Experimental study of performance of digital variable multiple air conditioning system under part load conditions
Introduction
The use of multi-connected air conditioning system to achieve occupant thermal comfort in buildings with low-energy demands has increased due to its several advantages, such as easier system maintenance, wide range capacity output, precise capacity control and high SEER (seasonal energy efficiency ratio), etc. There have been several studies relevant to multi-connected air conditioning system in the literature. Jang [1] described an experimental study on the convective heat transfer inside the scroll compressor. Cuevas and Lebrun [2] gave some experimental results as to inverter and compressor performance. Park et al. [3] analyzed the multi-type inverter air conditioner with variations of operating frequency of compressor, cooling load imposed on the system and cooling load fraction among rooms. Aprea [4] evaluated experimentally the energy saving obtainable by varying the scroll compressor speed to control the refrigeration capacity instead of the classical thermostatic control. Shi et al. [5], [6], [7] studied the control strategy of variable refrigerant volume air conditioning system by experiments and simulation. The study on DVM (digital variable multiple)air conditioning system is seldom done.
DVM air conditioning system is a kind of air source heat pump which has one outdoor unit and several indoor units. The structure of DVM air conditioning system is simpler and it is easier to install, more flexible to control than centralized air conditioning system. Each indoor unit is controlled independently. The outdoor unit is smaller and occupies less space than air conditioner of centralized air conditioning system. Digital scroll compressor is used in DVM air conditioning system. Digital scroll compressor is turned on and off cyclically by PWM (pulse width modulation) valve. And the capacity of the compressor is modulated by changing the proportion of load time in cycle time.
Hu and Yang [8] discussed the relationship between the degree of opening of the electronic expansion valves and the compressor output ratio. Yang [9] studied the operating characteristics of DVM air conditioning system under cooling condition by experiments. And the field test of operating characteristics and defrosting characteristics in winter was studied by Zhu et al. [10], [11]. Up till now, seldom has a paper been published regarding the performance of DVM air conditioning system under part load conditions, which has been analyzed in this paper.
Section snippets
Experimental scheme
In this paper, the load for infiltration and heat gains of occupants and equipment are kept constant in the experiment. Therefore, heating/cooling capacity of indoor units will change with the variation of solar radiation and on-unit ratio when indoor air temperature, outdoor air temperature and inner heat source are constant. On-unit ratio is defined as the ratio of running indoor units’ capacity to all indoor units’ capacity.
Experiments are focused on different on-unit ratio conditions of
Experimental system
The lab of DVM air conditioning system is shown in Fig. 1, which consists of four rooms: room 1 (l × w × h = 7.2 m × 6.0 m × 3.5 m), room 2 (l × w × h = 3.6 m × 6.0 m × 3.5 m), room 3 (l × w × h = 3.6 m × 6.0 m × 3.5 m) and room 4 (l × w × h = 6.0 m × 6.0 m × 3.5 m). Outdoor unit is located on the terrace. The wall thickness is 240 mm.
As described in Fig. 1, DVM air conditioning system consists of one outdoor unit, four indoor units, four control panels, refrigerant line, condensate water line and signal control line. DVM air conditioning system
Measuring instruments and test points distribution
In this experiment, dry bulb temperature, wet bulb temperature and air velocity of both outlet and inlet air, electricity consumption of compressor and fans, temperature and pressure of refrigerant at both inlet and outlet of the units are measured.
Temperature is measured by platinum resistance thermometers. The measuring range is from −200 to 200 °C and the measuring precision is 0.5 grade. The extending wire is three-wire model. All platinum resistance thermometers are calibrated by constant
Definitions of PLR, HEER and HHPF
Part load value (PLR) is used to describe the capacity of air conditioning equipment under non-nominal condition. There are two kinds of definitions of PLR, one is used to design or choose air conditioning equipment, and the other is used in energy consumption analysis.
In ASHRAE handbook, PLR is defined as the ratio of the actual capacity to the maximum capacity under this condition. This definition is usually applied to building dynamic energy simulation. Power and capacity under full load
Influencing factors analysis
In heating and ventilating system, heat loss of building envelope occupies the maximum proportion of heating load in office building. Meanwhile, experimental data show that hourly heating capacity and hourly work remain constant with the variation of solar radiation intensity because of north external wall. Therefore, in experimental system, the main factors that affect heating capacity and energy consumption are outdoor air temperature and indoor air temperature.
Cooling load includes two
Results and discussion
DVM air conditioning system usually operates under part load condition. Performance under part load conditions determines energy saving potentiality. Variations of HHPF with indoor and outdoor air temperature under heating condition, when each indoor unit is open, are shown in Fig. 2. Variations of HHPF and HW with PLR under heating condition are shown in Fig. 3, Fig. 4. Variations of HEER and HW with PLR under cooling condition, when each indoor unit is open, are shown in Fig. 5, Fig. 6. From
Conclusions
- (1)
HHPF decreases when indoor air temperature increases or outdoor air temperature decreases under heating condition. HEER decreases and HW increases when outdoor air temperature increases under cooling condition.
- (2)
Variations of HW and HEER present concave and convex distribution with PLR.
- (3)
Under heating condition, the increasing tendency of HHPF with PLR is more obvious under higher outdoor air temperature. Under cooling condition, the increasing tendency of HEER with PLR is more obvious under lower
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2020, EnergyCitation Excerpt :There has been in-depth research on the variable refrigerant flow (VRF) heat pump system, which has the advantages of indoor and outdoor space utilization and low installation costs by connecting several indoor units to one outdoor unit. Zhang et al. conducted an experimental study on the performance of a digital variable multiple air conditioning system under part load conditions in hot summer and cold winter zones [9]. The results indicate that digital variable multiple air conditioning system can keep economical and reliable operation under part load conditions.