Heat Transfer Fluid - Vapor Phase Indirect Heating Effectiveness at PreAtmospheric Boiling Point Temperatures
Un der s t an di ng te mp er a tur e imp l ic at ions dur ing w ar m - up an d c ool- dow n o f p r es sur e v es s els .
Ch a d M . W al l , Jim Ha m il t o n , Mar k We ch s l er
Ren ew a bl e F uel T e ch no logies - O ctob er 1 9 , 2 01 1
www.renewablefueltech.com
Summary:
The purpose of this evaluation was to characterize vapor phase heat transfer effectiveness of heat transfer fluid
products such as Dowtherm A, prior to reaching the fluid’s atmospheric boiling point of 257°C. This
characterization is a critical element for understanding warm-up and cool-down thermal expansion stresses on a
pressure vessel using heat transfer fluid to provide indirect heating.
An apparatus was built to measure temperature changes at various vertical distances from the liquid heat transfer
fluid, within an evacuated pressure vessel. Temperatures were monitored as the heat transfer fluid liquid was
warmed from room temperature to 300°C, and during subsequent cool-down.
The tests indicate there is significant vapor phase temperature change occurring well below the fluid’s atmospheric
boiling point (as low as 60-80°C), depending on the distance from the heated liquid. Additionally, uniform heat
transfer throughout a pressure vessel within 22 vertical inches from the liquid heat transfer fluid, was observed at
temperatures as low as ~150°C. These characteristics allow development of warm-up and cool-down protocols,
minimizing thermal expansion stresses.
Introduction:
1
3
The literature notes that Dowtherm A and similar heat transfer fluids have a vapor density of 4 kg/m at the
atmospheric boiling point of 257°C. The vapor phase, at this temperature and above, uniformly heats the vessel
within a few degrees. It is presumed that there is vapor phase heat transfer at vapor densities (Figure 1) below
the atmospheric boiling point. Understanding the vapor phase effectiveness at lower temperatures can provide
insight into the thermal expansion stresses on the vessel during the warm-up and cool-down phases.
Figure 1
Page 1
Equipment Description:
An apparatus (Figure 2) was designed to test the vapor phase effectiveness of heat transfer fluid. External TCs
were mounted at various locations to measure temperature change as the liquid heat transfer fluid was warmed
with external band heaters in the large reservoir at the base of the apparatus.
Figure 2
The apparatus consists of a 4” diameter ~18” tall reservoir, with a 72” long, 1” diameter pipe, which is flange
mounted to the top of the reservoir. The top of the 1” pipe has a tee connection, with a ½” pipe returning to the
bottom of the reservoir, serving as a liquid return. In addition, the ½” return has a tee connection to a ½” tilted pipe
that dead ends to an evacuation valve.
Thermocouples (TC) are mounted at liquid level; 6” above the liquid level on the main reservoir; at ~14”, 22” and
72” above liquid level on the 1” diameter pipe; and at two locations on the tilted ½” diameter pipe (24” and 60”
from the one inch pipe, respectively). There are also two TCs mounted on the ½” liquid return pipe at the liquid
level, and at 72” above the liquid level.
Procedure/Results:
The locations of the TCs were located to observe changes in temperature at specific distances from the liquid
level of heat transfer fluid. The heater temperature was raised in small increments, over time (Figure 3). Pressure,
measured with a gauge, was tracked and added to the graph.
As the temperature in the first TC location above the liquid level rose, the heaters were shut off, until the
temperature rise at the TC at 6”, then 14”, then 22” from the liquid level tapered off. Using this technique, the
initial temperature observed at the closest location (6” from the liquid level) began to rise at ~80°C and the areas
Page 2
within 22” of the liquid level reached uniform temperature at ~145-160°C. The remainder of the vessel (within 72”)
reached uniform vapor phase temperature control between ~160 and 180°C.
Subsequently, the heater temp was stepped up to 300°C, demonstrating tight, uniform temp control with indirect
vapor phase heating, from as far away as 11 feet from the liquid. After 30 minutes, the heaters were turned off
and cool-down was monitored. The worst-case distance began to drop off rapidly (relative to the temperature of
the liquid heat transfer fluid) at ~160°C, followed by locations, in order, the furthest away from the liquid.
Figure 3
Figure 4 shows a detailed plot of the warm-up phase, for TCs within 22 vertical inches of the liquid level.
Figure 4
Page 3
Conclusion:
Heat transfer fluids such as Dowtherm A provide effective, uniform vapor phase at temperatures below the fluid’s
atmospheric boiling point. Pressure vessel temperatures of locations up to 22 vertical inches from the liquid level
can be controlled within ~160°C delta, using warm-up and cool-down protocols. There is a temperature gradient,
correlated to distance from the liquid heat source that indicates vapor phase heat transfer begins at even lower
temperatures. At six vertical inches, vapor phase heating was observed at temperatures as low as 80°C. This
suggests that delta temperatures of adjacent pressure vessel surfaces within six vertical inches from each other,
can be maintained within a range of 80-90°C prior to reaching operating temperatures.
References:
1
Dowtherm A Heat Transfer Fluids – Product Technical Data – Dow Chemical
http://www.dow.com/webapps/lit/litorder.asp?filepath=/heattrans/pdfs/noreg/176-01337.pdf&pdf=true
Page 4