Basically, we can understand them as the definition of an oven. The heat is directly transferred to the product of the process, basically hydrocarbons or chemical solutions. For this reason, in petrochemical plants or petrol industry refineries the expression Process Heater is sometimes used. Depending on the fuel used, we can find a Direct Gas Heater or Direct Fuel Heater
Direct Fired Heaters are used for specific applications. We could consider Direct Fired Heaters basically as custom-designed equipment, given that the heat is directly transmitted to the product of the process, and any change in this characteristic could render the equipment ineffective for the new conditions of the process or even destroy the product or equipment due to unsuitability.
Their main applications are:
Within the petrochemical sector and refineries:
- Heating of crude oil to separators
- Gas pipe heating
- Control systems for the dew point of combustible gases associated with gas turbine plants
- High-pressure hydrocarbon gas flow heating at pressure reducing stations
- Steam super-heaters
- Cracking processes
- Aromatic furnaces
- Vaporization of liquids and gases
- Oxygen heaters
- Regeneration gas heaters
Other sectors, specific cases:
- Air heating – construction sector –
- Food industry
The Direct Fired Heater used in the petrochemical sector corresponds with that shown on Fig.1.
In these cases, it is usually high-power equipment exceeding 6000 kW in the majority of cases and extremely expensive, as its design is practically unique for each specific application.
It is very voluminous equipment, which means in most cases it is executed vertically, although for power between 40000 kW and 125000 kW horizontal execution is used, due to its large dimensions. Obviously its size depends on the equipment’s power, however radiation areas with heights exceeding 15 meters are not uncommon, nor are sets of equipment exceeding 40 meters in height – see Fig 2. –
Within this petrochemical sector, Direct Fired Heaters as Heat Transfer Fluid Heaters are occasionally used, or as indirect heaters. The good operation of the maintenance service of chemical plants and refineries of Direct Fired Heaters and the uniformity of spare parts are the main causes of this uptake, because as we will see later on, Heat Transfer Fluid Heaters in their specific functions are renowned for their advantages.
In the Direct Fired Heaters, or as they are commonly named in the petrochemical sector, Heaters, the basic layout is indicated in Fig 1.
One radiation zone, easier understood as a furnace with a coil (8), which can be horizontal/cylindrical as shown in the figure, but also vertical, where the final heating part of the product is carried out, which was previously heating in the convection zone, which also has a coil (4).
The temperatures reached in the radiation zone can be significant, given that the coil in this zone is not airtight with regards to combustion gases, with the furnace walls reaching high temperatures and requiring full covering in this area with refractory concrete, as well as the coil’s fittings.
This involves greater thermal inertia in the event of stopping the equipment, although the applications for which this equipment is designed involve continuous operation – 24 hours/day, 365 days/year -. Maintenance stoppages must be scheduled well in advance, and given they are not very frequent, involve long stoppages, as there are many malfunctions to be repaired after prolonged service times.
A particular characteristic of this equipment, consists of the use of the damper to control the temperature of the radiation zone – furnace -. In fact, its function is to prevent the inlet of excessive combustion air by closing the damper. Obviously, this pull increase in the chimney makes the burner fan work at higher air pressures, and therefore at a lower flow rate.
While with Indirect Fired Heaters the only variable to be controlled is the intermediate fluid outlet temperature – Fluid Transfer -, always considering not exceeding the film temperature of said intermediate fluid, with Direct Fired Heaters this variable is obviously also very important and critical, but should also share its importance with the existing temperature of the radiation zone. This requires a more sophisticated equipment control system than with Indirect Fired Heaters.
Air heating in construction sector
A specific application of Direct Fired Heaters, which although it involves specific design is not very sophisticated, as its configuration is very different to that shown in Fig.1, is that of heating for drying or heating in construction works – see Fig 3-. This method is commonly used, especially in the USA, on large works and their commencements, when the manufacture process is usually in open spaces. It is also used for heating large department stores.
The air (1) is let in at the same time as the combustion air needed for the combustion and product of the process, for cement drying and even for heating working areas, obviously with some significant needs for ventilation and the control of expelled gases so that they are not hazardous for employee health.
As advantages we can note:
- Small units compared to indirect fired heater – Fig. 4 – for the same functionality and therefore easy transport
- More economical
- Less maintenance costs
Its disadvantages as aforementioned are of a sanitary nature, with strict control of air renewal and combustions with the aim of preventing intoxication, in addition to the combustion adding humidity and carbon monoxide to the environment.
On the contrary, indirect heaters provide 100% dry and clean air, and can operate in closed spaces – when the construction works is at an advanced stage -.
Another specific application of Direct Fired Heaters and requiring a specific and particular design, is in the mining industry known as Elution Heaters.
In analytical and organic chemistry , elution is the process of extracting one material from another by washing with solvent, on almost all occasions with the provision of heat.
In the extraction of some minerals, such as gold for example, this process was and still is carried out using exchangers through which a mineral and solvent solution circulates on one side of the exchanger, and a heat transmitting fluid on the other – usually thermal fluid – and therefore Indirect Fired Heaters were used.
Nowadays, in some countries like South Africa, experimentation is being carried out with Direct Fired Heaters, with a configuration very like that of Heat Transfer Fluid Heaters – see Fig. 5 -. The filters needed for good operation of Elution Heaters is the greatest challenge in this application of Direct Fired Heaters.
Especially in frying trains – potatoes, snacks, precooked – the method used is via Heat Transfer Fluid Heaters through which thermal fluid circulates that is heated in the equipment and its heat is transferred in an exchanger to the oil that fries the aforementioned products.
Here also, like in the mining elution process, some installations have been carried out where the frying oil circulates directly through the boiler, which is then used as a Direct Fired Heater. Like in the mining process, the need for very bulky filters prevents it from being a commonly used system. The fast oxidation of oil used for frying is a factor that adds to the complexity of the system.