Process Heating - Steaming into first place01 October 2004
There are many viable ways to provide heat for processing: these can be divided into direct and indirect systems. A direct heating system is one where heat is released directly into the product - for example a gas flame heating a small cooking vessel. Indirect heating is more complicated in that a heat transfer medium is heated, and the medium is then applied to the process. An everyday example of indirect heating is the way in which a central heating system uses the boiler to heat water, which is then pumped through a heating coil in the hot water storage tank.
Most industrial process applications will benefit from an indirect heating system because of the greater controllability this provides, and/or the lower applied temperature of the heat transfer medium.
Therefore, when specifying a process, the temperature to which the product must be heated, and the need to control its temperature, must be carefully considered.
The choices usually fall between:
- direct heating using electric heating element(s);
- direct heating using an oil or gas-fired burner;
- indirect heating using hot water;
- indirect heating using steam; and
- indirect heating using thermal oil.
Because of the high price of electricity in the UK, electric heating is suited only to small processes, or those that are only carried out infrequently. Roughly speaking, electricity will be four times as expensive as natural gas or oil. It is nevertheless a very convenient system to use.
It is more likely that a direct heating system will use either oil or gas. Generally, the clean nature of natural gas will make this the first choice for most users, although availability of an industrial supply may be a problem in certain areas. Obviously there may not be an available supply in rural areas; however some industrial estates may already be saturated with gas users, meaning that obtaining an adequate supply might be expensive or problematic.
In such cases, the choice is therefore between LPG or oil, both of which will require storage facilities on site. LPG is, not without good reason, generally felt to be cleaner than oil, although it is more expensive.
However, it may be that direct heating is either inappropriate or undesirable for the process. The main problem with a direct heating system is that, because of the intense heat of combustion, the product being heated may suffer from localised overheating. Much will, of course, depend upon the product which is being manufactured. For example, if you are baking bread, a gas-fired oven is very effective. On the other hand, if the product is easily damaged by slight temperature excesses, an indirect heating system is preferable.
Although low-temperature indirect applications can be satisfied using hot water, most industrial applications requiring indirect heating make use of steam. This is largely due to the fact that steam is very controllable because of the direct physical link between its pressure and temperature. For example, when steam surrenders its heat energy it immediately condenses and no further heat is applied to the process. A mechanical valve can therefore be used to regulate the flow of steam (and therefore energy), ensuring that, when the valve is closed, no more energy can flow into the process.
However, this is not the case if, say, hot water were being used. When the flow of water is stopped, heat will continue to be delivered into the process until the retained water has cooled down.
Steam is the preferred process heating medium for many production processes. For example, most breweries prefer to use steam for heating. If a directheating system is used, localised overheating can change the flavour of the beer. Although this is considered to be an important characteristic element of a limited number of beers, it is not required in most and if not closely controlled can result in an unwanted burnt flavour.
Also, the majority of manufactured sauces are produced using steam. The cooking vessels are jacketed pans - typically 500-10,000 litres capacity - which are supplied with low-pressure steam typically at only 2 bar and therefore less than 135°C. Consider the direct heating alternative of using a gas burner, where the applied temperature could easily exceed 200°C or more, with the risk of overheating the sauce.
As steam is only water, and is inherently sterile, leaks will not present a contamination risk. They simply condense, producing a water spillage that is easily cleaned up.
Steam will generally be produced in a centralised boiler facility with energy distributed by a simple pipework system to the points around a site where energy is needed. Compare this to a site that has multiple processes that are direct-heated. It will require a fuel (oil or gas) distribution system that creates the potential for dangerous leaks, and will need a safe flue system for each process.
It is usual to produce steam in a fuel-fired boiler, although electric boilers are available for smaller loads. Steam can be produced in specialised boilers by indirect heating, using high-pressure hot water or thermal fluid.
Special 'thermal storage' boilers are also available to meet the need of certain process equipment such as autoclaves, which present transient peak demands. These boilers have a water content considerably greater than their standard counterparts, which enables a short-term supply of steam to be released should the demand cause a significant drop in boiler pressure. Such pressure drops will cause the formation of 'flash' steam as a proportion of the water spontaneously evaporates.
In some instances, where a very high temperature is needed, it can be provided using high-pressure steam. However, such systems can be very expensive because the cost of the boiler will be greater and the pipework and valves will need to be of heavier construction. The safe working limit on steam of most steel valves rated to PN40 will be 32 barg, which equates to a steam temperature of 239°C.
Thermal fluid systems are a very cost-effective alternative to high-pressure steam boilers because, while they normally operate at low pressures of typically around 2 barg, the fluid leaves the heater at temperatures of 300°C to 400°C.
However, due to the excellent searching property of hot thermal oil it is not possible to use screwed pipework, so either a flanged or fully-welded system must be used. Bellows-sealed valves should also be specified, as the glands of conventionally packed valves will not hold.
The temperature of a thermal fluid system is usually controlled using a three-way control valve that allows the circuit to flow continuously from the heater at a constant temperature with a proportion being allowed to flow to the process vessel. This is a rather more complicated arrangement than is needed for steam systems, but one which does provide accurate control.
An alternative is direct steam injection, but this is only appropriate to manufacturing processes where the inclusion of condensate from the steam is an acceptable ingredient in the finished product. This should not be a problem, even in the food processing industry, because the condensate is essentially sterile. However, it will be necessary to remove any debris such as scale and rust that might be liberated from the steam pipework. Culinary grade filters are therefore needed, and it is good practice to use stainless steel pipework downstream of the filter.
With so many options available steam is, even today, an effective and appropriate heating medium. Most steam boilers are gas-fired, although LPG or oil boilers can also be used. Electric steam boilers, although expensive to run, usually benefit from low maintenance costs and are very clean, so it is possible to locate them in a production area.
Having committed to a steam boiler, it is good business practice to consider other applications for it. For example, at the end of a day's production in a food factory it is necessary to wash down and/or clean in place (CIP) and, as production has stopped, it makes sense to use steam for CIP where practicable. Hot wash water can easily be produced using a heat exchanger and there is no need to have a separate hot water boiler and storage tank. Indeed, stored hot water can provide disappointing results once the tank temperature drops off following demand, whereas a heat exchanger heated with steam will continue to deliver a relentless supply of hot water.
It is even worth considering steam for space heating and because many space heaters are mounted at high level, low maintenance makes them a good choice. Finally, a steam boiler can provide affordable humidification control to a building - especially the office areas where air conditioning and modern ventilation systems can cause a dry atmosphere, resulting in respiratory problems for some people. A steam boiler can therefore be a healthy investment for your employees, as well as enabling a reliable production process.
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