The volumetric flow rate of heat transfer oil circulating through a hot oil heater is crucial. Let’s consider its effect in heaters that have fired burners and helical coils such as those shown in figure 1. Also known also known as thermal fluid heaters, oil-and-gas producers call them heat medium oil (HMO) heaters. These heaters are extremely versatile and are used in a variety of industries.
No matter the industry, all hot oil heaters have same basic setup: A burner heats heat transfer oil that circulates through the heater coil and through heat exchangers in other equipment. As the name implies, the oil transfers heat from the burner gases to user equipment that requires heat. A single heater can heat one or more fluid circuits with multiple pieces of equipment.
Factors that Impact Thermal Fluid Flow Rate
Flow rate usually is stated as gallons per minute (gpm), but it also may be expressed as cubic feet per second (ft3/s) or cubic meters per second (m3/s). Flow rate is impacted by several design factors such as coil configuration and pump size. Coil configuration involves coil diameter and length as well as whether it is split into two or more circuits.
Even though a heater may be suitably designed, things under the control of the operator also influence flow rate. For example, valve openings and strainer condition significantly impact flow rate. Regardless of the cause, if the flow rate is inadequate, the heater may not provide the amount of heat needed. Worse yet, its coil may overheat.
The Perils of Overheating Heat Transfer Fluids
Overheating the coil can have serious consequences, and the importance of a suitable flow rate cannot be overstated. Overheating could cause the coil to crack or rupture, allowing heat transfer oil to enter the heater’s combustion chamber and cause an uncontrolled fire. The fire could cause death, serious injuries and property damage.
Overheating also will cause the heat transfer oil to breakdown, which causes the oil to coke and deposit residues inside the coil. Residues can build up and clog the coil, thereby reducing heat transfer and leading to further damage. Moreover, the fluid will probably have to be replaced — at a considerable cost.
Protection Against Abnormal Flow in Thermal Fluid Heaters
Most hot oil heaters incorporate controls to protect against abnormal flow. But, because the heat transfer oil in the heater is heated to high temperatures, ordinary flow meters cannot be used to sense its flow rate. However, flow rate in a hot oil circuit can be determined by measuring pressure drop or the decrease in pressure from one spot in the circuit to another. Pressure drop, also known as differential pressure (DP), is caused by the inherent resistance to flow along its path. Accordingly, most hot oil heaters include devices to sense DP. These systems also include controls that will shut the heater down if the DP is outside its normal range.
Normal pressure drop varies with the heater and its application. Therefore, DP settings must be calculated for each heater based on information furnished by the user or customer. These settings are normally specified on the engineering drawings furnished with the heater for the specific application. If actual operating conditions are different from those expected, the heater may not run when recommended settings are used. Or it might run — but without adequate flow protection.
One way to sense DP is to design the heater with an orifice plate installed directly in the hot oil line. The orifice plate has a precision hole that restricts flow to provide a suitable pressure drop and facilitate its measurement. Two pressure gauges or sensors are also installed, one on each side of the orifice plate. The difference between the pressures registered by the two gauges is the pressure drop across the plate, and this value is precisely related to flow rate in the system. One drawback of an orifice plate is the extra length of hot oil line needed before and after its location to ensure laminar flow through the plate. Another drawback is that the section of line that includes the orifice plate cannot be cleaned using a “pig.”
Another way of sensing and controlling flow rate is with a differential pressure switch and gauges tapped into the inlet and outlet of the heater coil. This method avoids adding any restriction in the coil.
Figure 2 shows one type of DP switch used to detect low differential pressure. Other switches work in a similar way. One side of the switch is piped to a connection at the inlet of the helical coil and senses the pressure of the heat transfer oil at that point. The other side of the switch is piped to a connection at the outlet of the coil and senses the pressure at that point. Also note the two gauges shown in figure 2. The one on the left indicates actual pressure at the coil inlet. The one on the right indicates pressure at its outlet.
The difference in the two pressures is the differential pressure, which is equal to the pressure drop across the coil. It is known as DP process variable. The DP switch has a display that shows the actual differential pressure, which should be equivalent to subtracting the pressure shown on the gauge at the outlet from that shown on the gauge at the inlet. Incidentally, if the pressure at the coil inlet is the same as the pressure at the coil outlet, the display would indicate zero (0) differential pressure, which indicates that there is no flow.
The switch shown in figure 2 also has a DP setpoint that must be manually set to the recommended value. The switch is closed while the DP process variable is higher than its setpoint. It will open and shut off the burner if operating conditions cause the DP process variable to go lower than the DP setpoint, but it will reset automatically when the DP pressure goes higher than its setpoint. However, the burner will have to be restarted manually after it is shut off by the DP switch.
The switch also has a setting known as deadband or DB. Deadband affects resetting the DP switch after it has opened because of low differential pressure. Its setting governs how much the differential pressure must exceed the setpoint to reset the switch.
Dealing with Abnormal Flow in Hot Oil Heaters
If the pressure drops or differential pressures are outside the normal range specified for a particular heater, the cause should be determined (figure 3).
Possible causes of abnormally low differential pressure include:
- Clogged pump strainer.
- Improper settings of one or more valves in the piping circuit.
- Excessive resistance to flow in the system being heated.
- Pump is either undersized or defective.
- Pump rotation is wrong.
Abnormally high differential pressure is another condition that may be encountered, but it is less apt to cause overheating than low differential pressure. Note that some types of DP switches do not shut down the heater due to abnormally high differential pressure.
Possible causes of abnormally high differential pressure are:
- Clogged helical coil (unlikely).
- Oversized pump.
- Viscosity of heat transfer oil is too high.
When the heat transfer oil in the system is cold, its viscosity is higher than when it is hot. Abnormally high viscosity can cause abnormally high differential pressure. However, the viscosity of the fluid will decrease after it heats up, and the differential pressure should return to normal.
Operators should routinely check that the flow rate in their heater is within recommended limits. Heating efficiency and safely depend upon it.