Tempco Blog articles

Logarithmic mean temperature difference in heat exchangers selection

12/07/2019

LMTD stands for Logarithmic mean temperature difference, being the logarithmic average of the temperature difference between the hot and cold feed on primary and secondary end of a heat exchanger. The value is fundamental for the calculation of the thermal exchange surface of a heat exchanger.

The LMTD is a crucial value also for the selection of the kind of heat exchanger most suitable for a certain application. Thermal transfer between two fluids presenting a short temperature gap is indeed very slow, while it will be much more faster and efficient in case of steam at 130° C to heat up cold water to a temperature of 70° C, for example. For the same reason, heat recovery is much more efficient having a small quantity of water at very high temperature instead of a lot of water at a mild temperature.

Plate heat exchangers allow to work with very narrow log mean temperature differences, thanks to possibility to work with countercurrent fluids and with turbulent flows, achieving high thermal exchange rates. Thermal transfer rates of shell and tubes exchangers are instead lower, thus requiring a higher LMTD. The Log mean TD is therefore even widened for air/water or air/steam finned pack exchangers, requiring very extended thermal transfer surfaces.

In general, the logarithmic mean temperature difference is inversely proportional to the thermal transfer surfaces. Even a half degree of difference has a strong impact on the heat transfer surface and the calculation of a heat exchanger, thus on its cost.
In Tempco we’re are willing to provide you some example of this kind of calculations!

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Cooling of soda solution in oil & gas depuration process

05/07/2019

We have recently realized an interesting application for the cooling of soda caustic solution employed in the separation and purification process in oil & gas facilities. The system employs a plate heat exchanger served by a chiller, that produces nonfreezing solution at the temperature of 0° C employed for the cooling of a caustic soda solution at 4%. The soda solution comes from the depuration process of the condensations of the petrochemical plant, with an inlet temperature in the plate heat exchanger of approx 45° C and outlet temperature of 25° C.

The treatment of condensations in an oil & gas plant is achieved using a bipolar cation and anion-exchange resins membrane process, that removes the ions present in the condensations cooled down at 45° C, in order to avoid possibile pollution by hydrocarbons. The exhaust resins are then regenerated with solutions of sulphuric acid and soda at 4%.

The heat exchangers has titanium plates, ensuring corrosion resistance due to possible presence of chlorides concentrations. The thermal power capacity is 500 KW, and all the connecting pipes are mixed rigid and flexible, in order to allow easy installation operations on-site.

The chiller provides the heat dissipation of the thermal energy removed from the soda solution, and must ensure 365/24 operations. The system is thus equipped with redundancy, with multi-compressor and multi-circuit, avoiding interruptions of the depuration process even in case of partial fault. In addition, the chiller has been sized for the most challenging summer design conditions, to guarantee the maximum efficiency in all variable weather conditions in winter/summer. A step regulation allows to adjust the power capacity to the season and to the effective thermal need required.

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First start-up of a thermoregulating unit

28/06/2019

The initial start-up of a thermoregulating unit is a very delicate monent, on which many customers ask for our support.

Industrial thermoregulation units by Tempco are supplied completely tested and checked. Once on-site, the units must be filled with the working fluid and started-up, and this operation is crucial for the safe and optimal functioning of the units.

The filling is indeed a very delicate step. In case of water or pressurized water as working fluids, the operation is simplified because water easily flows within the hydraulic circuits, giving no problems related to air bubbles. The operation becomes more delicate with diathermic oil thermoregulating units: the heat transfer oil has indeed a high viscosity, capturing and holding air bubbles.

It seems a trivial tip, but the very first thing to do proceeding in the first start-up of a thermoregulating unit is to check the rotation direction of the pump. This is important for two main reasons, the first being to ensure the correct functioning of the unit to achieve maximum efficiency, and then because a wrong pump rotation direction can damage the spring of mechanical seals of the pump.

Let’s then start filling up very slowly the unit, taking care of completely eliminate the air within the hydraulic circuit. Once the air is completely removed, is it possible to proceed with the first start-up of the thermoregulating unit, turning on the pump and checking the pressure indicator on pump delivery. The pressure indicator must be stable, indicating the nominal project value of the working pressure of the pump. Otherwise, if the indicator is shaking, it means there is still some air inside the circuit, that must be removed. The cycle must be repeated until the pressure is completely stable.

Once the air bubbles are totally removed, the temperature can be raised, setting an lower set point on the thermoregulator compared to the nominal final working temperature, checking that the pressure indicator is stable. The operation must be repeated until the final working temperature is reached.

The first start-up of a complicated industrial thermoregulating unit working with diathermic oil can even take a whole working day, but it’s very important to carefully achieve it. In particular, the first heating up run must be done gradually and very slowly, to ensure that there is no air bubbles within the hydraulic circuit, avoiding serious damages on the mechanical components of the unit, such as mechanical seals and heating resistances.

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Brazed plate for renewables and electric vehicles

21/06/2019

Talking about brazed plate heat exchangers, the new R series of heat exchangers offers heat transfer efficiency increased by 10%, and is optimized for applications in heat pumps and HVAC systems. Brazing material is copper, for a maximum working pressure in a range of 30-45 bar and maximum working temperature of 200° C.

The BPHE are available with two formats, with maximum number of plates of 120, available with different combinations of plate pattern, H, L or M.

The application fields are various, starting with HVAC systems and cooling and heating in industrial processes, such as machining centers and machine tools and plastic injection and extrusion systems. Especially interesting are the applications in the field of renewable energy, for example in gear boxes and hydraulic units in wind power generation and as evaporator, condenser and heat recovery in cogeneration and ORC plants. But also in transportation, not only for motor oil cooling in engine systems but also in battery cooling systems in electric vehicles, cars or buses.

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Brazed plate heat exchangers, copper or nickel?

14/06/2019

Brazed plate exchangers are a particular kind of plate heat exchangers that can be employed in applications with high pressures and a wider range of temperatures. This is due to the peculiar manufacturing process involved, that gives brazed plate heat exchangers a high mechanical resistance: plates in this kind of exchangers are made of cold-pressed stainless steel, and during the assembly of the heat exchanger the plates are piled in top one of each other with cold-pressed foils of copper or nickel between them.

The plate heat exchangers is then placed in a vacuum furnace where, at high melting temperatures respectively for the copper or nickel, the two metals melt, flowing for capillarity in the contact points welding them. All the brazed plate exchangers are checked with hydraulic pressure test, ensuring there are no pressure drops and to eliminate the defective ones.

When is better to choose copper vs. nickel? The selection must consider two factors, the need to work with corrosive fluids and the pressure level involved in the application. Copper is indeed an excellent material, but it is not compatible with steam containing amine and ammonia, being corrosive on the metal and leading in a short time to leaks and pressure drops.

A copper brazed plate exchanger on the contrary can stand pressures up to 30 bar and more, while a nickel brazed exchanger offer lower working pressure limits, 15 bar for a standard version and up to 25 bar for special versions. On the other hand, nickel is corrosion resistant to aggressive fluids that can cause corrosion to copper.

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Dimple jacket exchangers in a seaport

07/06/2019

We have developed last year an application for the dissipation of the heat produced by heat pumps, using TCOIL dimple jacket exchangers immersed in the Lake of Como. After less than a year, we are working on a new similar application that is now transferring us from the lake environment to the sea.

The solution employing TCOIL dimple jacket heat exchangers is indeed perfectly suitable for another customer, who requires the cooling/heating of water serving heat pumps in a seaport in the Italian Liguria region, using the presence of sea water. The customer at the present employs traditional plate heat exchangers, that due to the presence of sand and an inefficient filtration system used to get often clogged, leading to conditioning/heating problems of the connected equipments.

The solution developed and designed by Tempco consists in two batteries of TCOIL exchangers immersed in the waters of the seaport, manufactured using chloride resistant materials, I.e. SAF 2507 (duplex), avoiding corrosion issues with the dimple jacket exchangers operating in continuous contact with the sea water.

The installation is planned for July 2019, just in time to the start of the summer season. On a second step, we will also supply the customer further additional heat exchangers, for the revamping and energy consumption optimization of the plants.

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Fouling factor in heat exchangers

31/05/2019

More about heat exchangers, and in particular let’s face the topic of fouling factor in heat exchangers. This is a relevant topic in order to ensure the proper thermal transfer efficiency in exchangers, as I’m often asked about it. Fluids flowing inside heat exchangers can indeed contain particles that lay down in sediments in the long term, sticking on heat transfer surfaces and lowering the exchanger’s thermal efficiency.

The fouling factor is a sizing up coefficient to be applied while engineering a heat exchanger. In fact, a higher margin must be accounted in the sizing of a heat exchanger, counteracting the fouling on heat transfer surfaces due to the settling of fouling particles.

The evaluation of the coefficient relies on the kind of flowing movements involved in the heat exchangers, impacting the quantity of particles that can buildup on thermal transfer surfaces. The fouling factor then varies depending on the kind of heat exchanger, being different for a shell & tube exchanger or a plate heat exchanger. The effective fouling factor values for a plate heat exchanger are ten times lower than the ones applicable to shell & tube heat exchangers, because fluids in a plate heat exchanger have a much more turbulent flow.

A turbulent flow of the fluid within plate heat exchangers allows to drag the particles that have the potential to settle on heat transfer surfaces, thus requiring a lower oversizing coefficient of the exchanger. Another reason is represented by the construction of the kind of heat exchanger itself: a shell & tube exchanger, once it’s manufactured, cannot be expanded, while a plate heat exchanger allows to vary and increase the number of plates, boosting its thermal transfer efficiency in case of fouling fluids or different operating conditions compared to design parameters.

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Heat recovery in food delivery

24/05/2019

Increase the standard of food delivery to taste the food fragrant as it comes out of the restaurant, employing an oven that recovers the waste heat from the engine of a scooter using a heat exchanger. That’s the really smart idea of the Italian start-up Hotbox, developed upon the know-how of a team of aerospace and industrial automation engineers, including the founder and ceo Anthony Byron Prada. The Hotbox start-up realized a heat recovery solution for food delivery that allows to keep the food warm and crunchy, with the catchy slogan ‘Taste the food, not the journey’.

Hotbox consists in a robust structure housing a heat exchanger, an air recirculating system and a de-humidifier, and it allows to keep the food warm at a temperature of 85° C, and crunchy thanks to the removal of the humidity, for over 40 minutes.

The solution employs the HotAir and SteamFree technology, removing the excess of humidity that gathers during the delivery, keeping the food warm and dry as if it was just coming out of the kitchen. The solution is ideal for restaurants and the many food delivery companies that are growing everywhere in the world, such as lobo, Deliveroo, Just Eat and Uber Eats.

The technology that leverages the waste heat of the engine in scooters is already compatible with three cargo scooter models among the most employed on the market (Kymco Agility Carry 50/125, Peugeot Tweet Pro 125, Sym Symphony Cargo 125), and it will be soon adapted to many other vehicles.

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Functioning of a chiller

17/05/2019

We’ve already talked about heat pumps in a video in our Tempco Youtube channel, maybe skipping a step. That’s why I’m dedicating a new tutorial to the functioning of a chiller (english subtitles available), responding to many requests I’ve received.

Let’s start saying that we all have a chiller in our own homes: a refrigerator is indeed a chiller, working on the same physical principles. The explanation of the thermal cycle of a chiller can be commonly found in many sources on the web, involving cycles of compression, expansion, evaporation and condensation. I wanted better to underline an essential concept, being the fact that a chiller is not a ‘cold maker’, but it simply removes heat, using a refrigerant gas, or freon, as fluid thermal vector.

The refrigerant enters the environment to be cooled through a compressor, and passing through an evaporator, that is a heat exchanger, the gas evaporates. To achieve the status change from liquid to vapor, the gas absorbs thermal energy, heating up, cooling down as a consequence the environment’s temperature.

The gas reaches then the external condenser, another heat exchanger, where it cools down condensing and returning to a liquid state, dissipating the heat into the external ambient. The complete thermal cycle is then accomplished as a simple heat transfer cycle, from an environment to be cooled toward an outdoor space.

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Thermoregulation in recovery and refining of precious metals

10/05/2019

The treatment of precious metals is a very interesting and challenging field for thermoregulation applications. Recently, we’ve supplied a couple of thermoregulating units expressly developed for a customer specialized in the recovery and refining of precious metals. These last two thermoregulating units supplied joined the approx. 12 machines installed at the customer’s facility through the years, working on reactors employed in the refining process of precious metals.

The two new thermoregulating units are the result of the longtime experience of the previous installments and of the co-engineering with the customer, that requires in its production process very extreme and reliable equipments, ensuring continuous operations with pressurized water at the constant temperature of 135° C.

This peculiar need required the implementation of specific components, such as a Grundfos pump with special liquid-cooled seal. The thermoregulating units work with brazed plate heat exchangers in impact version for high pulsating pressures, specifically engineered for employ with steam and pressurized water. Modulating control valves and on/off pneumatic valves have also been installed, equipped with metallic actuators in order to stand extreme thermal stresses.

The units finally have a flowmeter monitoring the flow rate, a crucial factor to ensure the thermal exchange efficiency in the reactors. Pipings are all made of stainless steel, with flanged connections (and welded wherever they’re not flanged) and spyrometallic gaskets resistant to high temperatures.

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