User:NajnaT/Inverter compressor

An inverter compressor is a compressor that operates with the inverter technology.

All builts of compressors (i. e. scroll compressors, reciprocating compressors, screw compressors, centrifugal compressors or axial compressors designed as open or semi hermetic devices) can be configured as inverter compressors. The significant characteristics is the use of a power inverter that changes direct current (DC) to alternating current (AC). By this, the compressor motor speed is controlled and optimized. Energy savings, precise temperature and humidity controls, greater comfort and a reduced impact on the power grid are considered to be advantages of the inverter technology.

Working principle
A variable frequency drive actuated by temperature sensors controls the speed of the compressor motor. The inverter compressor generates a modulated refrigerant mass oil flow and thereby a modulated cooling output. The motor continuously adapts speed, producing more or less cooling capacity to match the exact cooling need.

Market needs for variable capacity
Many refrigeration and air conditioning systems require reliable processes which are more efficient, compact, environmental friendly, easy to install and to maintain. The cooling requirements vary over a wide range during the day and over the year due to ambient conditions, occupancy and use, lighting etc. HVAC and refrigeration systems are usually designed for peak demand which represent only a small percentage of the actual operation. Such oversizing leads to efficiency losses and extra costs for oversized equipment. Capacity modulation is a way to match cooling capacity to cooling demand thereby matching these application requirements.
 * In comfort cooling, there may also be the need for a stable and accurate temperature and humidity control in areas such as hospitals, IT & telecoms, process cooling. In applications such as schools, restaurants and office buildings, it is important that the cooling system is able to adapt to wide daily shifts in load. [1]
 * In process applications such as fermentation, growing tunnels and industrial processes, accurate temperature settings are required to secure production quality.

Different modulating technologies
There are several ways to modulate the cooling capacity in refrigeration or air conditioning and heating systems. The most famous common in air conditioning are: on-off cycling, hot gas bypass, manifold configurations of multiple compressors, mechanical modulation (also called digital) and inverter technology. Each has advantages and drawbacks.


 * On-off cycling: results in switching off the fixed-speed compressor under light load conditions and could lead to short cycling and the reduction in compressor lifetime. Efficiency of the unit is reduced by pressure cycling and transient losses. The turndown capacity is 100% or 0%.


 * Hot gas bypass: involves bypassing a quantity of gas from discharge to the suction side. The compressor will keep operating at the same speed but thanks to the bypass, the refrigerant mass flow circulating with the system is reduced and thus the cooling capacity. This naturally causes the compressor to run uselessly during the periods where the bypass is operating. The turndown capacity varies between 0 and 100%.


 * Manifold configurations: several compressors can be installed in the system to provide the peak cooling capacity. Each compressor can run or not in order to stage the cooling capacity of the unit. The turndown capacity is either 0/33/66 or 100% for an even trio configuration and either 0/50 or 100% for a tandem.


 * Mechanically modulated compressor: this internal mechanical capacity modulation is based on periodic compression process with a control valve, the 2 scroll set move apart stopping the compression for a given time period. This method varies refrigerant flow by changing the average time of compression, but not the actual speed of the motor. Despite an excellent turndown ratio – from 10 to 100% of the cooling capacity, mechanically modulated scrolls have high energy consumption as the motor continuously runs.


 * Inverter compressor: uses a variable frequency drive – also known as inverter drive– to slow down or speed up the motor that rotates the compressor. This method varies refrigerant flow by actually changing the speed of the compressor. The turndown ratio depends on the system configuration and manufacturer. It modulates from 10% up to 100% at full capacity (depending on the compressor model) with a single inverter.

Applications
Variable speed technology can be implemented in HVACR, close control and process cooling applications and as diverse as packaged or split air-conditioning units, rooftops, chillers, precision cooling, VRF and condensing units.


 * Rooftop: is a very common unit type. The rising cost of energy means that air conditioning manufacturers must develop new generation of high-efficiency, cost-effective air conditioners for commercial buildings that meet or exceed a part-load efficiency standard of typically 18 IEER.


 * Air handling units with integrated cooling are used in commercial applications for air conditioning and humidity control in diverse ranges of buildings such as small office buildings, fitness and medical centres.


 * Modular chillers: a typical modular chiller installation uses multiple fixed-speed. These units share the same water system to supply the building with cooled or heated water. Hybrid tandem, associating one inverter and one fixed-speed compressor, can better match the capacity requirement compared to a modular chiller with fixed-speed tandem compressors.


 * Close control units are used in the cooling of IT and electronic equipment used in data centres, telecommunications and in manufacturing industries. Power management, energy consumption and heat loads are major challenges. Maintenance of a stable temperature and humidity control, compactness of the system and overall efficiency are key design challenges in these applications for ensuring data safety and availability.


 * Process cooling In many industries the machinery and processes generate a large amount of heat which requires cooling, to protect the equipment and / or to ensure that the product being manufactured is of the required quality.


 * VRF - Variable Refrigerant Flow VRF units are very popular cooling or reversible systems (heating and cooling). They combine flexibility for building owners and occupants alike, with energy efficiency, high comfort, and ease of installation, without compromising on reliability. VRF systems already extensively use inverter technology.

Benefits
Inverter compressor technology offers great opportunities for air conditioning systems, first of all in terms of reduced energy consumption and lower running costs. Additionally, the technology improves power factor leading to better power supply quality as well as better system diagnostics and open communication protocols which facilitate servicing. Continuous adaptation to cooling demand provides higher energy savings and accurate temperature control. The compressor matches power input with cooling capacity. The unit copes effectively with changes in temperature and runs at partial load which can represent over 30% reduction in the energy bill every year compared to a fixed speed or mechanically modulated compressor. The stable temperature control secures the processes in an optimum way and provides greater comfort.

The capacity modulation attenuates power demand peaks which also contribute to increasing the grid reliability and quality of power supplied. The soft start control leads to a near zero inrush current and improves EER:


 * DOL (Direct-on-line) compressor start draws 5-6 times the nominal current until the compressor running speed is attained. Traditional fixed-speed compressor in a system can make 8-12 start/stop cycles and every start will draw high current from the supply, which leads to high power consumption and stress on supply, and on compressor mechanical parts.


 * Most inverter scroll compressors are soft starting which avoid torque surges. It prevents high mechanical stress on the machine, and generates higher service costs and increased wear. Low in-rush current also helps in saving fixed costs charged by utilities (peak current calculation) and reduces mains and power backup loads.

Additional benefits are of inverter compressors are:

 * Better humidity control makes the unit suitable even for spa installations thanks to adjust the capacity of the unit and thus better control of the evaporating temperature.


 * Less noise than with conventional on-off systems during partial load operation

On top of the standard benefits for all inverter compressors and technology, Danfoss Commercial Compressors has prequalified inverter compressors and drives designed to work together.

This adds to the long list of benefits to manufacturers, consulting engineers and end-users in terms of:
 * Reduced number of components in the system
 * Higher reliability and continuous running
 * Easier to implement technology
 * Reduced applied costs
 * Faster time to market

Challenges in adopting inverter scroll in HVAC systems
Importance of the inverter drive: The compressor and drive need to be qualified to work together and for dedicated applications. The drive modulates the compressor speed and prevents it from operating out of the compressor operating limits. The inverter frequency drives need to use algorithms developed specifically for heating, ventilation and air conditioning (HVAC) or for refrigeration. They ensure that the system will run within the application constraints. The drive can also manage other devices such as oil injection valves or multiple compressors. As the compressor rotational speed changes, the amount of refrigerant — and oil — flowing through the compressor increases or decreases. The drive ensures that the compressor is optimally lubricated at all compressor speeds.

OEM manufacturer skills in inverter system integration: Not all OEMs are yet prepared to adopt inverter technology due to following reasons:
 * 1) Competence level and experience engineering OEM in implementing inverter technology. The strong experience in mechanical must be complemented with electrical engineering skills as well as controller development and programming.


 * 1) Inverter compressors continuously keep changing speed to match the load, which makes the oil management of the system more complex compared to traditional technology. Oil management mastering is a key issue – when not using an oil-free compressor type - where compressor manufacturer experience makes the difference.


 * 1) The application support for OEMs by compressor and inverter drive manufacturer is critical in system integration.

Misconceptions about variable speed technology

 * Inverter technology reacts too slowly to load changes: Some inverter scrolls ramp up/down speed range is between <0.1s; 3600s>.
 * Full load inverter losses 5-10%: At full speed the inverter car lose 3% of efficiency. As full load is not the dominant profile, the loss will be minimal in the overall unit consumption.
 * Oil circulation increases at high frequency operation: Oil circulation rate electronically controlled by some inverter scrolls can be less than 3% at full speed.
 * Oil management is more complex with inverter compressor systems: Some inverter scrolls control oil circulation at both low and high speed requiring minimal design accommodations.
 * Inverters can’t be used where EMC (Electro Magnetic Compatibility) is a concern: Some inverter solutions offer 2 levels of EMC filtration which meet the most stringent European standards related to electromagnetic interference.