How to control electric heaters in thermoregulating units?

Which are the possible solutions to control and manage electric heaters employed in thermoregulating units? There are in fact three possible options, offering a more and more fine and accurate control of temperatures.

The first option for the control of electric heaters in thermoregulation is to use an electronic thermoregulator, that ensure the fine regulation of the temperature set-point by commanding a contactor, hence electric resistors, with a certain frequency. Clearly, if the resistors have a high frequency of intervention, especially in case of high power resistors, the service life of the contactor will be limited.

Contacts in a contactor are indeed designed to ensure a certain operating life, with a number of cycles, and beyond it the contacts get damaged. It happens that contacts get welded together inside the contactor, so that they don’t respond anymore to the thermoregulator, pushing the thermoregulating unit up to excessive high temperatures.

There are obviously some safety systems, such as safety thermostats, that provide the interruption of power and then halting the system. In this first option, thermoregulation has then some physical limitations, due to the maximum number of intervention allowed by the electro-mechanical contactor.

 

 

A second option is to employ a static relay, which is electronically managed avoiding electro-mechanical contacts. This solutions also enables to operate micro-openings and micro-closings, achieving a more fine and sophisticated temperature regulation.
Anyway, even a static relay can get damaged and thus remaining in a closed status, pushing the thermoregulating unit over the maximum safety temperature. Also in this case, the safety thermostat will operate, and usually in our Tempco thermoregulating units we implement a line contactor managed by the safety thermostat, providing the safety halting of the system when maximum temperatures allowed are exceeded.

At last, a third option is to employ a SCR (silicon controlled rectifier) static relay, with power adjustment. These device allows an even more fine and precise control of the temperatures. Instead of working with on-off activation of the thermoregulator, an SCR allows indeed a modulation of power supply, from a minimum value up to a maximum temperature value. Exactly as if it was an inverter, but applied to electric heaters.

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Thermal energy management guide Tempco is online

The strong and consolidated expertise of Tempco in the field of thermal energy management led a few years ago to the making of a book called ‘Thermal energy and industrial processes’, which is now being released also in English and made it available in a dedicated section of the Tempco website.

We have indeed decided to make these resources available online, in order to help operators and companies navigate through all the thermal machines available on the market, as well as getting more familiar with some essential concepts such as the evaluation of the thermal duty, a fundamental step for the correct engineering of temperature regulation, heating and cooling systems.

The main section of the technical manual is focused on the definition of the several types of thermal machines existing, and how to select the right one based on one’s individual production requirements. From classical heat exchangers, evaporative towers and chillers through thermoregulating units equipped with the most advanced IoT condition monitoring solutions, and up to smart systems aimed to further increase energy saving leveraging free cooler and dry cooler.

The manual finally includes a section offering a wide range of industrial applications realized by Tempco over the years, within a great variety of industrial sectors such as pharma, chemical and food & beverage, oil & gas, steel mill and machine tools, automotive industry, cogeneration and power generation, and also the latest innovative solutions for immersion cooling in data center cooling.

Contents will be published weekly, starting with November and going on for the following three months. So enjoy the reading, I hope it will prompt to further questions and more in depth insights to deploy and explore together!

Thermal energy and industrial processes

Tempco Energia termica

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Characteristics and applications of double wall heat exchangers

Prompted by a great and interesting video published by Kaori, our long partner for brazed plate exchangers, on its Youtube channel, let’s go talk about double wall heat exchangers.

Double wall heat exchangers are security type exchangers, having a peculiar construction with a double plate, hence their name ‘double wall’. This ensures that even in case of breaking or cracking of a plate, due to corrosion or other causes, the mixing between the primary and secondary fluid is avoided.

This is a kind of construction that can be applied both with brazed plate exchangers and gasketed plate exchangers, but also in shell and tube exchangers. This is employed wherever the mixing between the fluids is absolutely to be prevented. That’s the case of food industry for example, where products that have to be cooled before bottling, such as mineral water, beverages, milk or wine, don’t have to mix with cooling water, maybe not even potable.

Further applications are with hydraulic oil and diathermic oil, where the presence of water within the oil could lead to damages or also be extremely dangerous. The cooling of oil in power converters is another application field, where the mixing of oil and water can cause very expensive damages.

 

In gasketed heat exchangers the breakage of a gasket doesn’t lead to fluids’ mixing, because leakages go outside of the exchanger. Fluids can mix only in case of a crack in a plate. With double wall heat exchangers, even in case of cracking of a plate the leakage flows outside of the exchanger, making it immediately visible. In case of double wall shell and tube exchangers the leakage is not so immediately visible. In this case, there is a collecting chamber where leakages flow, and it can be equipped with special sensors alerting the maintenance team or the plant manager that a leakage happened, and thus that there is a cracked tube.

Clearly, double wall exchangers involve much higher costs, because at least the construction materials are doubled. In addition, the air space between the two walls decrease the thermal exchange coefficient, thus requiring an increase of the thermal transfer surface. This is even more impacting on shell and tube exchangers, yet usually offering lower thermal transfer rates, which further decrease with the addition of a tube in tube and an additional air space.

Anyway, double wall exchangers are a mandatory choice in all of those applications where it is necessary to avoid the mixing between the fluids.

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Cooling towers in high power converter testing

Pictured here is a cooling system we deployed for a testing chamber for power converters we’ve just supplied to a customer. The company is a manufacturer of IGBT, thyristors and diode type high power converters, that employ advanced electronics and digital regulation solutions.

Torre evaporativa test convertitori

The dissipation system dedicated to the testing of power converters we have supplied is a prefabricated skid package solution, that includes:

Evaporative tower
– Pumping group
Plate heat exchanger
Water softening and dosing system
– Electrical control cabinet with inverter on tower fans

In particular, in inverter allows to adjust the speed of fans based on external ambient air temperatures, ensuring the maximum cooling efficiency with the lowest power consumption possible.

Scambiatore test power converters

Tempco scambiatori test convertitori

The cooling in test operations for power equipments is a very well consolidated field of application for Tempco, thanks to the expertise and know-how built in several projects developed. Not too long ago, we’ve indeed already talked about a quite identical application for heat dissipation in the testing of inverters, made with a dry-cooler for lower power capacities.

 

Tempco test convertitori

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Advantages and characteristics of monofluid thermoregulation

Monofluid thermoregulation is a solution that offers important benefits, especially in application within pharma and chemical industry. These industries need to control temperatures in a bunch of production equipments, such as reactors, pressure filters, blenders and mixers. All of these machines require to maintain some rising or descending temperature ramps, involving a thermal range for the processed products that vary from low to high temperatures and viceversa.

An example is a reactor employed for the production of an active pharmaceutical ingredient. Starting from ambient temperature, the product has to be heated, and then maintained at a certain temperature. Then it has to be cooled, and then maybe again heated and so on.

Usually jacketed reactors are employed, with half-pipe coil or with thermal transfer jackets. Based on the temperature level required for the product inside the reactor, a hot or cold fluid is inserted within the jacket.

In the past, but still employed very much today, to achieve heating a hot fluid was inserted inside the jacket, vapor of diathermic oil for example. When the process needed a cooling phase, then vapor and condensate got completely discharged, and the jacket was then loaded with refrigerated water or glycol water. The ensemble of these discharging and loading operations clearly take some time, and thus can slow down the productivity. In addition, this kind of solution only offers a relatively precise control of temperature levels.

Hence, monofluid technology developed along the years. This technology employs in fact the same fluid as utilities – vapor, diathermic oil, hot water, refrigerated water, glycol water -, but they all flow within an array of heat exchangers. A unique fluid then flows inside the exchangers, which is able to withstand the temperature range required by the overall thermoregulation process.

This fluid flows then within the reactor’s jacket. There are clear advantages: downtimes are eliminated, related to discharge and loading operations of the different fluids. But most of all, the risk of mixing of the fluids is avoided. Which means there is no more risk of having antifreeze water flowing inside the boiler, or condensate ending up in the chiller, diluting the glycol water increasing the freezing risk.

Advantages are then significant, and clearly there are on the other hand some disadvantages. There is a slight loss in thermal exchange, indeed, due to the fact that the temperatures of utilities are a little bit higher and lower than the temperature of the fluid that flows within the jacket. But from an operative point of view, benefits are huge making it a largely implemented solution. Finally, there is no comparison in terms of temperature control that can be achieved: it is possible to operate using a switching valve, or bypass valves and regulating valves systems, that allow to obtain an extremely fine and accurate temperature regulation.

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Fouling factor in heat exchangers and nanocoatings

Never tired to speak about fouling factor on thermal transfer surfaces in heat exchangers, as we’ve already did in the past. This is indeed a crucial aspect in the design and engineering of every cooling and heating system that employs heat exchangers, and it can be defined as the theoretical resistance to heat transfer due to the deposition of layers of dirt and substances upon thermal transfer surfaces. In order to correctly evaluate the fouling factor it is essential to determine the kind of application, the temperatures involved within the process and, of course, the kind of fluids that are supposed to flow inside the exchanger.

Tempco sporcamento scambiatori

There are four types of fouling mechanisms, that can be classified as chemical, biological, caused by deposition and related to corrosion events.

Chemical fouling
This category includes the fouling caused by the deposition of substances generated by chemical reactions that are trigger in fluids when certain temperatures are reached. An example is the formation of salts and carbonates that precipitate at the temperature of approx. 55° C. Or also, in the dairy industry, milk protein got burned at certain temperatures, settling a film upon the thermal transfer surfaces.

Biological fouling
Micro organisms can grow within the fluid, such as algae, fungi and bacteria, that stick on thermal transfer surfaces lowering the transfer efficiency, and also fostering corrosion. The selection of the right materials of the plates, which inhibit the growth of these micro organisms, is here a feasible solution.

Deposition fouling
Cooling and heating tasks in production processes often involve the presence of process fluids containing suspended particles. It is thus necessary to adopt some design precautions, such as the right section path of the flowing channels, the vertical mounting of the exchanger (so that gravity helps draining the particles toward the bottom) or to guarantee a suitable speed flow rate. Otherwise, these particles can settle and grow on thermal transfer surfaces, compromising the thermal efficiency.

Corrosion fouling
A wrong selection of plates’ materials can lead in the long term to severe corrosion effects, with the formation and deposition of oxide layers on thermal transfer surfaces, causing an isolation action that lowers the heat transfer.

In many cases, cleaning activities through chemicals or also mechanical brushing can be a solution, removing scaling and layers of dirt that in the long term can lead to the complete clogging of the exchangers, and all the way to damaging and breaking the plates.

Tempco corrosione piastre scambiatori

But it is also possible – and of course preferrable – to prevent fouling in heat exchangers, thanks to:

  • Proper selection of the type of exchanger based on the application
  • Correct selection of the exchanger’s design and materials of the plates
  • Softening and filtration of fluids, improving their chemical/physical conditions
  • Study of special corrugation of the plates to ensure a suitable turbulence of the fluids

Finally, an interesting innovation comes from nanocoating, with the development of nanotechnology applied to HVAC and refrigeration. These are nano-tech solutions studied at the Saarbrücken Leibniz Institute for new materials, for instance, offering antimicrobial, anticorrosion and anti adhesion properties, avoiding the deposition of dirt on treated thermal transfer surfaces, therefore reducing the costs and efforts related to periodical cleaning and maintenance operations of heat exchangers.

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How to prevent cavitation in centrifugal pumps

Centrifugal pumps and cavitation, an extremely dangerous bundle that absolutely has to be avoided. Cavitation is an event that happens when steam bubbles develop within a fluid, which imploding cause a characteristic noise inside the machinery, lowering its performance and damaging its components, such as pumps’ impellers and the pump itself. This is a very common phenomenon in ships’ screws and centrifugal pumps, indeed. Let’s then see what to do in order to avoid cavitation in centrifugal pumps.

Centrifugal pumps come with characteristic performance curves, the most common ones being the flow rate hydraulic head of the pump, as well as the NPSH – Net positive suction head curve, related to the maximum suction head of the pump. We’re talking about pumps having a NPSH, thus not self-priming pumps which automatically fill themselves sucking water. NPSH pumps have to be filled with water, respecting a suction head not surpassing the indicated NPSH value.

When forcing a pump operating at a lower suction value, we will surely end-up with cavitation. With cavitation, for example on a ship’s screw, air bubbles develop near the edges of the impeller, reducing the performance and imploding. The same with connection edges of the pumps’ impellers. Vapor bubbles develop when the vapor pressure is surpassed, otherwise when the pressure level decreases so much that the liquid starts evaporating. Let’s bring a basic physical principle here: at atmospheric pressure, water starts boiling at 100° C. Putting water in a vacuum vessel, the boiling temperature decreases a lot.

When a lower pressure area is generated near the pump’s impeller, it will ignite the evaporation of the fluid. Evaporating water causes micro-implosions that damage irreparably the impeller.

 

 

In addition to a wrong NPSH value, cavitation can also be generated by a wrong installation of the pump: mistakenly sized suction collectors foe example, or with sharp edges. Furthermore, putting a angle-pipe or a T-pipe very near to the suction of a pump, we will create angles that can cause lower pressure areas leading to vapor generation. Vapor spreads all over the impeller, increasing the low pressure area in the suction area of the pump, literally destroying the impeller due to cavitation.

While installing a centrifugal pump is thus fundamental to respect its NPSH curve, its suction head, or putting the pump under a positive suction head, by creating a positive pressure on the pump’s suction. At the same time, a careful design and engineering of suction collectors is important, and carefully sizing components such as foot valves, check valves and suction valves.

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How to combine efficiency and noise levels reduction in cooling towers

We’re to talk about noise levels and efficiency, referred to cooling towers applications. This is a topic we’ve already faced in the past more than once, because evaporative towers while offering high cooling capacities are also a source of noise generated by the moving parts in fans employed to obtain the required air flow.

Noise emission levels became surely a big problem in the case of a Poland gas distribution company that a technological partner of ours, Wentylatory Wentech, grabbed to our attention. The energy sector company had to face back in 2015 the risk of a class action from residentials and recreation centers that along the years were builded in the ponds near the plant.

Tempco ventilatori Wentech torri evaporative

The problem was then to adopt solutions aimed to decrease disturbing noise emissions able at the same time to ensure to same high air flow rates required to maintain the efficiency of the extensive fan cooling towers system installed.

ventilatori torri raffreddamento

The high noise emissions of the evaporative towers have been eliminated thanks to the solution offered by the company Wentylatory Wentech. The supplier verified the gas distributor situation by taking measurements at several dozens points in the plant and the adjacent residential areas. The customer was then offered the quietest 14” blade model fans on the market, which combined the high efficiency rates required with significantly lower noise emissions.

Tempco ventilatori Wentech bassa rumorosità cooling towers

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How to proceed to the correct start-up of a centrifugal pump

We talk again about centrifugal pumps, and in particular about the correct start-up procedure of a pump in order to ensure its proper functioning. This is a very common operation during the commissioning of our thermoregulating units.

The first step in the start-up procedure of a pump is to fill the circuit with the working fluid, water, glicol water or oil, using as reference the minimum level, or just above it, or otherwise the pressure level specified in the technical manual. The next step is to give a small power to the pump, in order to verify the right rotation direction of the pump. This is important to avoid damages to the mechanical seal. If the pump rotates in the wrong direction, one of the three power cables must be switched (we’re clearly talking about three stepper motors).

Next step is to power up the pump, throttling the gate valve installed on the pump’s inlet. While powering up the pump we check the value of the pressure indicated on the pressure gauge on the inlet. The value must be stable. During the start-up of a plant, there will be easily the presence of air bubbles within the circuit, and therefore the measurement will be quite wobbly.

We thus have to open partially the gate valve on the pump’s inlet, stop the pump and proceed to discharge the air from the vents located on the pipes. Then the pump can be re-started. The operation has to be repeated until the pressure level on the inlet gets stable.

Now the pump must be set on it characteristic curve. Read the working pressure specified on the technical manual, and let’s measure the amperes absorbed by the pump. What we have to check is in fact that the pressure drops generated when opening the gate valve on the inlet are such as to maintain the pump in absorption mode. In case the absorption parameters of the engine are exceeded, the motor can indeed get damaged.

These operations are quite easy on plants working with water, but they get a little more complicated when using hydraulic oil or diathermic oil. The higher viscosity of oil requires indeed a longer time to fulfill all the spots of the pipes in the circuit. Very likely there will then be presence of air bubbles in the circuit for a few hours still. In this case, the overall operation must be repeated until the pressure value on the pump’s inlet gets finally stable.

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Renewables and efficiency in the underwater data center Microsoft

An important confirmation of the advantages of data center cooling exploiting the cold and constant temperatures of sea water recently came from Microsoft, with its Natick project, an underwater data center module that was installed in 2018 at a depth of 35 meters on the seafloor in the North Sea. After two years of operations, the data center has just been brought back to the surface and showed a failure rate of servers of approximately one-eight compared to an identical data center structure installed on the ground, therefore with a reliability eight times higher.

underwater-data-centers-microsoft

 

This is another main example of the potential of energy saving and cooling efficiency that can be achieved using water basins, not only for data center cooling but also for example for heat pumps, as showed in the Tempco applications of TCOIL heat exchangers at the Marina di Loano seaport and on the Como lake.

Microsoft datacenter subacqueo

Microsoft’s Natick project was based on the idea of putting the data center in an environment with constant and low temperatures, avoiding fluctuations between day and night that are detrimental for electronics components. And therefore using the cold seawater of the North Sea to cool servers achieving a big energy saving. The data center has also been powered at 100% by wind and solar energy. The structure was then filled only with nitrogen, eliminating oxygen and humidity that cause corrosion on data center equipment. An environment not suitable for human operators, but very much favorable to electronic components.

project_natick_undersea_microsoft_datacenter_1528380408

 

The project achieved very important goals, in terms of higher reliability with lower energy consumption for server cooling, lower risks of damages due to shocks and people moving among the servers. And most of all, an increased sustainability of data centers, since the Microsoft underwater data center was completely powered using renewable energy. A crucial step forward in the direction of future and sustainable data centers, to satisfy the increasing demand related to the spread of cloud and AI applications worldwide.

 

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