ASHRAE Journal - May 2020 - 88

ASHRAE RESEARCH UPDATE

transfer surfaces, according to the report. However, coils
that are fouled with substances that are not water soluble, such as grease, potentially could benefit from being
cleaned with detergents.

FIGURE 3 This graph shows the "Effect on condenser capacity as a function of
fouling level for all field fouled coils" summary results of how some coils have
improved performance, and some have reduced performance-particularly the
most highly fouled coil.

Creating Realistic Fouling

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M AY 2020

1.10

1.05
Normalized Capacity

The researchers used their new understanding of real
fouling to develop a method to realistically foul coils in a
laboratory.4
"We developed a standardized fouling material that
simultaneously imposes the same resistance to airflow
and heat transfer that the original field fouling did," he
said. "This took a great deal of trial and error, using various materials in varying quantities and ratios of fiber
and particulate matter, but we eventually found a mix
that works extremely well."
The first fouling the researchers examined was the
fouling on the air-conditioning systems from people's
homes in Omaha, Nebraska, and Las Vegas, said Yuill.
The researchers sent the fouling material to other laboratories to examine.
ASHRAE wanted to develop a new way to realistically
foul a coil in the lab, said Yuill. Therefore, the researchers created a new kind of fouling material that matched
the performance of what they found in the field, in
terms of resistance to airflow and the rate of heat
transfer.
"We initially thought (the fouling) could act like a blanket, not only reducing the airflow rate, which reduces
heat transfer, but also reducing the heat transfer
because it could act like insulation," he said.
The researchers developed a custom set of particulates,
based on the size distribution of particulates in field
samples from Las Vegas and Omaha. They also tested
several fibers, such as those used to test air filters (cotton linters), which did not match what ws experience
outside. Yuill said they tried many different fibers and
ratios of fiber to particulates. They finally found that
feathers worked best for the fiber part, he said.
"One thing we had learned from our examination
was that particulates don't really stick to coils, but they
do stick to fiber. So you have to have fiber as well as particulates for realistic fouling," said Yuill.
The researchers tested the synthetic fouling material's
effect on six different condenser designs: three plate fin
coils, two microchannel coils and one spine fin coil. The
results showed that increases in airside pressure drop

1.00

0.95

0.90
0

10
Fouling Mass (g/ft2)

after fouling were smallest for spine fin coils and larger
for denser coils, according to the research report.

Surprising Results
"This project yielded an astonishing result that initially none of us believed. It showed that fouling can
increase the heat capacity of a condenser, even though
it reduced the airflow rate. In fact, in our small sample
of coils, on average there was no improvement in heat
capacity from cleaning the coils," Yuill said. "I doubt
anybody reading this will believe that this result could
be correct, but we repeated our experiments several
times, and exhaustively switched out sensors before we
finally started to understand how these results could
happen."
The research report said the results of testing the
original field-fouled coils show that fouling has negligible effects on total system performance for most of
the coils, even with heavy apparent fouling. Sometimes,
the fouling even improved the system's heat rejection
performance. Figure 3 summarizes these results, showing
no clear trend between fouling level and heat transfer
capacity. The average fouling effect on the heat rejection
capacity of the tested coils used with single-speed fans
was negligible-less than the measurement uncertainty,

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ASHRAE Journal - May 2020

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