2.2. Digital scroll capacity control**

**2.1. On/off capacity control**

The on/off capacity control is the simplest method adjusting the predetermined temperature (set point) using a thermostat. After the temperature reaches the set point, thermostat stops the compressor and circulating refrigerant in the cycle. Since the secondary fluid continues to circulate, the temperature of the water or air gradually raises. When the thermostat detects,

**No Year Author Objective Compressor Control**

and PID control

compressors

system

system

system

control

fault diagnosis variable speed refrigeration system

 refrigerant for variable speed refrigeration system

comparison for refrigeration

comparison for HVAC systems

for automobile air-condition

comparison for refrigeration

controlled-refrigeration system

and heat pump variable speed

control for HVAC system

on compressor and electronic

with EEV and TXV

driving methods

expansion valve

Reciprocating Conventional

Scroll Conventional

Scroll Conventional

Reciprocating Fuzzy/PID

Automobile Fuzzy/PID

Reciprocating Fuzzy-on/off

Scroll Fuzzy/on/off

Scroll Fuzzy

Reciprocating On/

On/off P-PI-PD-PID

Fuzzy/PID

off-PID-fuzzy

Fuzzy/PID

Fuzzy/PID

52 2005 Kim and Kim [70] Experimental study for artificial

53 2006 Nasutin et al. [59] Energy saving of variable speed

55 2008 Cuevas and Lebrun [60] Drawback of variable speed

56 1996 Cheung and Kamal [73] Fuzzy and conventional control

59 2004 Aprea et al. [74] Fuzzy control and on/off control

60 2006 Aprea et al. [75] Fuzzy-controlled and thermostat

61 2006 Aprea et al. [76] Fuzzy control for chiller

62 2006 Wang et al. [78] PID, fuzzy, and fuzzy-PID

63 2006 Navale [79] Performance of fuzzy and PID

64 2010 Ekren and Kucuka [72] Performance of fuzzy control

**Table 1.** Researches on capacity control methods and control applications.

57 2003 Rahmati et al. [77] Fuzzy and PID control

58 2004 Li et al. [80] Fuzzy and PID control

54 2007 Cho et al. [71] CO2

Electrical control applications

124 Refrigeration

In this method, scrolls force to separate and compression of refrigerant stops. This modulates refrigerant flow without changing or stopping the compressor motor. The separation of the scrolls is realized by using an external solenoid valve; also, there is a bypass connection line between the discharge chamber and the intake gas. The upper scrolls can separate from the bottom scroll by 1 mm vertically. A piston is fixed to the top of the upper scroll and lifts up the upper scroll when it moves up. There is a modulation chamber at the top of the piston that is connected to the discharge pressure through a bleed hole. An external solenoid valve connects the modulation chamber with the suction side pressure. If the solenoid valve is closed, the scrolls operate as a classical scroll compressor. If the solenoid is opened, the discharge chamber and intake gas pressure connect each other and decrease the discharge pressure. This leads to less pressure holding the piston down, thereby causing the piston to shift upward, which in turn lifts the upper scroll. This action separates the scrolls and results in no mass flow through the scrolls. De-energized external solenoid valve again loads the compressor fully, and the compression is resumed (**Figure 3**) [8].

During the loaded state, the compressor operates like a standard scroll and delivers full capacity and mass flow. However, during the unloaded state, there is no capacity and no mass flow through the compressor. The scrolls are separated in a periodic cycle (20 seconds) to obtain a time-averaged compressor capacity based on the ratio of loading and unloading times. This allows the digital scroll to achieve infinite capacity modulation between 10 and 100%.

**Figure 3.** Digital scroll piping.

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 very close capacity modulation results [9].

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] discussed the application of digital scroll compressor in high-accuracy constant temperature and humidity air-conditioning. Analysis and comparison between digital scroll and inverter technology were studied by Shi [16] and Ma and Sun [17].

Huang et al. [18] conducted an experimental study on operation characteristics with variable air volume under refrigerating and heating condition of ducted air-conditioning (AC) unit with digital scroll compressor and conventional scroll compressor. The effects of air volume on the refrigerating (heating) capacity, input power, EER (COP), outlet temperature, discharge pressure, discharge temperature, and suction pressure were studied. As a result, the authors reached that the unit with digital scroll compressor was well adapted to the variable air volume system and good for the economical and reliable operation of the ducted AC unit.
