Ahead of steam?
15 Mar 2011
Most engineers seem to think that steam is the only option for heating industrial processes: it is a very established and proven technology after all.
Many steam systems are likely to have either been in place for some time and so have no capital costs, or for a new installation could require a lower initial capital investment.
As usual, there is no one-size-fits-all answer, but one option that is increasingly being used for temperature control of processes is thermal fluid heating. The potential for reduced plant insurance premiums, maintenance gains, reliability and accuracy can often bring overall cost improvements for a customer using thermal fluid over a traditional steam system.
Indeed, a number of ’big name’ processing and manufacturing organisations have recently moved to thermal fluid temperature controllers, with companies in the food, pharmaceutical and general process manufacturing/engineering industries particularly active in this area.
Traditionally using oil to reach high temperatures - without the issues around steam pressure - thermal fluid units now also provide either water or a combination of the two for consistent lower or mixed temperature requirements.
Thermal fluid temperature control units lead to precise yet still highly flexible, cost-effective options on the temperatures needed throughout a process. Process manufacturers can change temperatures quickly at the push of a button, or ensure consistent accuracy so the material is processed within tight parameters.
Efficiency
Heat efficiency can be improved by using thermal fluid temperature control units, though this does depend on the level of maintenance and insulation used with a steam system.
However, as the temperature control units are small enough to be deployed right next to, or under, the process element they are heating, there is much less piping needed, fewer valves, less risk of leaks and minimal exposure to ambient temperatures.
These systems also usually have low-volume tanks, reducing the liquid volume in circulation and resulting in a reduced power input for heating. And, as thermal fluid units are relatively compact, working alongside the process, users can just power down, unplug and move the unit when the production line changes.
In terms of maintenance, newer thermal fluid units offer self-cleaning or non-ferrous parts and self-diagnostic systems. Manufacturers are also reducing the number of moving parts, for example using solid-state relays to reduce issues of wear and tear.
Heavy treatment
Boiler feed water is often heavily treated prior to use within a steam system, causing issues when it needs to be discharged, or if it leaks. Steam also requires high pressure to get high temperatures, which needs to be carefully managed, usually with certified boiler staff and inspection visits.
In contrast, thermal fluid heaters operate at much lower pressure, need less ’treatment’ for their fluid and do not need certified staff to manage them. Production line staff can be trained on operations and user interfaces, and the control units can guide staff through what they need to do.
Another point is the requirement to mitigate the risk of contamination within the processes or ensure specific temperatures to control chemical reactions or bacterial risks.
Thermal fluid units can be supplied with a range of options; these include food grade types, hygienic stainless steel and/or an IP (Ingress Protection) standard of 55 and above to minimise contaminants getting into systems.
Steam versus thermal fluid
Carl Knight at Fulton Ltd details the key points for those considering a move away from tried-and-tested steam:
- If the system’s required temperature is above the freezing point of water (0ºC) and below approximately 180ºC, the choice is usually steam. However, if the required temperature is below 0ºC or above 180ºC, thermal fluid - also commonly known as “hot oil” systems, “thermal oil” systems and “thermal liquid” systems - may be a better solution. Thermal fluid heater systems can be designed with maximum operating temperatures to 400ºC.
- Steam carries about 1,000btu/lb of useful energy. Hot water and thermal fluid carry much less energy (1-100Btu/lb). Steam does not require a pump to transfer the energy. The purchase cost of steam systems may be less than thermal fluid systems.
- Thermal fluid heater efficiencies can be 5-8% higher than conventional steam systems. They require no water treatment and are subject to less fouling due to the considerably lower heat flux. Also, with flash losses of a typical steam system (including steam trap losses) at some 6-14%, plus a blow down loss of up to 3%, and de-aerator losses of some 2%, the difference in efficiency becomes pronounced.
- Thermal fluid systems can be up to 20% more efficient - excluding additional heater and steam boiler efficiencies.
- Steam systems require precise use of pressure to control temperature - limiting accuracy to around ± 2ºC. Evenness of heating can also be an issue due to varying rates of condensation and condensate removal in the heat user.
- Heat transfer fluid systems should control temperatures to ± 0.5ºC or better: the metering and mixing of supply and return fluid, with high rates of turbulent flow, ensures even thermal transfer across the entire user surface.
- UK law often requires that full-time competent operating personnel supervise the operation of high-pressure-fired steam systems. The annual cost per engineer might be well over £40,000.
- Most thermal fluid systems operate at atmospheric pressure and are vented to atmosphere at the expansion tank, so they seldom require qualified or “competent engineering staff” to operate them. Pressures are limited to the pump discharge pressures (typically 2.5 Barg to 3.5 Barg) that are necessary to keep the fluid in turbulent flow, while overcoming frictional losses in the piping.
- Steam systems are well known for corrosion problems, due in the main part to poor water treatment. Air in combination with hot water, salts and other reactive contaminants presents an extraordinary potential for metal corrosion. Add the scale and deposits arising from the minerals found in most water supplies and system problems quickly compound.
Keeping soup off the boil
A leading UK soup manufacturer needed to maintain the temperature of a soup product during its transfer from the cooking vessel to the filling machine: but it was essential to maintain the soup at no higher than 90ºC so that it would not cook any further.
The design criteria stipulated a thermal duty of 6.3kW using mains water, with a supply temperature of 95ºC sited internally within a wash-down area.
Equipment supplier Tricool built a temperature controller capable of generating and extracting some 9.0kW/16,000kCals/hr@70ºC, in-line with the customer’s specifications, based on a start temperature of 21ºC, a finish temperature of 90ºC and a cycle time of one hour.
The unit, which was installed in the roof in order to free-up floor space, supplies hot water to the pressure vessel via jacketed pipe work on coaxial tube at 95ºC.
“Water doesn’t become steam until it reaches 100ºC, therefore the [temperature controller] was ideal for this type of application, where accurate temperature control of hot water is required,” said Dave Palmer, Tricool’s European process temperature control director.
