Digital Scroll vs. DC Inverter: Cooling Performance Under High Ambient Temperature

DC Inverter technology has been widely used by Japanese A/C brands in VRF system since the end of 2003. Depending on the manufacturer, the range of frequency delivered by the inverter driver can be from 20 Hz to 120 Hz, which means that the compressor can spin from 1,200 rpm to 7,200 rpm. Therefore, a lot of end-user / installer or even consultants would expect that DC Inverter driven VRF systems can deliver higher cooling capacity simply by “speeding up” under severe condition (e.g.: high ambient temp for cooling, low ambient temp for heating, or IDU / CDU combination ratio is near 130%).

Digital Scroll vs. DC inverter (Scroll or Rotary based) technology: Compressor Operating Envelope

A compressor only sees the suction temperature/pressure and the discharge temperature/pressure. The saturated suction and discharge pressures are plotted as the x-axis and y-axis (see Figure 1). The Digital Scroll compressor can operate anywhere within this operating envelope without any concern on reliability.

Figure 1: Operating envelope of Copeland Scroll

The operating envelope of the inverter scroll is much more complicated (see Figure 2). There are specific areas in the map that the inverter scroll cannot operate due to the constraint on bearing lubrication, discharge temperature, etc. This is the main issue in the design of the inverter scroll/rotary – it is very complicated and it needs complicated algorithm to ensure reliable operation.

Looking closely at the operating map, it can be seen that the full operating envelope is only available between 45 Hz and 70 Hz.  At all other operating speeds, the envelope becomes much shorter and restrictive (Figure 2).  One of the features generally promoted for inverter scroll / rotary is that they can run at a higher RPM and deliver more capacity when the ambient is hot.  But in careful observation of the operating envelope, the 120 Hz operating speed has a low condensing temperature. This means that when there is a demand for higher capacity during a hot summer day, the inverter will be unable to run at a high speed. 

Figure 2: Typical inverter scroll operating map