In recent years, transcritical CO₂ systems have largely become standard practice in commercial refrigeration, such as in supermarkets and medium-sized cold stores. The systems are increasingly becoming integrated system solutions for low and medium temperature applications as well as air conditioning and heating. Coordinated interplay and control of the individual system components pave the way for a stable, reliable operation mode for the entire year.

BITZER analyses countless systems in close cooperation with customers and partners. Building on tests in its laboratory and demonstration systems, BITZER supports its customers from planning and building new commercial refrigeration systems to retrofitting pre-existing ones. By equipping compressors with VARISTEP mechanical capacity control, it becomes possible to build efficient refrigeration systems with excellent capacity graduation in the smallest of spaces, as demonstrated by a few case studies below.

The challenges of refrigeration in supermarkets etc.

Supermarkets are characterised by their highly changeable need for refrigeration depending on the time of day or customer footfall. Besides the well-known basic requirements such as observing the permissible operating conditions and ensuring functional oil management, the capacity control must be tailored to the application for compressors to operate smoothly. The purpose of capacity control in parallel compounding is to cover minimum loads in order to minimise on-off cycles, especially in the lead compressor, and achieve a high control accuracy (CF) with minimal capacity changes per step, so as to lower operating costs and increase the reliability of the system.
In recent years, rationalisations in systems and installations have, in practice, often led to conflicting requirements in terms of high efficiency, low system complexity and low investment costs. Common consequences include unfavourable system performance and poorer operating reliability, such as due to:
// a reduced number of compressors and/or excessively large compressors per suction group,
// active liquid injection with regular subsequent injections in lieu of an external desuperheater for the low temperature stage,
// low temperature capacity control with on-off cycles instead of capacity control with a frequency inverter,
//even stricter heat recovery requirements,
// heat recovery systems without storage tanks on the hot water side,
// less time needed for production, installation and commissioning,
// a reduced number of filter and oil changes.
The discharge gas temperature illustrates the effects on the lead compressor. A standard compound control system monitors the following variables independently of one another and features a safety cut-out for the high pressure, discharge gas temperature, suction gas superheat, oil level and motor temperature. The permitted discharge gas temperature in particular depends on the pressure ratio, suction gas superheat, operating frequency, operating time and dynamics of the operation. A lower operating frequency and higher suction gas superheat affect the thermal load of the compressor and lower its application limit. In image 1, dashed line 3 shows the maximum permitted discharge gas temperature (t max.) for an operating frequency of 25 Hz with suction gas superheat of 30 K. Highly unfavourable operating conditions for the lead compressor in the medium temperature stage are characterised by:
// Daytime operation with lots of on-off cycles in the secondary compressors and unstable (fluctuating) operating conditions brought about by low control accuracy,
// Night-time operation with low operating frequency and regular pump-down cycles with a high pressure ratio and high suction gas temperatures,
// Night-time operation with low operating frequency and a high number of on-off cycles, characterised by an active liquid injection before the lead compressor was shut down causing an excess of liquid refrigerant on the suction side during the restart delay.
The conditions shown as examples indirectly affect the tribology of the compressor drive gear and can result in increased wear on the bearings.
An excessively low control range and significant load or capacity changes result in instability in the overall system, especially when the control range of the lead compressor is unable to compensate for the drops in capacity caused by other compressors switching on and off. Control accuracy is used to describe this relationship. It is the difference between the capacity of the lead compressor at maximum and minimum frequency, divided by the capacity of the subsequent compressor (source: ASERCOM, see also Technical Information on parallel compounding of BITZER reciprocating compressors).

Case studies: Improved control accuracy and reduction in the minimum possible part load with VARISTEP capacity control

Supermarket refrigeration systems are built with a primary focus on low investment costs and small system dimensions, as shown in the following example. The lead compressor 4JTE-15K-40P is equipped with a frequency inverter that can operate the compressor at between 30 Hz and 60 Hz. Two additional 4FTE-20K-40P fixed-speed reciprocating compressors have also been added to the compound system. A mechanical capacity control system has not been fitted.
// The minimum capacity of the system in winter is 19.1 kW
// and its maximum capacity in summer is 88 kW.
A flash gas bypass system with the following operating conditions was selected for the design:
Summer:
Operation mode: Transcritical
Evaporation temperature -8°C
Gas cooler outlet temperature 40°C
High pressure 98.6 bar
Winter:
Operation mode: Subcritical
Evaporation temperature -8°C
Condensing temperature 15°C
The low temperature stage has not been included in order to keep the following examples simple. The control accuracy of this system configuration is just
// 36% in summer
// and 34% in winter.
These values need optimisation. In this case, the control accuracy (CF) does not allow for good coverage of the refrigerating capacity required by the evaporator.
// Constant switches between minimum and maximum lead compressor operating frequency,
// frequent cycle rates of secondary compressors,
// strongly fluctuating suction, medium and high pressures and
// cyclical operation with high and low suction gas superheat
are all typical symptoms of a system operating with low control accuracy. The operating behaviour of such a system during opening hours on a typical winter’s day at our latitudes is depicted in image 2.

Frequency range expanded

In the first step, the frequency range of the lead compressor is expanded to the standard range. This way, the control accuracy (CF) can be
// improved to 36 and 55
in a relatively quick and easy manner. At this point, it is necessary to check whether the thermal load is still permissible for the lead compressor, given the suction gas superheating and operating conditions. Likewise, the motor power reserve needs to be inspected if the frequency-controlled motor has star wiring.

VARISTEP retrofit

An effective way to improve control accuracy is to retrofit a secondary compressor with a mechanical capacity control (CR100% or CR50%). Keeping the initial situation the same, the second compressor in the compound system is equipped with the VARISTEP mechanical capacity control system from BITZER. This optimises the control accuracy to
// 109% in summer and
// 107% in winter,
because it avoids capacity jumps between the lead compressor and the subsequent compressors. It is necessary here to check whether the compressor rack controller is able to energise another capacity-controlled compressor.
However, the use of VARISTEP capacity control opens up many more options. In the example in question, for instance, the compound system could be planned even more beneficially. The lead compressor 4HTE-15K-40P is equipped with VARISTEP for virtually stepless capacity control. The integrated IQ MODULE makes it easy to operate. The subsequent compressors are a 4FTE-20K-40P with stepped capacity control (50% /100%) and a fixed-speed 4FTE-20K-40P. This means that the part load efficiency of the system increases significantly thanks to VARISTEP capacity control. The minimum capacity in winter is now 4.4 kW and its maximum capacity in summer is 88.5 kW. The control accuracy has been optimised to

• 122% in summer and winter.

This combination of compressors makes it possible to realise the largest capacity control range with just three compressors (image 3).
Coverage of partial load conditions all the way to minimum load, such as by the lead compressor, guarantees a stepless mass flow, stable suction and high pressures and stable suction gas temperatures. As such, it becomes possible to effectively prevent reduced system efficiency, potential wet operation, reduced oil return from the system, fluctuating control circuits and unfavourable operating conditions for compressors. This makes it prudent to control the refrigerant mass flow in a stable, controlled manner: Control of the system components that is tailored to the circumstances of the system and not excessively dynamic results in a stable, controlled refrigerant mass flow with no fluctuating control circuits.

Start-stop cycles

The number of compressor start-stop cycles must be taken into account in this context. For compressors without frequency inverters,
// 6 starts per hour and
// at least 10 minutes between two starts are recommended.
High cycle rates are a challenge for the compressor motor (thermomechanical) and the drive gear (mechanical). In contrast, high cycle rates can easily lead to mechanical stress on the drive gear when a compressor is operated with frequency inverter. The danger of a lack of oil arises when a compressor is operated exclusively at a low frequency and with periodic on-off cycles. To ensure that the drive gear is sufficiently lubricated, the compressors should be operated at a frequency of ≥ 40 Hz for ≥ 10 s in the starting phase, in order to lubricate the drive gear properly before the control is released. When evaluating systems in operation, it is necessary to take into account that the operation with higher cycle rates is largely focused on operation at night or outside of opening or operating hours. Generally speaking, cycle rates of
// 120 per day can be considered critical and cycle rates of
// 160 per day can be considered inadmissible.

Oil return

Additionally, operation that is as stepless as possible with low compressor cycle times promotes a good oil return from the system. For example, this prevents cold oil mixed with refrigerant from suddenly being returned from the evaporators to the compressors after defrosting phases or under high refrigerating capacity. Moreover, stable operating conditions allow the oil barrier in the oil level switches to settle and align with the level in the drive gear.

Minimum load coverage

The following measurements demonstrate how favourable stepless minimum load coverage during compressor operation is.
Before it was converted to stepless capacity control with VARISTEP and IQ MODULE, the lead compressor was operated with a frequency inverter. In this case, the necessary refrigerating capacity was far lower than expected. The consequences for the lead compressor were cycle rates per day of between 120 and 160 in winter operation, long shut-off periods and operation with cyclical pressure and temperature fluctuations. The visualisation of the operating conditions in the application limit illustrate this.
The VARISTEP capacity control with IQ MODULE made it possible to stabilise the operation and largely avoid the start-stop cycles of the lead compressor. This is also shown by the time-weighted mean operating condition values. For example, the average evaporation temperature is 2 K higher and the average high pressure is 3 bar lower – with a positive impact on energy consumption.

Retrofitting refrigerated display cabinets in supermarkets

Exceptional care is needed when refrigerated display cabinets are to be retrofitted with glass doors. Most systems are ordered and delivered with considerable capacity reserves. Retrofitting glass doors lowers the necessary refrigerating capacity by an additional 40 - 50%, depending on the temperature class and evaporation temperature. This exacerbates the deviation between the installed and necessary refrigerating capacity, which has a significant impact on the part load behaviour of the systems. Cycle rates will rise rapidly. If a system’s lead compressor has high cycle rates, it is necessary to check whether the additional installation of glass doors can be combined with a conversion of the compound system. Suitable measures would include:
// Select a lead compressor with a smaller displacement volume and
// install a secondary compressor with stepped capacity control.

A look ahead

Compound systems with CO₂ as a refrigerant have become commonplace in supermarkets in recent years. For one, the practical experience supported by extensive data analyses shows that some system concepts are only partially able to meet the different load requirements for various reasons. The goal must be to reconcile high efficiency, low complexity and low investment costs. The control accuracy of the system and the coverage of minimum load conditions play an important role in this context. It is demonstrably possible to improve both significantly with mechanical capacity control systems in the compressors. We can expect the systems to be optimised further in the future.

Further information:
ECOLINE reciprocating compressors for transcritical CO₂ applications
Application guide for the use of CO₂ / R744
Technical Information on parallel compounding of BITZER reciprocating compressors
BITZER Software for designing CO₂ systems

(published 2023)