In gas analysis, sample gas coolers protect valuable analysers from condensate and other substances. Reason enough to be careful when selecting it. In addition, the characteristics of the available cooler types vary greatly; the correct cooler will ensure a good analytical result, the wrong one makes it questionable. The high performance version is not always the preferred choice. The option to optimise scrubbing as offered by Bühler Technologies opens up new opportunities for improvement in some applications.

Cooling and condensing are closely related. The fine drops on the outside of the window show: It’s not only chilly outside, but also humid. The decrease in temperature causes humidity to condense, which is the same principle used by all sample gas coolers. The hot sample gas the gas probe removes from the process stream, sometimes with the help of a pump, is cooled below its dew point. So the humidity and other vapours inside the sample gas already condense inside the cooler, not in the analyser. There they can in the best case alter the analytical result, or worst case even damage the analyser.

There are two important principles to choose from – depending on the required performance range: Peltier coolers and compressor coolers. These are typically optimised for performance. This also applies to the wide range offered by Bühler Technologies. However, as of late this manufacturer also offers so-called washout optimised versions. Dr. Claus-Peter Jellen, Gas Conditioning Product Manager at Bühler, explains: "The new scrubbing-optimised coolers are particularly of interest for emissions measurement, where the Sulphur dioxide content can be falsified due to scrubbing this gas inside the cooler."

But let’s start with the "classics" which were a herculean task to select with regards to this application. Until now. After all, a number of criteria and factors need to be considered to find a suitable and durable cooler with sufficient power for the task, yet not oversized, thus unnecessarily expensive. A sophisticated calculation program Bühler calls the "Gas Cooler Finder", which is quite the understatement, now makes this a lot easier. Considering the large range of gas coolers offered by Bühler, this program "stocked" with key parameters not only narrows down the choices of suitable coolers. It also shows users to which extent the suggested cooler will be running near cooling and heat exchanger capacity under the respective conditions and whether there is any room left. So 98 % cooling capacity means: The gas cooler would be running near capacity. Jellen states: "We would not recommend this type of cooler, even if it would be nominally sufficient. It would always be running at under high load and be correspondingly sensitive. This would not serve the user, as there should be a higher cooler capacity reserve to accommodate fluctuations in the process and the environment. When operating the sample gas cooler under lower loads, a longer service life can be expected."

When considering these types of tips, the calculator will easily narrow down the selection - to two or three base models which will then need to be compared. Important parameters are the ambient conditions, for one. For applications in zone 1 explosive areas, after entering all of the parameters the Gas Cooler Finder will then for example suggest the EKG 2A Ex, an ATEX and GOST approved compressor cooler with a nominal capacity of 615 kJ/h, available with up to four gas paths. As with all Bühler gas coolers, the user can choose from a stainless steel, glass or PVDF heat exchanger. The high performance cooling principle of this model also covers applications where large volumes of relatively humid sample gas are moved to the analyser or which are to be operated in ambient temperatures of up to 45 °C.

Since the range of Ex-safe coolers is limited, the Gas Cooler Finder will also quickly determine if the user is asking the "impossible", such as a gas cooler for Ex areas with four gas paths, two heat exchangers and a gas pressure of 6 bar absolute. The calculator will in this case straight away limit this parameter to the available 0.5 to 2 bar. Likewise with maximum gas temperature and inlet temperature options. From the very start, the program will only provide project developers with sensible values to choose from. The alternative would be trial and error – the program prevents the user from this and the results page also clearly indicates if the parameters may result in a possible but oversized solution. This is where Bühler is upfront: If the application would require the Ex-safe cooler working at only 4 percent capacity after all, the user would possibly also accept the extremely low utilisation of capacity.

The second principle Bühler uses in line with its process analysis range is Peltier cooling, so thermoelectric cooling, which offers less cooling capacity but will suffice for many applications in process and emissions analysis. The TC (Thermoelectrical Coolers) series, just as the EGK compressor cooler series for safe areas, covers a wide range of gas coolers. The project developer will typically be "open"; the calculator may also suggest EKG and TC coolers which can be used for his application. But even if the result shows some cooler types which would be overloaded by the selected parameters, it can guide the project developer in the right direction. Some slight modifications with a marginal redesign of the analytical process could possibly increase the options. Then, e.g. in addition to the higher priced compressor coolers, a Peltier cooler may be an option after all.

Event the heat exchanger material choice affects the subsequent parameters to choose from and ultimately the results provided by the calculator. The material for maximum cooling capacity is stainless steel thanks to its excellent thermal conductivity. However, if corrosion resistance is important, it’s better to choose glass or PVDF, the least conductive material. Acidic components inside the gas will typically require this. If you only need to supply one analysis measurement with sample gas, you will have numerous cooler with single gas path and one heat exchanger to choose from. If the user is looking for several gas paths, the calculator will specify the number of heat exchangers to choose from: for eight gas paths, for example, four heat exchangers, for four gas paths two to four heat exchangers. Here the lower dew point can be specified as an improvement of +2 to +20 °C, corresponding to the gas cooler settings. The heat exchanger material also limits the gas pressure options – a steel or glass heat exchanger for example will handle a higher gas pressure than PVDF. With respect to the lower dew point, electric gas coolers can be adjusted between +2 and +20 °C, with a factory setting of 5 °C. Other variables are the maximum gas temperature, the humidity or the inlet dew point of the gas, and of course the maximum volume flow. The ambient temperature also needs to be specified as another key factor for cooler selection; a high ambient temperature requires a higher cooling capacity.

Doing so as shown in Fig. 2a will return a list of 5 coolers (see Fig. 2b) designed for maximum cooling capacity. However, a closer look will show the TC-MIDI 6111 would already be at 97 percent capacity; it may be advisable to select the TC-MIDI 6112 utilising only 82 percent of the cooling capacity, or one of the compressor coolers shown. The "View" button will then provide the project developer with detailed information about the respective cooler type including data sheet and operating instructions, as well as available spare parts.

He will further have the option to determine if there is a cooler optimised for maximum scrubbing which fits the selected parameters, or which parameters he may need to change for a match. For example, this would require the option "2 heat exchangers", as the principle of scrubbing optimisation is based on two heat exchangers connected in series

The available models have a + at the end of the model name, so e.g. TC-STD+, TC-Double+ or EGK 2-19+.

The calculator makes project planning considerably easier. "The planner can determine the effects e.g. a change in the ambient temperature or flow rates beforehand. So he can take certain contingencies into account from the start", Jellen concludes. The Technical Support for the Gas Cooler Calculator provides lots of tips and information about the special features of the different cooler types, the choice of materials, and gas parameters.

A carefully selected and configured cooler will provide high quality analysis results, not least due to Bühler coolers being known for a high dew point stability of 0.1 K within the specified range. Jellen illustrates: "Substantial fluctuations are undesirable, as they would produce errors in the analysis results."

The range of coolers offered by Bühler will cover virtually any conceivable application in emissions and process analysis. The TC-Mini is advantageous for applications where saving space is important, however the cooling capacity is limited to 55 kJ/h. In addition to the being able to set the dew point at 3, 4, 10 or 15 °C, it does have a Delta-T control option. This guarantees maximum condensing out of the sample gas. As with most Bühler gas coolers, a fine filter or a moisture detector monitoring the function can be built in. "Moisture penetration from the process could temporarily overload the cooler. The moisture detector will then give the alarm, thus protecting delicate analysers."

The larger and more powerful TC-Midi Peltier cooler is available with one or two gas paths. In addition to a moisture detector and filter, it’s also available with peristaltic pump for the condensate, and integrated sample gas pump. Being powered by the cooler is also beneficial with respect to investment costs. The integrated layout also benefits maintenance: The tubes on the condensate pump are quick to change - without tools. The pumps can be stopped from the menu for this type of maintenance but the cooler itself continues to run to quickly return to analysis.  This results in a compact, ready to connect solution to aid with safe process control in the chemical industry, petrochemistry, and biochemistry. The display, which shows temperatures and alarms, can be used to set dew points and alarm thresholds. An alarm output is used to connect to higher-level control systems. The powerful cooler, available in versions for 40 and 50 °C, can even temporarily be operated at 60 °C. "With one case in the Middle East, for example, where the cooler was installed inside a temperature-controlled cabinet, higher temperatures had to be expected when opening the cabinet", Jellen tells. "The electronics on the TC-Midi will easily handle this overrun and cooling will even then still be guaranteed."

Other Peltier coolers are the TC-Double with double cooling system and the TC Standard base unit, both of which are also available in a scrubbing-optimised version, thus also suitable e.g. for emission control. These can be used in automated measuring systems (AMS) per EN 15267-3.

The Bühler compressor coolers preferred starting at certain operating parameters include the very common EGK 1/2. Apart from the compact design it also features a built-in peristaltic pump option. Compressor cooler are available for wall-mounting and desktop operation, or as 19" cooling units. Depending on the version and model they have between one and four, and upon request even up to eight gas paths.  In addition to built-in peristaltic pumps, the EGK 2-19 also has the option of a built-in fine filter and moisture detector as well as an analogue output for the temperature. The robust compressor sample gas coolers generally feature more cooling capacity than Peltier coolers, particularly at higher ambient temperatures.

On of the most powerful coolers is the EGK 4S featuring a cooling block with four cavities for plug-in heat exchangers in various materials. These can be used to control multiple analyses. Its performance is only topped by the EGK 10, suitable for very high flow rates of up to several thousand litres. Dr. Claus-Peter Jellen advises: "This sample gas cooler is an option for those requiring very fast response times. It can further also be used for very warm, humid gas found for example in steel plants."

Functional principles

Two cooling principles to choose from

Compressor cooling: With a few exceptions, compressing gas will raise the temperature. Expansion will lower it. This effect named after Joule-Thomson is used in compressor-based cooling. Furthermore, the temperature of a pure liquid remains constant whilst boiling/evaporating. Another basic principle of physics: Evaporating a liquid (phase change) requires a lot of energy – which is released again during condensation. This is used in the compressor cooler. Low pressure gas (evaporated coolant) moving from the evaporator is condensed. This raises its temperature. The hot gas is cooled to and condensed in the heat exchanger of the cooling system. The liquid coolant partly evaporates due to the decrease of pressure; cooling the other part. It then reaches the cooling block in the sample gas cooler. Here the coolant is again heated by the warm sample gas. The heated, partly evaporated coolant is transported to the evaporator, where it heats up more and evaporates. The cycle then repeats.

Peltier cooling (thermoelectric cooling): This type uses the Peltier effect (named after Jean Peltier, 1785 – 1845) of certain semiconductors. The stream produces a temperature difference (and vice versa). The effect is based on based on different energy levels of the so-called conduction bands of n- and p-doped semiconductors. Electrons flowing from one semiconductor to the other absorb energy, resulting in cooling. 

Dr. Claus-Peter Jellen, Gas Conditioning Product Manager at Bühler Technologies: "Using the Gas Cooler Finder the project developer can easily determine the effects of a change in the ambient temperature or the flow rates, for example."