Heaters and heating systems

Fires caused by heaters and heating systems can be divided into two key areas those that are caused by faulty systems including poor design, inappropriate installation or lack of planned preventive maintenance and those that are caused by inappropriate use or misuse. 

The preceding paragraphs detail the hazards associated with electrical and gas systems which are also common to those sources of energy used by heaters and heating systems. As has already been discussed, the poor design, inappropriate installation and lack of planned preventive maintenance are all causes of fire. It is therefore appropriate to ensure that systems are designed, installed and subject to planned preventive maintenance. 

The installation of any heaters and heating systems must be undertaken by competent engineers installing to the design specification. It is also likely that these engineers will be retained to undertake the planned preventive maintenance of the system to ensure that it remains safe and fit for its intended purpose. 

Most heat transfer systems are of a closed loop design that circulates a heat transfer medium between heaters and heat exchangers. Circulation pumps provide flow and regulating valves are used for process control. The heat transfer medium is usually steam, a high flash point oil, or in process plants flammable liquids and gases. Inherently steam is a safer medium to use and is preferred over other mediums. When steam supplies are unavailable high flash point oils (organic or synthetics) are sometimes used. 

The cone calorimeter is also used to quantify the corrosivity of products of combustion as described in ASTM D 5485. The Cone Corrosimeter uses the same load cell, specimen holder, retainer frame, spark igniter, conical heater, and exhaust system as the cone calorimeter. A heated stainless steel sampling tube is connected to a funnel placed on top of the conical heater. A gas sample is continuously drawn from the tube at a rate of 4.5 L/min. The sampling tube is connected with silicone rubber tubing to the pump via an 11.2L exposure chamber, a filter, and a flow meter. A target is placed in the exposure chamber at the start of the test and exposed to the corrosive atmosphere of the gas sample for 60 min or until the specimen has lost 70% of its total mass loss, whichever occurs first. 

Once a decision is made to eliminate these two bottlenecks (fired heater and separation system), the analysis of reactor behavior could be modified to consider a number of secondary effects. The case study did not consider the effect of increasing reactor flowrates on the heat transfer coefficient (gas-phase resistance dominates). The inpact of the increased pressure drop over the reactor was not examined. These options were not necessary to confirm that the reactor is capable of providing a 50% increase in acetone production. The option of increasing the HTM flowrate was not appraised. 

Find a way to overcome the constraint while still maintaining the areas. This is often possible by using indirect heat transfer between the two areas. The simplest option is via the existing utility system. For example, rather than have a direct match between two streams, one can perhaps generate steam to be fed into the steam mains and the other use steam from the same mains. The utility system then acts as a buffer between the two areas. Another possibility might be to use a heat transfer medium such as a hot oil which circulates between the two streams being matched. To maintain operational independence, a standby heater and cooler supplied by utilities is needed in the hot oil circuit such that if either area is not operational, utilities could substitute heat recovery for short periods. 

A homogenizer or rotary positive pump may be used as a timing or metering pump to provide a positive, fixed flow through the pasteurization system (Fig. 6). The pump is placed ahead of the heater and the holding section. Various control drives assure that the pasteurized side of the heat exchanger is at a higher (7 kPa (1 psi)) pressure than the opposite side. 

Multiple-Effect Evaporators A number of approximate methods have been published for estimating performance and heating-surface requirements of a multiple-effect evaporator [Coates and Pressburg, Chem. Eng., 67(6), 157 (1960) Coates, Chem. Eng. Prog., 45, 25 (1949) and Ray and Carnahan, Trans. Am. Inst. Chem. Eng., 41, 253 (1945)]. However, because of the wide variety of methods of feeding and the added complication of feed heaters and condensate flash systems, the only certain way of determining performance is by detailed heat and material balances. Algebraic soluflons may be used, but if more than a few effects are involved, trial-and-error methods are usually quicker. These frequently involve trial-and-error within trial-and-error solutions. Usually, if condensate flash systems or feed heaters are involved, it is best to start at the first effect. The basic steps in the calculation are then as follows ... 

Heat recovery water heater and the saturated air preheater. Balance-of-plant equipment and systems, including interconnecting piping, valves, controls, etc. 

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