Demands to
increase performance of modern heat exchange systems are constantly being made;
requiring the removal of larger rates of energy, using process units that
occupy a smaller unit footprint. Heat
transfer enhancement plays an important role in improving energy efficiency and
developing high performance thermal systems. Heat transfer processes that
involve boiling are typically efficient modes of heat transfer; however the desire
to increase efficiencies of those processes have prompted the development of
enhanced heat transfer surfaces for boiling
processes. A wide variety of industrial processes involve the transfer of heat
energy during phase change and many of those processes employ old technology.
These processes would be ideal candidates for a redesign that could achieve
improved process performance.
Utilization
of enhanced heat transfer tubes is an effective method used in the development
of high performance thermal systems. Vipertex™ enhanced surfaces, have been
designed and produced through material surface modifications, which result in
flow optimized heat transfer tubes that increase heat transfer. Considerations
in the development of the optimized, three dimensional, enhanced heat transfer Vipertube
design include the maximization of heat transfer; minimization of operating
costs; and minimization of the rate of surface fouling. This study details the 1EHT
bundle boiling results over a wide range of conditions. Results for the 1EHT
bundle show combined overall bundle increases of shellside and tubeside heat
transfer up to 97% when compared to the heat transfer enhancement of a smooth
tube bundle using typical fluids ( n-Pentane, p-Xylene and water); for midpoint
shellside Reynolds number values in the
range of 2800 to 20400; with effective
mean temperature difference (EMTD) values between 15.4 OF ( -9.2 OC) and 118.3 OF ( 47.9 OC).
Enhanced heat transfer tube bundles that are capable of producing efficiency
increases in excess of 90% are important options to be considered in the design
of high efficiency processes. These enhanced tube bundles recover more energy
and provide an opportunity to advance the design of many heat transfer
products.
1. Introduction
Boiling heat
transfer is used in a variety of industrial processes utilizing various heat
exchangers, evaporators, chillers, refrigeration systems, power generation
components, desalination systems and petrochemical applications. Many
industrial processes that involve the transfer of heat energy employ old designs;
making those processes ideal candidates for a redesign utilizing enhanced
surfaces that improve system performance.
Gough (2012) discusses the increased demand on the performance of heat
exchangers and the need to enhance their performance. Enhanced heat transfer
techniques are still relatively new and the results are not always predictable.
Additionally, experimentation is
difficult and modeling can be complicated; with meaningful results sometimes
difficult to obtain. Enhancements in boiling heat transfer processes are vital,
and make typical industrial applications more energy efficient. Rough or
enhanced surfaces with cavities on the surface promote nucleate boiling and
reduce the required wall superheat necessary for nucleate boiling to occur. Heat
transfer enhancements for boiling applications in tubular products are
available; however the limited options of enhanced tubes used in that flow
regime are sometimes limited in range and/or enhancement; and in addition, expensive
to operate and produce.
Nucleate
boiling is a type of boiling that takes place when the material surface
temperature is hotter than the saturated fluid temperature and the heat
flux is below the critical heat flux. When boiling is initiated for a
range of temperature differences, bubbles form at nucleation sites
and flow from the surface, producing fluid mixing near the surface, accompanied
by an increase in heat transfer. It is shown that the Vipertex 1EHT enhanced
tube used in such applications would provide more nucleation points than an
equivalent smooth commercial tube, increasing heat transfer and advancing the
designs of those products. Processes expected to benefit the most include those
using flooded-type or full-liquid type evaporators, in which a fluid boils on
the outside of a tube bundle; these have been widely used for desalinization,
solar-powered absorption chillers, and other types of industrial heat exchange
applications.
Advantages
of enhanced designs include an increase in the heat transfer coefficient;
smaller unit footprint; more economic operation costs and a prolonged product
life. There are a few heat exchanger optimization scenarios to be considered including:
a one-for-one replacement of smooth tubes with enhanced tubes of equal length
producing heat transfer increases for a constant fluid flow rate; typically increasing
the pumping power requirement of the enhanced tube heat exchanger. Another
scenario to consider is constant pumping power designs; in this case the
required tube length could be reduced. Finally for the case of constant heat
transfer in the same unit footprint; the use of enhanced tubes will reduce the
pumping power requirements. The current study evaluated the latest generation of
enhanced 1EHT boiling tubes for a variety of conditions.
Visual
observation and statistical methods were used by Ulbrich and Mewes (1994) who
proposed a classification of flows that include bubbly, intermittent and
dispersed flows showing liquid and vapor superficial velocities. Noghrehkar et
al. (1999) pointed out that the use of only visual observations as a flow regime
indicator could lead to false conclusions. They used the probability density
function (PDF) of local void fraction fluctuations to indicate the flow regime
and they identified flow patterns near the
shell wall; those patterns varied from the patterns in the bundle core.
Burnside et al. (2005) and Iwaki et al. (2005) used particle image velocimetry
to characterize the velocity fields inside the bundle. They tested a very short
bundle butted up against a plexiglass end plate in order to view the flows.
Aprin et al. (2007) ran a series of void fraction measurement experiments in a
tube bundle with a variety of test fluids
( n-pentane, iso-butane and propane). Ribatski and Thome (2007) show that void fraction are one of the most important
parameters inside tube bundles. Their analysis unveils important discrepancies
in some methods using visual observations that are not backed up by more
objective measurements of the flow regime. It is difficult to accurately model the heat transfer (evaporating fluid, layout of
the bundle, etc) and fluid flow on the outer tube surfaces of a bundle of tubes for processes
characterized by two phase flows. Furthermore, little heat transfer data exists
for some fluids typically utilized (i.e. hydrocarbons and other process fluids).
Due to these reasons, excessive safety margins are often utilized in thermal
design and they typically oversized the heat exchanger in terms of process area.
Cornwell et
al. (1980) shows that the heat transfer characteristics in a tube bundle are
significantly different from those obtained from a single tube. The strong
interactions between two-phase flow and tubes, combined with the reduction of
the cross-section area and the disturbance due to the vapor production on
adjacent and upstream tubes strongly modify the heat transfer mechanisms. Additionally,
an understanding of enhanced heat transfer begins with an understanding of the
fluid properties involved and their interaction with the heating surface and
tube bundle. Browne and Bansal (1999) have presented an overview of the heat
transfer characteristics of the flooded tube bundle evaporators and boiling on
the outside tubes and tube bundles. They discuss the influence of tube position
and configuration. Upper tubes within the bundle can significantly increase
nucleate boiling heat transfer more than the lower tubes at low heat fluxes;
this is known as the bundle effect. In the high heat flux regions of fully
developed nucleate boiling, these influences disappear and the data for all
tubes merge onto a pool boiling curve of the single tube. However, the designs
of some systems require enhanced tubes to be used in order to increase boiling
heat transfer at low heat fluxes. In desalinization devices and some heat
exchangers making use of low quality heat energy, both wall temperatures and
heat fluxes are generally quite low and cannot cause boiling on smooth heated
tubes under common large tube spacing conditions. Therefore in order to advance
those designs, enhanced surfaces must be used.
Government
legislation and specific energy conservation targets have been set for overall
energy reduction on a national basis by many countries. Additionally,
government incentives are available to reduce energy usage and environmental
impact. Gough (2012) points out that recent events in Japan have prompted the
Japanese government to take a more active role in its serious drive to reduce
energy use. Recently, additional countries (i.e. United States, Korea, Denmark,
etc.) have been promoting energy
efficiency, making the development of enhanced heat transfer tubes and other
enhanced heat transfer technologies even more important.
Vipertex
optimized several enhanced heat transfer tube designs which increase heat
transfer on both the inside and outside surfaces. These tubes have been
designed and produced through material surface modifications, creating flow
optimized heat transfer tubes. Kukulka
et al. (2011) evaluated enhanced tubes under fouling conditions and detailed transient
results of the 1EHT tubes are presented. In another study, Kukulka and Smith (2012) evaluated the surface
geometry of enhanced tubes and that study formed the groundwork for the tubes
utilized in the present study. None of the conditions and/or geometries from
the previous enhanced studies was the same as the present study; therefore no
direct comparison could be made to previous enhanced works.
2. Experimental Details
Figure 1
provides views of the enhancements on the inner and outer surfaces of the 0.625 inch (15.875
mm) outer diameter Vipertex 1EHT enhanced stainless steel tube that was
evaluated in this study. Boiling heat transfer characteristics of the enhanced
Vipertex 1EHT heat transfer tubes were evaluated in water and then verified
with in an experimental study at the Heat Transfer Research, Inc. (HTRI)
Research and Technology Center. Additional 1EHT bundle evaluations took place in
the HTRI study using shell side fluids of n-Pentane, water and p-Xylene with
steam utilized inside the tube. Tube OD was 0.625 in (15.875 mm) with a 0.7813 in (19.85 mm) pitch and 30 deg tube bundle
layout angle. One row of dummy tubes (same outer surface on the dummy row as in
the middle rows), producing the continuous pattern necessary to simulate the
flow pattern, in the center of the boiler. Both the smooth and 1EHT bundles
were studied in cross flow in the same shell (11.75 in (298 mm) Shell ID).
Experiments
were carried out with a staggered tube bundle with three fluids under a wide
variety of operating conditions. These conditions are representative of typical
industrial processes. Heat transfer coefficients deduced from the results are
compared to smooth tube bundle correlations (Xace 7.0) from the HTRI database
developed previously by HTRI from experimental work performed using the same
apparatus.
3. Results
The overall
heat transfer coefficient (tubeside heat transfer resistance, metal resistance
and shellside heat transfer resistance) is measured for a variety of fluids
with a wide range of flow and thermal conditions. Average duty is calculated
based upon the average of: (i) steam condensate flow rate and the change in
steam enthalpy; and (ii) test fluid flowrate, test fluid condensate rate and
change in test fluid enthalpy. A comparison of overall heat transfer is
necessary since the 1EHT tube is enhanced on both sides, making it difficult to
differentiate the heat transfer on the tube or shell side. Additional future
testing will allow a tubeside/ shellside differentiation.
Figures 2a
(n-Pentane), 3a (water) and 4a (p-Xylene) compares the variation of the
mid-shellside Reynolds Number with the overall heat transfer ratio, U* (overall
heat transfer coefficient for the Vipertex 1EHT enhanced tube bundle ,U1EHT, divided
by the overall heat transfer coefficient in the smooth tube bundle, USmooth). Overall
1EHT bundle enhancement, is shown in
Figure 2a for n-pentane under pressure; showing a maximum enhancement of 99% and a
minimum of 49% when compared to a smooth tube bundle. Local maximum enhancements
appears for approximate Reynolds Numbers of 10000 and 20000, with the minimum
enhancement (50%) occurring for Reynolds Numbers near 15000. Overall bundle
enhancement, shown for water in Figure 3a, is almost 200% and appears to be
constant with no trend apparent over the limited Reynolds Number range
evaluated. Overall 1EHT bundle enhancement, is shown for p-Xylene (for
pressures close to ambient pressure) in Figure 4a; and average enhancements of
approximately 24% are seen.
Figures 2b
(n-Pentane), 3b (water) and 4b (p-Xylene) compare the variation of overall heat
transfer ratio, U* (overall heat transfer coefficient for the Vipertex 1EHT
enhanced tube bundle ,U1EHT, divided by the overall heat transfer coefficient
in the smooth tube bundle, USmooth) with the Effective Mean Temperature
Difference (EMTD) of the heat exchanger. Overall 1EHT bundle enhancement is
shown in Figure 2b for n-pentane under
pressure; showing a maximum
performance, when compared to a smooth tube bundle at EMTD values greater than
13 OC. Overall bundle enhancement, shown for water in Figure
3b, is almost 200% for the limited range evaluated. Overall 1EHT bundle
enhancement, is shown for p-Xylene (for pressures close to ambient pressure) in
Figure 4b; and average enhancements of
approximately 24% are seen.
Figure 2 Variation of the Overall Enhancement
Ratio, U*, (ratio of the overall heat transfer coefficient for the Vipertex
1EHT, Type 304 L stainless steel Enhanced Tube Bundle, U1EHT ,to the
overall heat transfer coefficient in the smooth tube bundle, USmooth),
for n-Pentane used as the shellside fluid and its variation with (a) Mid-Shellside Reynolds Number (Re) (b) Effective Mean Temperature Difference
(EMTD), OC.
Figure 3 Variation of the Overall Enhancement
Ratio, U*, (ratio of the overall heat transfer coefficient for the Vipertex 1EHT,
Type 304 L stainless steel Enhanced Tube Bundle, U1EHT ,to the
overall heat transfer coefficient in the smooth tube bundle, USmooth),
for Water used as the shellside fluid and its variation with (a) Mid-Shellside Reynolds Number (Re) (b) Effective Mean Temperature Difference
(EMTD), OC.
4. SummaryFigure 4 Variation of the Overall Enhancement
Ratio, U*, (ratio of the overall heat transfer coefficient for the Vipertex
1EHT, Type 304 L stainless steel Enhanced Tube Bundle, U1EHT ,to the
overall heat transfer coefficient in the smooth tube bundle, USmooth),
for p-Xylene used as the shellside fluid and its variation with (a) Mid-Shellside Reynolds Number (Re) (b) Effective Mean Temperature Difference
(EMTD), OC.
The purpose
of this study was to characterize the bundle boiling of a Vipertex 1EHT
enhanced tube bundle using n-Pentane under pressure, in cross flow boiling on
the outside of the tubes. Additional testing was performed on p-Xylene, at
close to ambient pressure, in single phase heating on the outside of the tubes.
Evaluation was carried out with tubes that had an OD that was 0.625 in (15.875 mm)
with a 0.7813 in (19.85 mm) pitch and a 30
deg tube bundle layout angle, producing a continuous pattern that simulates the
flow pattern in the center of the boiler. Both the smooth and 1EHT bundles were
studied in cross flow, using the same size shell (11.75 in (298 mm) Shell ID). Experiments were carried out with a staggered
tube bundle, with n-Pentane, water and p-Xylene under a wide variety of
operating conditions. These conditions are representative of typical industrial
processes.
A number of conclusions can be drawn from the
results of this study:
1.
Results for
the 1EHT bundle show combined overall bundle increases of shellside and tubeside
heat transfer up to 97% when compared to the heat transfer enhancement of a
smooth tube bundle for midpoint shellside Reynolds number values in the range of 2800 to 20400; with effective mean
temperature difference (EMTD) values between 15.4 OF ( -9.2 OC) and 118.3 OF ( 47.9 OC).
2.
Overall bundle enhancement (1EHT) for n-pentane
(under pressure); shows a maximum performance
enhancement of 99% and a minimum of 49% when compared to a smooth tube bundle.
Local maximum enhancement appears for approximate Reynolds Numbers of 10000 and
20000, with the minimum enhancement (50%) occurring for Reynolds Numbers near
15000. Overall 1EHT bundle enhancement for water is almost 200% and appears to
be constant with no trend apparent over the limited Reynolds Number range
evaluated. Overall 1EHT bundle enhancement for p-Xylene (at pressures close to
ambient pressure) produce average enhancements of approximately 24% .
3.
Overall bundle enhancement (1EHT) for n-pentane
(under pressure); shows a maximum bundle
enhancement of 99% and a minimum of 49% when compared to a smooth tube bundle.
The 1EHT bundle shows a local maximum enhancement, at EMTD values greater than
13 OC.
All this
leads to an important and exciting advancement in process design. The patented
Vipertex surface enhances heat transfer, conserves energy and minimizes cost.
Additional studies at lower Reynolds numbers will take place as it is unclear
as to the level of enhancement. Additional bundle testing over a larger range
of flows, temperatures and fluids may provide additional insight to the problem.
Further studies of new Vipertex surfaces are currently under way.
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