As winter approaches, processing plants and engineering firms begin to turn their thoughts toward preparing for the extreme weather conditions they will soon be facing. Typically, when individuals working in the processing industries think of “weatherizing” their plants, they conjure up images of heat tracing. However, proper winterizing efforts require a broader, more thoughtful approach. Of the many options available, the use of electric process heaters, although not well known or understood, is a practical, effective approach to winterizing.
Although both heat trace and electric process heaters are used to achieve the same goal -- protect equipment and processes from extreme temperatures -- they are not comparable approaches. Heat trace is used to maintain heat within a pipe, valve body or other piece of equipment. Process heaters are used to maintain a specific temperature as well as drive heated processes, prevent condensation, provide boost heating in flow lines and other applications.
At a large, intricate industrial complex or processing plant, winterization often is treated as a low priority. However, improper heating outputs can have significant negative impacts on facility operations, even leading to a plant shutdown. Heaters (or the lack thereof) are comparable to the smallest ring gear in a transmission. Although they may be cheap and easy to ignore, when they fail, the entire transmission fails.
Here are just a few examples of applications where process heaters can have a substantial positive impact.
Process Heating ApplicationsLarge process facilities -- refineries, electric power plants, and pulp and paper mills, for example -- primarily use either steam or fired types of heat sources (gas, coal or oil) for large bulk heating and processing requirements. However, there are many instances where electric heating should be employed to overcome the limitations of these systems during periods of exceptionally cold weather. Two good options are electric steam superheaters and trim heating systems. In both cases, these systems consist of inline circulation heaters coupled with temperature control systems. Steam superheaters prevent steam from reaching its saturation point within long, remote pipe runs by boosting the steam temperature well into the superheated region. This same inline concept is used for electric “trim” heaters. These heaters are used in conjunction with steam systems to provide steam at the process point with the correct temperature and pressure. An electric heater and control system respond much faster to load variations and can be controlled much more accurately. Additionally, when steam heat loads reach plant capacity, an electric heater system should be considered as an alternative solution vs. expanding the facility’s boiler capacity or adding a new boiler system.
Calendar rolls, used within pulp and papers mills, are a good example of an application where electric process heating can help. Calendar rolls are located at the ends of paper machines and are used to increase paper smoothness, improve final surface finish and act as a final thickness gauge. Fluctuations in temperature naturally cause the roll to expand or contract, and therefore impact the quality of the finished product. To avoid temperature fluctuations, which can be significant in winter months, many paper mills use either electrically heated hot oil heat transfer systems or forced ducted air systems using tubular or finned heating elements. These types of electrically heated systems, mated with the proper control systems, can achieve temperature uniformity within 1.8oF (1oC) or better.
One benefit of forced air systems is that they do not suffer the lag times associated with steam or hot oil heat transfer systems. In addition, if inclement weather causes plant temperatures to drop, forced air systems usually have a 10 to 20 percent reserve built into the heaters that should be able to handle the lower ambient temperatures. Steam systems may have difficulty if they are already operating at close to capacity.
Operating Temperature MaintenanceAs the thermometer plummets, oils become increasingly viscous, especially fuel oils, making pumping difficult. In addition, as temperatures fall below 32oF (0oC), oils such as diesel #2 have the added problem of paraffin wax precipitating out of the oil solution, causing wax buildup and deposits and potentially clogging fuel nozzles on boilers, engines and other equipment. One potential solution is an “O”-shaped over-the-side heater. These electric heater assemblies employ rigid tubular heater elements in an “O” shape attached to an unheated riser mounted to a flange plate. Either a local or remote control box can be used with the heaters. The system uses a low wattage surface loading to warm the oil enough to avoid precipitation and viscosity increases but low enough to avoid element coking and oil degradation. A high limit temperature sensor is attached to the heater element bundle, and a high limit control circuit is set to trip below the fuel’s flashpoint. Using electric heaters within tanks is a simple solution to ensure that fuel oil is ready to flow for main system use, or that backup fuel tanks, used by standby generator systems, are ready on demand.
Another place where cold can wreak havoc is on tank farms. Crude tank farms typically employ some type of temperature maintenance system -- either steam loops or some form of internal electric heating. Some lesser-known electric heating designs may offer energy savings over these methods. For example, one alternative to steam loops is the use of pipe-insert (or bayonet-type) electric heaters. These heaters can be removed and serviced without having to drain a tank’s contents. For tank farms, it is recommended that additional tubular elements are installed in the heater design to function as spares should any of the elements fail. This reduces downtime by allowing maintenance to simply rewire to the spare tubular heaters without having to remove or replace the heater bundle.
Pipe-insert heaters are composed of pipe vessels mounted through the sides of storage tank walls. Screw-plug or flanged immersion heaters then are inserted into these bayonet-like wells. The wells may remain dry or be filled with a heat transfer fluid to improve heat transfer to a tank’s contents. The exact design selected depends on the type of oil, desired temperatures, tank capacity and other variables. These heaters normally need to be AMSE code stamped or obtain a CRN to ensure proper operation in hazardous locations.
Suction heaters, although not well known, are another alternative for heating storage tanks. Similar in design to pipe-insert heaters, suction heaters only heat the liquid pumped rather than the entire contents of the tank. This reduces energy consumption (as heat is only applied as oil is drawn out) and the pumping system requirements, which translates to savings on capital equipment investments.
The setup of a suction heater system usually involves bolting a flanged immersion heater directly to the storage tank through a pipe section. As the liquid closest to the pump is heated, it easily flows and is drawn through the outlet nozzle attached to the pump suction side. The power densities of these heating systems are kept low (0.6 to 1.5 W/cm2tubular heater surface loading) to ensure minimal oil degradation and heater element coking. The use of a valve on the intake side of the pipe section is recommended to facilitate removal and service of the heater bundle as needed.
Hydraulic and lube oils used in process equipment and mobile heavy equipment are usually lighter weight and have low viscosity. A good heater system is a critical component of winterizing efforts and will ensure that equipment operates properly and efficiently, regardless of the weather. Hydraulic oil heaters, like lube oil heaters, are usually screw-plug or flanged-style immersion heaters, and are inserted into oil tanks and reservoirs to ensure the maintenance of the proper oil temperature. If hydraulic oil is allowed to reach very low temperatures (-4oF [-20oC] or lower), the result often is excessive leakage, sluggish performance and lower efficiencies. At most facilities, hydraulic oil reservoirs normally are maintained between 86oF and 140oF (30 and 60oC) to account for the loss of heat within oil lines as they get further away from the reservoir and are exposed to low ambient air temperatures. On occasion, inline circulation-type heaters can be installed along long hydraulic lines to keep the oil within a specified temperature range.
One final benefit of heaters in lube oil systems is that they play a key role in oil/water separation. When oil is heated to around 122 to 140oF (50 to 60oC), emulsified and free water begin to vaporize more readily. Oil/water separation also ensures entrained gases and other undesirable volatiles are vaporized as much as possible. Ridding lube oil of entrained gases means there is less opportunity for cavitation within bearings and other key equipment components.
Heat Tracing AlternativesHeat trace begins to lose effectiveness as equipment mass begins to increase and temperatures climb into the 212 to 302oF (100 to 150oC) range. Alternatives to heat trace, while initially more expensive, may be better suited to overcome problems with temperature maintenance and system heat loss.
For example, consider this brief case history: a manufacturer of carbon pitch was seeking a way to insulate the mating flanges on its pitch pipelines. The pipelines, which operate at around 700oF (370oC), were insulated, but all attempts to insulate the mating flanges using traditional heat tracing had failed. A flat tubular heater element, curved to fit the flange circumference, was installed. Appropriate terminal also boxes were installed on the heater ends, which were designed to bend up at a 90oangle from the flange surface to ease installation and wiring. The solution proved to be simple, relatively inexpensive, and more reliable and controllable than heat trace.
Heat tracing also may have limited value in high temperature or high mass valving. Traditionally, heat trace installers coil or wrap a large amount of heat trace cable onto valves to ensure they get enough heat energy. Even when this approach is effective, it increases the maintenance times required to service or remove a valve. An alternative is to use small-diameter cable heaters (non-self limiting) with controller packages and internal temperature sensors. The heater, control and installation expense is higher than a solution that uses heat tracing. However, the investment may prove worthwhile as the heater package provides a higher power density, higher temperature capabilities (to 1,400oF [760oC]) and the ability to shape the cable to the contours of the valve.
Freeze ProtectionConveyor systems, including chutes and hoppers, also can have problems when temperatures drop far below freezing. Specifically, moist or wet particulate materials, even when covered, have a good chance of bunching up on or freezing to conveyor belts or a conveyor’s chains and rails. Freezing can cause belts and chains to snap, and chutes and hoppers to become blocked or clogged. If moisture is within the conveyed materials as well as from Mother Nature, a robust, water-resistant heating solution is required. A flat tubular heater with ends seals against moisture and contamination ingress is one alternative. Clamping bars can be used to ensure good physical contact between the heaters and conveyor rails. In applications such as this, heaters also can be placed at points along, and in direct contact with, conveyor belts.
Within industrial cooling towers, used to shed excess heat from industrial processes and HVAC systems, the primary winterization concern is preventing a tower’s catch basin (sump) from freezing, which can result in pump failure, frozen lines and, potentially, complete system failure.
For towers used within refrigeration and industrial applications, an immersion heater can be immersed in the cold water sump to keep the water above freezing. Factory-installed heaters are normally through-the-side immersion heaters with mounting flanges and gaskets. Add-on installers also may use this type or, if clearances permit, utilize an over-the-side type to avoid penetration of the basin. An adjustable thermostat, complete with bulb and capillary, also is installed to energize the heater element contactor and to maintain a basin water temperature of at least 40oF (4.4oC).
A float switch wired into the control circuit ensures that the heater cannot energize unless completely submerged in water. And, to disconnect the main power supply in case of excessive current draw (caused by a short to ground somewhere in the heater, the heater controls, or the wiring) a circuit breaker is necessary.
An alternative to immersion heaters is the use of a bypass circulation system. In this system, two bypass lines are utilized to create a circulation loop independent of both the main pump and the cooling tower water distribution system. One bypasses the cooling tower riser, diverting flow directly into the cold-water basin. The other line is installed in a warm area and connects the tower suction to the tower return, thus bypassing the main pump-process system. The line includes a small auxiliary pump complete with a magnetic starter, a small instantaneous water heater with a globe-valved bypass, a gate valve preceding the auxiliary pump and a check valve following it. The remaining components consist of an immersion thermostat, preferably located in the basin or suction line near the sump, a thermostat and heating cable system to protect the makeup line, and a globe valve at the discharge point of the interior bypass.
The bypass circulation method, like the immersion heater system, effectively protects the tower’s basin from freezing. However, the bypass method has the additional benefit of also protecting the tower’s exposed piping.
Overall, the use of electric heating technology offers process industries such as pulp and paper, oil and gas, refining, electric power, and chemical processing and an effective solution to prevent freezing, improve processes normally impacted by cold temperatures and overcome the limitations of steam systems used with process heating applications. Although every application and industry is different, the use of electric heaters for winterizing can solve recurring maintenance problems and other headaches caused by the effects winter has on process facilities.