Circulators

No. The temperature accuracy of heating baths is tested at an ambient temperature of 20°C using water as the circulating liquid (70°C).

The temperature accuracy of refrigerated circulators is also tested at an ambient temperature of 20°C using ethanol as the circulating liquid (–10°C).

The temperature accuracy at higher and lower temperatures will deviate from the specified value. In addition, the temperature accuracy is adversely affected if bath liquids other than water or ethanol are used.

A portion of the heating thermostat pumps' electrical energy is lost to the bath liquid in the form of heat. Pump output flow, thermostat insulation, and the type of bath liquid used may affect the increase in bath temperature.

Some bath temperatures can increase as much as 70°C above ambient temperature; however, Lauda^® Ecoline Circulators only heat the bath slightly above ambient temperature when the lowest pump flow setting is selected.

Yes and no. Due to the high cost of producing a small, laboratory size explosion-proof circulator, Lauda offers only larger, industrial-sized circulators for explosion-proof applications.

A simple, cost-effective solution for small-scale laboratory applications is to place the circulator outside the hazardous area and run tubing into the designated area. For more information, please contact the Brinkmann BioSystems Applications Lab at 800-645-3050 ext. 2258, or e-mail apps@eppendorf.com.

Technical data can only be specified under accurately defined measurement conditions. The temperature control accuracy is dependent on a constant ambient temperature. Variations in ambient temperature will have a negative effect on the circulator's temperature control accuracy.

Many thermostats have a port for connecting an external temperature probe to control the temperature of an external bath. If the circulator is set to external control, then the display will indicate the temperature of the external bath rather than the internal bath. Depending on the model, it is possible to display the external temperature continuously or with the touch of a button.

The following formula can be used to estimate the time it will take to heat or cool a given bath volume by ΔT.

The example illustrated below applies only to the circulator. When working with a closed loop external system such as a jacketed reaction vessel, it is important to add the volume of the tubing as well as the reaction vessel to the bath volume.

Cooling/Heating Capacity =	mass (kg) x specific heat x ΔT (°C)
	Time (sec.)

Example:	Lauda Heating Circulator Model E-203
	Bath Volume = 3 L
	Circulation Liquid = Water
	Specific heat of water = 4.2 J kg–1 K–1
	E-203 heating capacity = 1.3 kW

Time =	mass x specific heat x ΔT	=	3.0 kg x 4.2 J kg–1 K–1 x 30 °K	= 4.8 min.
	Heating Capacity		1.3 kW x 60 sec min–1

 

An open system consists of a circulator and an open external bath. As the circulator pumps liquid into the external bath, liquid must also be pumped out at an equal flow rate. This prevents the bath from overflowing (see Figure 1). A pressure/suction pump or a duplex pump can be used for an open system.

Pressure/suction pumps require a level controller accessory to protect against bath overflow. This is necessary because the flow rate of pressure and the flow rate of suction are never exactly equal. A duplex pump has a built-in level controller.

Figure 1

A closed system includes a circulator and jacketed reaction vessel or condenser (see Figure 2). The circulating liquid is pumped through a closed loop system. A simple pressure pump is sufficient for this type of application.

Figure 2

No. The discharge pressure, not the suction pressure, determines the flow rate. The true benefit of using a pressure/suction pump in a closed system as compared to using a pressure pump is that the flow rate is not reduced as the tubing diameter "narrows."

No. A closed system will not drain when air is not being introduced into the system.

Yes. However, this may potentially lead to flooding in the lab. There are a number of preventive steps that can be taken to avoid this situation. One solution is to disconnect the hoses at the highest point to vent the system after shutting off the pump. Another solution is to install reverse flow solenoid valves at the highest point of the hoses. These valves will ventilate the system without disconnecting the hoses.

Yes. The new Lauda Ecoline Circulators offer a selection of five different flow rates.

The volume of the external circuit should not exceed 50% of the bath volume. Therefore, special care should be taken when selecting a suitable bath. If the external volume is too large, too much time will be spent cooling the liquid in the external circuit. The subsequent temperature differential between the external circuit and internal bath impedes the circulator's capacity to maintain accurate temperature control.

Another consideration is the effect of volumetric expansion. In a closed circuit, the thermostat must withstand the total volumetric expansion of the circulating liquid, as well as the liquid inside the thermostat. When the external closed circuit is 50% of the bath volume, the volume is increased by 13% per 100°C. Over a larger temperature range, the expansion can quickly exceed the difference between the circulator's maximum and minimum volume, also know as the "buffer volume" Lauda offers circulators with an especially large buffer volume, such as Model K12-KS, Model K12-KP, and Model US-6.

The application as well as the temperature should be considered when choosing a suitable bath liquid. For example, a clear bath liquid should be chosen for applications where the sample would be monitored.

For the purpose of controlling the temperature with an internal bath, Lauda Circulators can maintain temperatures as high as 300°C. The Lauda Model USH-400 is suitable for closed loop systems and can maintain temperatures as high as 350°C.

Unfortunately, the fumes emitted from circulating liquids are unavoidable. There are a number of steps that can be taken to contain the odor. A covered bath is recommended; when an open bath is required, an exhaust fan at the liquid surface can minimize fumes. When possible, circulators should be placed in a fume hood.

If a thermostat is required solely for circulation and operates continuously at high temperatures, it is advisable to use the high-temperature thermostat USH-400, specially designed to contain emissions.

A program segment is simply a temperature setting for a given time interval. For example, one might program a circulator to hold 97°C for 20 minutes, followed by 44°C for 15 minutes, and finally 72°C for 60 minutes. Each temperature and time interval represents a segment.

Lauda has developed the PC-compatible Wintherm software for this purpose. The software can create, edit, copy, and store curves or profiles. The profile can be transferred and executed via an RS-232 interface.

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Lauda A Class circulators

According to the DIN 58966 standard part 1, the control accuracy of a circulator is designated and defined as temperature stability. Temperature stability as well as the corresponding specifications in the literature are based, in the case of a heating circulator, on an operating temperature of 70°C and water as the thermostating liquid, with an ambient temperature of 20°C. In the case of a cooling circulator, they are based on an operating temperature of –10°C and ethanol as the thermostating liquid, with 20°C ambient temperature.

Under constant conditions it can be said that the temperature stability in both cases, measured with water and with ethanol as well, varies only slightly at higher and lower temperatures. Worse results are generally found when changing to a different bath liquid, especially at higher operating temperatures.

The temperature stability is defined according to the DIN 58966 standard parts 1 and 2. Technical data can only be specified under accurately defined measuring conditions. This question involves ambient temperature. The temperature stability is measured at an ambient temperature of 20°C and under constant conditions. If there is constant solar radiation, it can be said that the specified temperature stability is largely maintained. If the solar radiation changes, this alters external disturbance and the temperature stability value declines.

Only specially designed circulators are suitable for operation in hazardous areas. The necessary components—such as EEx-protected driving motor for the circulating pump, electrical heater, operating and control devices—are unsuitable for conventional laboratory circulators because of their size and cost. EEx-protected circulators are therefore usually much larger than laboratory units but also more powerful.

It is possible to use laboratory circulators by placing them outside the hazardous area and passing the thermostating liquid through tubing into the hazardous area. Technical advice by a specialist is always necessary

A closed external circuit does not drain as long as absolutely no air can pass into the system.

This does happen and has often led to flooding the laboratory. The only remedy is to “vent” the connecting hoses after the circulator has been switched off. A more elegant solution is to fit a so-called reverse flow protection, which vents the connections at the highest point with the aid of solenoid valves

The terms “quickly” and “accurately” in this question are mutually exclusive. Either an approximate value of the required heating and cooling capacities can be calculated quickly, or an accurate time-consuming calculation is necessary. When calculating the required performance, different situations must be taken into consideration. In many cases, the circulator is being used for cooling an external system of which a certain amount of energy has to be dissipated. If this amount of energy is known, the circulator must at least provide the required cooling capacity at the appropriate operating temperature.

Applications often involve cooling down or heating up within a certain period of time. If the circulator is used only as a bath circulator, it is important to know whether there is some material inside the bath, which has to be thermostated. In that case, the thermal capacity of the thermostated material has to be included.

Working temperature range and operating temperature range are concepts defined in the DIN 58966 standard, part 1. The working temperature range is defined there as the temperature range reached at an ambient temperature of 20°C by the circulator alone, using only the specified energy sources and not using any additional devices. In practice the energy source is nearly always electrical energy.

In a heating circulator the working temperature range starts approximately 3°C above the so-called intrinsic temperature and in most cases ends at the upper limit of the operating temperature. The intrinsic temperature is produced when the heating is switched off, due to the mechanical energy input, and it depends on the pump output, the insulation of the circulator, and the bath liquid used. If, for example, the bath cover is not included in the standard accessories, the upper limit of the working temperature range is restricted to the operating temperature, which can be reached by the circulator without a bath cover at 20°C ambient temperature.

The operating temperature range, by contrast, is limited by the permitted lowest and highest operating temperatures. For example, for a heating circulator the specification includes temperature ranges below the intrinsic temperature of the unit. This is always the operating temperature range since in this case an external cooling device is required.

It has in the meantime become the practice for heating circulators with a built-in cooling coil to include in the working temperature range the temperatures that can be reached by tap water-cooling. However this has to be indicated.

For a low-temperature circulator, which always incorporates a cooling unit, only the working temperature range is specified.

ACC range (active-cooling control area according to DIN 12876 standard) is the operating temperature range during operation with an active-cooling unit. Example: working temperature range: –30°C to 150°C, ACC area: –30°C to 100°C. This information implies that the cooling unit cannot work continuously at temperatures of above 100°C. The working temperature range is equal to the ACC range in all Lauda devices.

This is half the average width of the operating temperature fluctuations, which is produced through the control action even under constant conditions. Definition and measurement are laid down in the DIN 58966 standard parts 1 and 2.

The temperature stability thus indicates only how the temperature at a particular point varies with time, and it does not specify the distribution of the average temperature within the circulator. Depending on the construction, on the bath liquid used and on the set operating circulator, there will be differences, which may be appreciably larger than the temperature stability. Lauda circulators generally provide such thorough mixing inside them that there are no important deviations at the bath opening. For the so-called Lauda calibration circulators, the spatial variations lie within the temperature stability range.

Temperature stability provides no information on the absolute temperature accuracy, for instance, how far the temperature indication on the display agrees with the reading of an absolutely accurate thermometer. In Lauda circulators, this accuracy is in the range of 0.10° over the range from 0°C up to 100°C. The exact details are contained in the operating instructions of the individual units.

Circulators without appropriate safety measures can represent a hazard for personnel and objects. One needs only to consider the possible dangers when temperatures outside the water temperature range involve the use of usually a flammable liquid. Lauda was the first manufacturer to provide special safety fittings on circulators for over temperature protection and low-level protection. Safety devices for circulators were defined for the first time in 1979 in the DIN 12879 standard. With the introduction of safety classes, emphasis is now on particular safety devices depending on the bath liquid being used.

In the course of internationalizing standards, DIN 12879 has been replaced by the Euronorms EN 61010-1 and 61010-2-010. Circulators must be equipped with different safety fittings depending on the flammability of the bath liquid.

Since the EN standard does not state any concrete requirements in this respect, the necessary requirements to be met are stipulated in the DIN 12876-1 standard: Units of safety class I are only suitable for use with nonflammable liquids, marked “NFL” by Lauda. The safe operation of these units is guaranteed by an integrated over temperature protection. Laboratory circulators of safety class III are suitable for use with flammable liquids, marked “FL” by Lauda. These units are fitted with low-level protection as well as with an adjustable over temperature protection.

Every circulator incorporates a circulating pump, which ensures thorough mixing of the bath liquid inside the bath and also pumps the bath liquid through an external circuit. The electrical energy taken up by the driving motor passes as thermal energy into the liquid, which leads to a slow temperature rise inside the circulator up to the so-called intrinsic temperature. The intrinsic temperature depends on the pump output, the insulation of the circulator, and on the bath liquid used. The intrinsic temperature of a heating circulator can sometimes be as high as 70°C or more. On the Lauda Ecoline circulators, the intrinsic temperature is only slightly above the ambient temperature if one of the lower five pump output steps is selected. Without cooling, the circulator can only be operated above the intrinsic temperature. The data in the literature refer only to water as bath liquid and to an ambient temperature of 20°C. With more viscous bath liquids and at higher ambient temperatures, the intrinsic temperature may be appreciably higher so that the specified working temperature range is restricted.

Silicone oils dissolve silicone rubber, which leads to problems with leaks after a while.

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