VOL 22 ISSUE #1
 
Good flow is essential to high heating efficiency and long life of the heater and oil.
 

The importance of good flow of hot oil in a hot oil heating system is often overlooked at hot-mix asphalt (HMA) plants. But good flow is essential to high heating efficiency and long life of the heater and oil. 

Heatec service technicians often discover inadequate flow at plants they visit. Such plants are likely to have poor heating efficiency. The heater and hot oil are also apt to have a short life. The remedy starts with learning how the system works and what causes the problem.

KNOW THE SYSTEM

Consider a hot oil system heated by a fired-heater. Most HMA plants use such a system to heat asphalt storage tanks and other components. The oil (also known as thermal fluid) is heated as it circulates through the heating coil of the heater. The coil is heated by hot gases from a burner that is fired either by natural gas or fuel oil. The amount of heat imparted to tanks and other heated components depends on how hot the oil gets and its rate of flow as it circulates through the coil. Restrictions in the system can reduce the rate of flow. Abnormally low flow rates can allow the coil to be overheated and damaged, causing oil to break down prematurely.

This is the UE differential-pressure switch now used on Heatec hot oil heaters.
This is the UE differential-pressure switch now used on Heatec hot oil heaters. It opens an electrical circuit to shut down the heater if differential pressure drops below a preset limit. Differential pressure results from pressures produced by the hot oil pumped through the heating coil inside the heater. It is the difference between the pressure at the coil inlet and coil outlet. It must remain within the preset limit to allow the heater to operate. A differential pressure lower than the preset limit usually means the liquid is not flowing fast enough to prevent the coil from overheating. Theoretically, insufficient flow could also be caused by an obstruction inside the coil, but this almost never happens.

FLOW OF HOT OIL

As part of Astec’s annual service schools for HMA plant operators, Heatec instructors demonstrate the effect of too little flow, and it’s done in a way that attendees are apt to remember. A mock-up piping system is used to simulate flow of hot oil. The school instructor directs the flame from a hand-held acetylene torch to a small area of the piping that contains liquid. Then, the oil pump is shut off so the oil does not flow through the pipe. After only 15 seconds, the pipe is so hot that water squirted on the coil turns instantly into steam. 

The instructor continues the demonstration by reheating the pipe for an extended period of 60 seconds, except now the pump is turned on so that liquid flows through the pipe. He then invites attendees to touch the pipe. Despite some reluctance, someone volunteers to touch the spot that was heated—only to find that it is not uncomfortably hot. This provides a memorable demonstration of how liquid flow affects the transfer of heat from the pipe to the liquid. Moreover, it illustrates how the heater coil will be overheated and ruined when heat from the burner is not carried away by adequate liquid flow.

MEASURING FLOW RATE

If there was a simple, easy way to measure the flow rate of hot oil circulating through the coil, the heater could automatically shut off if adequate flow were lacking. Unfortunately, devices to measure flow rates directly are not well suited to hot oil systems. That’s why an indirect method is used that relies on pressure measurements. 

Pressure is measured at the inlet to the coil and at the outlet of the coil. The difference in these two measurements is known as differential pressure (DP) and is an indirect method of measuring flow rate. Thus, a switch that reacts to DP is used on all Heatec hot oil heaters. The switch is preset at the Heatec factory to allow the burner to operate as long as the differential pressures sensed by the switch remain within limits. The limits are established by Heatec’s engineers for each heater model.

Heatec now uses an improved version of the UE One Series electronic pressure switch, used from 2010 to mid-2016. The improved version is available from Heatec as a replacement for the obsolete version. Details on the new switch are covered in two Heatec Tec-Notes: Installing differential pressure switch on Heatec thermal fluid heaters HC & HCS series, Publication 7-16-301 and Setting differential pressure switches on Heatec thermal fluid heaters HC & HCS series, Publication 7-16-299. These Tec-Notes are available online at heatec.com.

DIFFERENTIAL PRESSURE

You may be wondering what problems or conditions cause inadequate flow and what effect they have on differential pressure. You can learn that from the accompanying illustration, which includes a list of problems and their effects on differential pressure. The illustration depicts a thermal fluid circuit with a heater coil. Please note that one of the problems listed is restricted piping. A clogged hot oil strainer and restrictions in hot oil piping are the most common causes of low flow. 

Piping restrictions are often caused by undersized hot oil jumpers used at pipe joints on heated asphalt piping. Many plants use ¾-in (1.90 cm) jumpers, which are much too small. We strongly recommend use of 1½-in (3.81 cm) jumpers. A 1½-in (3.81 cm) jumper increases flow about three-to-five times more than a ¾-in (1.90 cm) jumper. 

Not only do undersized jumpers reduce flow, they also cause unusually long heat-up times at a cold startup. Because of this, many plant operators avoid shutting off their heating systems overnight for fear of delaying production the next morning, while waiting for asphalt lines to heat up. Consequently, they miss fuel savings from shutting off the system overnight. Unfortunately, larger jumpers are not easily retrofitted to existing systems.

This diagram illustrates the various problems that can cause abnormal flow in a thermal fluid circuit and how they affect differential pressure (DP) across the heater coil.
This diagram illustrates the various problems that can cause abnormal flow in a thermal fluid circuit and how they affect differential pressure (DP) across the heater coil. Low flow rate is the usual cause of overheating and coil damage.

ENSURE ADEQUATE FLOW

So, no matter whether your hot oil heating is old or still in the planning stages, you should do all you can to ensure adequate flow. It really pays to consider flow—right from the start, before construction begins. But the quest never ends. You must always be alert to maintain operating conditions that ensure good flow. 

You should also consider the important advantages of using a multi-line heater system, instead of a single-line hot oil system. Multi-line heaters significantly improve flow through all components, including the heater coil.

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