Product News Letters

Product News Q1: Modular Heating Cooling and Chilling the Thermal Control Units -

  • This stream needs chilling. How to do it?
  • How do I cool the contents in my jacketed reactor?
  • How do I get heat to my forming press?
  • The answer is.....?

Modular Heating Cooling and Chilling the Thermal Control Units

Fluid Heating, Cooling and Chilling packages have become the established method for providing heat transfer services to process users. These can include reaction vessels in the chemical and pharmaceutical industries, presses in the forming industries, fryers in the food industries and waste heat recovery in many different guises. The list is endless and they are applicable across a wide range of industries. They all have the common need of heat removal or addition or both.

Equipment Selection

Accurate Control of Temperatures at all stages in the process cycle is, in most cases, vital to ensure the quality of the end products. The process users may be operating over temperature ranges anywhere between -80 C and +350°C and only specifically selected heat transfer fluids will allow this to be done with accurate control at all stages. (We have discussed the selection of heating fluids before in these pages. If in any doubt, give us a call).

Accurate control of temperatures

The performance of any process is very dependent on the Correct Selection of the Heat Transfer Fluid and on the circulation rate around the jacket to suit the full operating temperature range. This is particularly important if a cooling or chilling stage is used. 'The prediction of the heat transfer performance is dependent on a correct assessment of the variations in heat transfer coefficients at all stages of the operating cycle.

Thermal Fluid Systems have developed programmes for Accurately Predicting Performance and have verified the reliability of these predictions from experience gained from the many installations executed over two decades. This experience allows the development of fully cost effective Thermal Control Units for all types and sizes of process user.

These modular skid units often have open construction and allow easy access to all working parts. However for onerous environments such as severe cold they can be manufactured with a stainless steel casing or within a process building (usually a container) and still be transported to site. And always they have the Integrity in Design demanded by the major Engineering Contractors and by the high level engineering standards of the end users.

Operating Range can be extended from -80 °C to +350 °C by including an electric heater and suitable chilling facilities. In larger installations a central gas or liquid fuel fired heater can be utilised supplying more than one TCU skid unit for heating and cooling a variety of different users.

Thermal Fluid Systems Ltd. can offer Thermal Control Units for operation on the full range of available heat transfer fluids plus water and water/glycol mixes. The Thermal Control Units are a fully proved solution to all Heating, Cooling and Chilling problems in the process industries.

If you think we can help with one of these or indeed any Thermal Fluid problem please don't hesitate to call

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Product News Q2 :Choosing a Heat Transfer Fluid

Choosing a heat transfer fluid can be daunting. Here we discuss the various properties to guide you in the right direction. At the end of the day it is viscosity that has the biggest effect on heat transfer efficiency. But of course the right thing to do is to give us a call and we'll make sure you make the right choice.

There are a huge number of Heat Transfer Fluids to choose from. One supplier alone has 15 different products. Multiply that by the number of suppliers and that makes the whole exercise a little daunting.

However we can start to whittle down numbers very quickly. Let's start with temperature of operation. If your process is involved in low temperatures then you need to consider the lower viscosity oils. Manufacturers go to alot of trouble to ensure they have an oil suitable for low temperature operation. But be careful to consider what your top end temperature will be. If it is high you may have operate at pressure as low viscosity oils can have a high vapour pressure at higher temperatures.

And comparing one oil with another. What are the advantages and disadvantages of each? Well it depends what you are trying to do.

For example, low viscosity improves heat transfer but low density reduces it. High thermal conductivity makes water efficient but most non-aqueous fluids have very low conductivity. The only way to compare thermal performance of 2 fluids is to calculate the fluid heat transfer coefficient (also known as the inside film coefficient) using some variation of the Seider-Tate equation. The way the science works in this equation, Viscosity has the most influence on heat transfer, followed by Density, Specific Heat and least of all Thermal Conductivity.

If cost is your major concern then the price of the oil is key. But also its heat transfer coefficient. Could the right choice here reduce the size and hence cost of your equipment? If so, an expensive oil could pay dividends, despite its intrinsic cost.

At high temperature stability becomes an issue and oils need to be designed for continuous operation at above 300 °C. Of course such versatility will come at an increased cost.

And finally don't let's forget water. It has the highest heat transfer coefficient of all the fluids. If you can use it you'll tend towards smaller equipment. Plus in and of itself, it is cheap. Water can operate up to 170 °C with glycol added, so well worth considering. However, water has the downside of being corrosive to steel and above 100 °C has to operate at elevated pressure, so that is where the balance comes.

Discuss the Choice of Fluid with your Thermal Fluid Systems engineer. He will advise the best path forward for you. Here at Thermal Fluid Systems we can advise on the whole range. Back to top

Product News Q3: Choosing a Heat Transfer Fluid - Part 2

A leading manufactured of packing materials decided to switch his old heaters to more efficient and reliable heaters. New dual fuel Bertrams heaters were installed in stages to minimise downtime because the process was contineous. TFS also undertook the piping modifications of the HTF lines to accomodate the new heaters.

In our last bulletin we concentrated on the physical properties and their effect on heat transfer. This month we consider temperature limits in more depth.

Under normal operation, where turbulent flow is used, the heat from your source must pass through the metal wall of the exchanger, be it an electric or gas heater, or a straightforward shell and tube unit. Then it has to pass through a thin layer of liquid which "sticks" to the tube wall or at least moves slowly in comparison to the bulk fluid. This is known as the boundary layer or inside film. Since it truly is adjacent to the hottest part of the equipment it is here where any damage is done to your heat transfer fluid. Though the average fluid temperature may be ok according to your fluid manufacturer's data, this film can exceed that temperature. The molecules then start to crack and you end up with carbon and low boiling species. These by-products can cause fouling of heat transfer surfaces and in extreme cases, blockages, and, increase your vapour pressure and reduce the flash point of the fluid.

Realistically this is mostly a problem with heaters. For example in a gas heater the radiant section can be extremely hot, much hotter than the average fluid temperature in any part of the surrounding tube. There is little resistance to heat transfer through the tube wall and hence the film is exposed to quite a high temperature. Generally the heater manufacturer will design for a high fluid flow within the tube to dissipate the heat flux quickly and to minimise the depth of the film layer.

The average film temperature can be calculated from the total heat transferred and the calculated film coefficient. But in reality it is an estimation, unless you want to go in for finite element analysis. And the maximum film temperature, ie. that bit closest to the wall, is even higher.

The average film temperature can be calculated from the total heat transferred and the calculated film coefficient. But in reality it is an estimation, unless you want to go in for finite element analysis. And the maximum film temperature, ie. that bit closest to the wall, is even higher.

Manufacturers will tell you the maximum film temperature they recommend. This can be 15 to 30 °C higher than the maximum recommended bulk temperature. However the more separation you have for operating temperature away from these maxima, the less problems you will have with your fluid ageing.

At minimum temperature the considerations are much different. First you have to be able to pump the material. For this to be possible with a centrifugal pump requires a maximum viscosity of around 400cps. Any more than this and it will become impossible. So you should always look to choose a fluid with a viscosity below this level at the minimum temperature you are liable to be operating at. If it's -5 °C on a bitter New Year's morning, you don't want to be called in because the "pump's not working"!

If you have to choose a fluid for some reason that is not always pumpable you need to think about heat tracing.

Viscosity also affects heat transfer. In fact, as we said in part I, it has the largest effect of all the fluid properties. This is especially important if you're designing for cooling as well as heating. And it compounds itself if you're also looking for a very high maximum temperature. Because, as you might guess, most fluids that are good at high temperature tend to fall down at low temperature. However manufacturers have designed some very nice solutions for this and some fluids are extremely versatile. But you must expect to pay a higher price for a more sophisticated oil. We have to pay for all that additional development, they really are more expensive.

It is difficult to get meaningful heat transfer above 20 cps.

Again, give us a call and we'll help you solve your issues.

Good Heating

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