Advances in recirculating chillers

12th December 2017

Testing an open external cooling bath
Fig 1. Cooling capacity required for aqueous solvents in relation to evaporation flask size (at a chiller temperature of 20 degrees)
Fig 2. Cooling capacity requirements in relation to the evaporation flask surface area for aqueous solvents
Fig 3. Power input and power consumption over a period of one day

The demands on recirculating chillers can differ widely from one application to another. Configurations are frequently inappropriately scaled, which may lead to disappointing results, unnecessary blocking of laboratory space, strains on allocated budgets, and excessive operating costs. Here, we describe how to determine the correct scale of an appropriate recirculating chiller for use with rotary evaporators.

With the launch of the RC 5, the IKA portfolio of recirculating chillers now offers four different models with cooling capacities from 400 to 1,400W. This means that efficient cooling down to a temperature of -30°C is available for a wide range of different applications.

A common question asked by customers relates to the selection of an appropriately dimensioned recirculating chiller for one or more rotary evaporators. To answer this question, we tested various complete systems by IKA, composed of one or more RV 10 control auto rotary evaporators and chillers.

The task of a rotary evaporator is the separation of mixtures of various solvents or suspensions. In this process, the solvent to be extracted is evaporated by the rotation of an evaporation flask in a heating bath; the vapour then passes through a glass condenser where it condenses and is collected in a receiving flask.

The required cooling capacity at the glass condenser is determined by the mass flow and temperature of the solvent vapour that is passed from the evaporation flask to the condenser.

The temperature and mass flow are dependent on the pressure setting, the temperature in the heating bath, and the effective surface available for heat transfer in the evaporation process. The rotation of the evaporation flask creates a thin film of liquid solvent that coats the entire inner surface of the flask and is thus available for heat transfer.  

In the following example, the target value to be determined is the cooling capacity required in relation to the size of the evaporation flask.

The required cooling capacity was calculated using measurements of the inlet and outlet temperatures at the glass condenser and the flow rate of the cooling medium. The tests were carried out at room temperature.

Reference experiment
Water was used as a reference solvent and as the heat transfer medium in the recirculating chiller. The reference distillation was carried out at a typical water bath temperature of 60°C. The temperature of the solvent in the evaporation flask is around 40°C. The pressure is reduced to 72 mbar and is therefore close to the boiling point. The temperature in the chiller should be a constant 20°C.

The required cooling capacity changes in the course of the distillation process. The respective maximum values are recorded. This ensures that for the future calculation of the maximum number of rotary evaporators per chiller, the ambient temperature in the chiller remains constant and vapour is not drawn into the vacuum pump at any time. The size of the evaporation flasks was varied (50-3,000ml).

The results can be seen in Fig.1, which shows the maximum cooling capacity required in relation to evaporation flask size. The larger the evaporation flask, the higher the cooling capacity required.

In Fig.2. the surface area of the evaporation flask is plotted above the required cooling capacity. As the power input from the heating bath to the evaporation flask is directly proportional to the flask surface area, the result displays an almost linear relation.

Procurement costs can be reduced and valuable laboratory space can be gained by taking into consideration the number of recirculating chillers required in relation to evaporation flask size. For example, a 400W chiller such as the IKA RC 2 basic can be used for up to four rotary evaporators at one time when these are fitted with 50ml evaporation flasks. At low water supply temperatures of the chiller, its cooling capacity is reduced.

In addition to lower cooling capacity, greater losses to the environment must be taken into account in the case of low temperatures of the incoming water supply. The results shown in Fig. 2 for a water supply temperature of 20°C cannot be attributed to deviations in water supply temperature.

The water considered as a solvent in this case corresponds to a worst-case scenario. Lower cooling capacity demands may be assumed for other solvents (up to 20%, depending on the solvent).

Energy efficiency
In the recent past, energy efficiency has also become an increasingly important aspect in the laboratory segment. As is the case with all other IKA devices, conservation of resources is also a key priority in the operating concept of the RC recirculating chillers. This is especially important in the case of chillers that are frequently in operation for several hours at a time.

The key targets here are, firstly, the reduction of the local CO2 footprint and, secondly, the reduction of operating costs.

Here, accrued “CO2 equivalents” and cost factors arising during the entire product life cycle must be taken into account.

The real costs are primarily determined not only by the price of purchase, but also, and above all, by the operating costs and the length of the product life cycle. The CO2 equivalents can be determined from the power consumption of the chiller and the specific CO2 emissions in the electricity mix.

An important aspect in the technical design of IKA lab equipment therefore lies in enabling demand-controlled operation based on smart control technology. This leads to a reduction in operating costs and increases the length of the product life cycle.

Thanks to the interaction of high-quality components and the smart control concept of the IKA RC recirculating chillers, this results in power savings of up to 60% in comparison with some conventional chillers. These savings are primarily achieved by the use of a speed-controlled compressor that adapts to momentary demand. For its recirculating chillers, IKA relies on a combination of a micro-channel condenser and a large, speed-controlled cooling fan for heat dissipation.

A standard feature is an electronic expansion valve that simultaneously regulates performance and ensures excellent temperature stability (up to ±0.05 K). The high-quality PU foam insulation of the heating bath ensures that the energy savings achieved are not unnecessarily lost to the environment.

The energy efficiency has been confirmed in tests simulating practical situations. The first step is the determination of the idle power consumption of the chiller being tested. The term idle power consumption describes the electric power that is required to maintain a constant temperature of 20°C in the heat transfer medium that circulates through the glass condenser. At this stage, the evaporation flask was not immersed in the heating bath.

The actual test involved a total distillation of 500ml of water in an IKA RV 10 control rotary evaporator. Every last drop was distilled in the evaporation flask at a water bath temperature of 60°C, an ambient temperature of 20°C, and a cooling water flow rate of 50 litres per hour. This simulates an eight-hour working day during which four distillations (500ml of water) were performed. The power consumption was measured and recorded throughout the entire testing period.

To enable a comparison, the tests were repeated with recirculating chillers made by other manufacturers. Fig. 3 shows the effectiveness of the demand-driven recirculating chiller (red) in comparison with an unregulated chiller (blue). The total power requirement in kilowatt-hours can be read from the area below the respective performance curves. The generally lower idle power consumption of the IKA chiller between distillations and the adaptation of the cooling capacity during the distillation process can be clearly seen.

With the launch of the RC 5 basic and control recirculating chillers, IKA is relying for the first time on the natural coolant R290 (propane) and thus consistently pursuing the development of environmentally friendly products. The global warming potential of R290 is 430 times lower than that of comparable synthetic coolants. The coolant volume required can also be considerably reduced in comparison with R134a.

Even in the case of the IKA RC 2 recirculating chiller based on R134a, a considerably lower volume of greenhouse gases were emitted than from some comparable unregulated chillers.

Practical handling
It is the sum of the small things that lead to making lab procedures safer and more convenient for users and, as a side benefit, make them faster. The moulded grips and transport rollers of IKA recirculating chillers make them easy to move and set up.

The baths have an integrated filling funnel. The current fill level is indicated by an LED display. Emptying is made particularly convenient by a bath outlet with a separate drain valve. If water is to be used as a heat transfer medium, the company recommends the use of a water bath additive that is not subject to labelling, e.g. IKA Water Protect, to prevent the development of microorganism films.

Maintenance tasks are limited to regular checking and cleaning of the air filter. This provides a guarantee of constant cooling capacity throughout the product life cycle.

Markus Schlegel is with IKA Werke.






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