6 4 ASHRAEJournal October 1998A SHRAEJ
OURNALntroducingfreshairintoair-conditionedbuildingsinhotandhumidclimatesrequirescarefulanalysis.
Bringing in fresh air canconstitute a substantial portion of the
totalenergy consumed by the building. For ex-ample, if this makeup
air is improperlytreatedandcontrolled,itcancauseel-evated humidity
levels.Aprimary/secondarysystemdesignwherein fresh air is
introduced and treatedviaadedicatedfreshair-handlingunit(AHU) and
then delivered to a second-ary system offers system flexibility
andimproved humidity control. This type ofsystem can also help save
energy. In thisarticle, the application of runaround
coilsconfigured in this type of a system is
de-scribed.Thisarrangementcansaveen-ergy while improving humidity
control.The dominant application of runaroundcoils is to exchange
heat between fresh airand exhaust air. In hot and humid
climatesmore than 50% of the load is latent. There-fore, cooling
coils have to over-cool the airto achieve a sufficiently low
sensible heatfactor. Thisusuallyforcestheuseofre-heat to avoid high
indoor humidity levels.For many of the buildings for which
analy-ses were conducted, the primary cause
ofhighindoorrelativehumiditywasoversizing. A secondary, but still
importantfactor, was improper treatment of introducedoutside
air.Figure 1 illustrates an application
oftherunaroundcoil,whereinoneofthecoils is placed before the
cooling coil andthe second after the cooling coil. The
re-coveryofenergyfromthehotoutdoorairisusedinreheatingtheovercooledair,
which achieves dual energy savings.This scheme simultaneously
reduces therequiredcoolingandreheatenergyasshown in Figure
2.Runaround Coil Energy SavingsA simplified analysis is used to
illus-tratethepossibleenergysavings.Theanalysisisbasedonthefollowingas-sumptions:
Constant volume of AHU operatingat 10,000 cfm (4719 L/s). Supply
fresh air properties at stan-dard conditions (70F [21C] and 50%
RHare
used).AnequationisusedtoexpresstherunaroundcoilscapacityQatvariousconditionsforthepreviouslyspecifiedAHU:Q
= F (Tal Twi)(1)Where Q is in Btu/h, F is the coil pro-Fresh Air
TreatmentIn Hot and Humid ClimatesIAbout the AuthorGeorge J.
Berbari is the manager of thedesign department of Tabreed, a
districtcoolingserviceproviderintheUnitedArab Emirates.By George J.
BerbariMember ASHRAESaving EnergyUsing Runaround Recovery
CoilsFigure 1: A schematic of the fresh air AHU with runaround
recovery coils.portionality factor, Tal is the temperatureof the
air entering the coil, and Twi is thetemperature of the water
entering the coil.A manufacturer product selection
pro-gramwasusedtoevaluatethecoilsatthree different ambient
conditions:DB/WB temp. = 115/85F (46/29C)DB/WB temp. = 95/80F
(35/27C)DB/WB temp. = 75/70F (24/21C)A polynomial was then utilized
to in-terpolate the equation variables F and Twi.The basic solution
is to equalize the logmean temperature differential (LMTD) ofthe
two runaround coils:1- Pre-cooling Runaround Coil3- Re-heat
Runaround Coil2- Cooling Coil 4- Supplementary Heating Coil
(Optional)Circulating
PumpFreshAirTreatedFreshAirExpansionTankPre-filter& Bag Filter2
3 4 1The following article was published in ASHRAE Journal, October
1998. Copyright 1998 American Society of Heating, Refrigerating and
Air-Conditioning Engineers,Inc. It is presented for educational
purposes only. This article may not be copied and/or distributed
electronically or in paper form without permission of
ASHRAE.Advertisement in the print edition formerly in this
space.Advertisement in the print edition formerly in this
space.Advertisement in the print edition formerly in this space.6 8
ASHRAEJournal October 1 9 9 8) d e u n i t n o c ( l i o C g n i l
o o C ) d e u n i t n o c ( l i o C g n i l o o C ) d e u n i t n o
c ( l i o C g n i l o o C ) d e u n i t n o c ( l i o C g n i l o o
C ) d e u n i t n o c ( l i o C g n i l o o C l i o C A . R t a e H
- e R l i o C A . R t a e H - e R l i o C A . R t a e H - e R l i o
C A . R t a e H - e R l i o C A . R t a e H - e R l i o C t a e H -
e R l i o C t a e H - e R l i o C t a e H - e R l i o C t a e H - e
R l i o C t a e H - e Rl i o C f f O l i o C f f O l i o C f f O l
i o C f f O l i o C f f O t a e H e l b i s n e S t a e H e l b i s
n e S t a e H e l b i s n e S t a e H e l b i s n e S t a e H e l b
i s n e S t a e H l a t o T t a e H l a t o T t a e H l a t o T t a
e H l a t o T t a e H l a t o T . p m e T B D . p m e T B D . p m e
T B D . p m e T B D . p m e T B D t a e H l a t o T t a e H l a t o
T t a e H l a t o T t a e H l a t o T t a e H l a t o T . p m e T B
D . p m e T B D . p m e T B D . p m e T B D . p m e T B D t a e H l
a t o T t a e H l a t o T t a e H l a t o T t a e H l a t o T t a e
H l a t o TW WWWWb l / b l b l / b l b l / b l b l / b l b l / b l
h / u t B h / u t B h / u t B h / u t B h / u t B h / u t B h / u t
B h / u t B h / u t B h / u t B F F F F F h / u t B h / u t B h / u
t B h / u t B h / u t B F F F F F h / u t B h / u t B h / u t B h /
u t B h / u t B5 9 0 0 . 0 7 6 7 , 1 0 3 7 8 4 , 5 2 5 1 . 4 7 3 9
7 , 1 9 1 1 . 4 7 05 9 0 0 . 0 0 0 7 , 9 6 2 0 8 3 , 3 9 5 0 . 2 7
0 6 8 , 9 6 1 2 7 05 9 0 0 . 0 5 0 6 , 7 3 2 5 6 9 , 7 2 5 0 . 0 7
5 5 9 , 7 4 1 0 7 55 9 0 0 . 0 2 0 4 , 5 0 2 2 6 9 , 1 7 4 0 . 8 6
8 5 1 , 6 2 1 0 7 2 0 8 , 1 25 9 0 0 . 0 0 1 0 , 3 7 1 0 7 7 , 5 1
4 0 . 6 6 0 5 5 , 4 0 1 0 7 0 1 4 , 3 45 9 0 0 . 0 7 4 3 , 0 4 1 7
0 3 , 9 5 3 0 . 4 6 3 1 2 , 3 8 0 7 7 4 7 , 4 65 9 0 0 . 0 1 3 3 ,
7 0 1 1 3 3 , 5 4 3 1 . 2 6 9 2 2 , 2 6 0 7 1 3 7 , 5 85 9 0 0 . 0
9 7 8 , 3 7 9 9 1 , 6 2 2 2 . 0 6 1 8 6 , 1 4 0 7 9 7 2 , 6 0 13 9
0 0 . 0 8 0 9 , 9 3 8 0 9 , 9 3 3 . 8 5 2 5 6 , 1 2 0 7 8 0 3 , 6 2
16 8 4 , 9 5 4 , 5 5 6 6 8 4 , 9 5 4 , 5 5 6 6 8 4 , 9 5 4 , 5 5 6
6 8 4 , 9 5 4 , 5 5 6 6 8 4 , 9 5 4 , 5 5 6 6 0 6 , 8 7 7 , 6 6 6 ,
1 6 0 6 , 8 7 7 , 6 6 6 , 1 6 0 6 , 8 7 7 , 6 6 6 , 1 6 0 6 , 8 7 7
, 6 6 6 , 1 6 0 6 , 8 7 7 , 6 6 6 , 1 4 3 2 , 3 9 6 , 9 8 3 4 3 2 ,
3 9 6 , 9 8 3 4 3 2 , 3 9 6 , 9 8 3 4 3 2 , 3 9 6 , 9 8 3 4 3 2 , 3
9 6 , 9 8 3 1 2 9 , 3 5 1 , 8 9 3 1 2 9 , 3 5 1 , 8 9 3 1 2 9 , 3 5
1 , 8 9 3 1 2 9 , 3 5 1 , 8 9 3 1 2 9 , 3 5 1 , 8 9 3Table 1:
Energy analysis of a 10,000 cfm fresh air AHU with runaround heat
recovery coils in Augusta, Ga.Table 1 continued.e d i s t u O e d i
s t u O e d i s t u O e d i s t u O e d i s t u O e d i s t u O e d
i s t u O e d i s t u O e d i s t u O e d i s t u O l a u n n A l a
u n n A l a u n n A l a u n n A l a u n n A e r u t s i o M e r u t
s i o M e r u t s i o M e r u t s i o M e r u t s i o M l i o C . A
. R g n i l o o C - e r P l i o C . A . R g n i l o o C - e r P l i
o C . A . R g n i l o o C - e r P l i o C . A . R g n i l o o C - e
r P l i o C . A . R g n i l o o C - e r P l i o C g n i l o o C l i
o C g n i l o o C l i o C g n i l o o C l i o C g n i l o o C l i o
C g n i l o o Cb l u B y r D b l u B y r D b l u B y r D b l u B y
r D b l u B y r D b l u B t e W b l u B t e W b l u B t e W b l u B
t e W b l u B t e W n i B n i B n i B n i B n i B t n e t n o C t n
e t n o C t n e t n o C t n e t n o C t n e t n o C W TW TW TW TW
T1 1111t a e H l a t o T t a e H l a t o T t a e H l a t o T t a e
H l a t o T t a e H l a t o T l i o C n O l i o C n O l i o C n O l
i o C n O l i o C n O l i o C f f O l i o C f f O l i o C f f O l i
o C f f O l i o C f f O. p m e T b l u B y r D . p m e T b l u B y
r D . p m e T b l u B y r D . p m e T b l u B y r D . p m e T b l u
B y r D . p m e T b l u B y r D . p m e T b l u B y r D . p m e T b
l u B y r D . p m e T b l u B y r D . p m e T b l u B y r DF F F F
F F F F F F s r u o H s r u o H s r u o H s r u o H s r u o H b l /
b l b l / b l b l / b l b l / b l b l / b l F F F F F h / u t B h /
u t B h / u t B h / u t B h / u t B F F F F F F F F F F 2 0 1 7 7 6
2 4 1 0 . 0 3 . 8 7 3 9 7 , 1 9 1 2 . 4 8 3 . 6 57 9 8 7 4 7 3 6 1
0 . 0 9 . 5 7 0 6 8 , 9 6 1 3 . 1 8 3 . 6 52 9 6 7 3 9 2 6 5 1 0 .
0 5 . 3 7 5 5 9 , 7 4 1 3 . 8 7 3 . 6 57 8 4 7 6 9 4 1 5 1 0 . 0 1
. 1 7 8 5 1 , 6 2 1 3 . 5 7 3 . 6 52 8 2 7 7 3 6 6 4 1 0 . 0 7 . 8
6 0 5 5 , 4 0 1 3 . 2 7 3 . 6 57 7 0 7 3 0 9 1 4 1 0 . 0 3 . 6 6 3
1 2 , 3 8 3 . 9 6 3 . 6 52 7 9 6 3 5 1 , 1 5 4 1 0 . 0 9 . 3 6 9 2
2 , 2 6 2 . 6 6 3 . 6 57 6 5 6 2 3 9 7 2 1 0 . 0 5 . 1 6 1 8 6 , 1
4 1 . 3 6 3 . 6 52 6 8 5 8 1 8 3 9 0 0 . 0 1 . 9 5 2 5 6 , 1 2 0 .
0 6 3 . 6 5L A T O T L A T O T L A T O T L A T O T L A T O T 2 1 3
, 5 2 1 3 , 5 2 1 3 , 5 2 1 3 , 5 2 1 3 , 5 4 3 2 , 3 9 6 , 9 8 3 4
3 2 , 3 9 6 , 9 8 3 4 3 2 , 3 9 6 , 9 8 3 4 3 2 , 3 9 6 , 9 8 3 4 3
2 , 3 9 6 , 9 8 3s r H 2 1 3 , 5 : s i y g r e n e c i r t c e l e
p m u p g n i t a l u c r i c d e t a m i t s E 6 4 7 . 0 : s i r e
w o p p m u p g n i t a l u c r i c d e t a m i t s E . r a e y / r
H - W k 1 7 4 , 5 = w K 3 0 . 1W k / p H m p g 7 2 1 ( 0 6 9 , 3 (
/ ) t f 7 + t f 5 . 1 1 . W k 3 0 . 1 = ) . f f e % 0 7October 1 9
9 8 ASHRAEJournal 6 9H U M I D I T YTwi = (Tal + Ta3) / 2 + 1.5303
Tal 0.029625Tal + 0.0000625Tal 2 F = 7277 + 52.78( Tal Twi)( Tal
Twi)2(2)WhereTa3 istheleavingairconditionofthecoolingcoilcontrolled
at 56F (13C).A simplified control system is assumed. The main
coolingcoil should dehumidify the air to a level of 66.5 grains
(.0095 Kg/Kg of dry air), which corresponds to that of the comfort
condi-tions of 76F (24C) and 50% RH. This can be attained via a
dewpoint temperature controller or a simpler DB temperature
con-troller set at 56F (13C).In this system the runaround coil is
operated all summer withan on-off control. A diverting valve can be
used to control theleaving air temperature of the second downstream
runaroundcoil, which should not exceed 70F (21C). An optional
supple-mentary reheat coil, if used, can control the final leaving
airtemperature which should not drop below 70F (21C).Two hot and
humid climates were chosen for the analysis:Augusta, Ga. in the
United States and Abu Dhabi in the UnitedArab Emirates. The Bin
Hour method was used to estimate theannual energy required.Heat
pipes are widely used now in place of runaround
watercoils.Heatpipesareslightlymoreeffectiveastheydonotrequire
circulating pumps. They use refrigerant phase changeas a heat
transfer media between hotter and colder deck. Run-around coils use
recirculating water for the same purpose.Heat pipe manufacturers
software was used to evaluate theheat pipes performance using the
same weather data for Au-gusta, Ga. The heat transfer results and
annual recovered
en-ergyforheatpipeswereincloseaccordancewiththoseofrunaround
coils.Benefits of Runaround CoilsThe runaround coils reduced the
cooling coils total annualcooling energy by 19% for Augusta (from
2056.4 106 Btu to1666.8 106 Btu [2170 GJ to 1758 GJ]) and by 21%
for AbuDhabi (from 4137 106 Btu to 3280 106 Btu [4364 GJ to
3461GJ]). The supplementary reheat coil total annual energy
wasreduced by 49% for Augusta (from 786 106 Btu to 398.1 106Btu
[829 GJ to 420 GJ]) and by 62% for Abu Dhabi (from 1303.9 106 Btu
to 495.7 106 Btu [1375.6 GJ to 523 GJ]). The Augustaresults are
displayed in Tables 1 and 2.A three-way diverting valve and a third
recovery coil can beused for winter heat recovery between exhaust
air and fresh air.Because water piping is much easier to install
than air ducts,exhaust and fresh air duct distance is not an issue.
Supplemen-tary reheat is generally not required if indoor relative
humidityis in the range of 60% to 65% at off-peak conditions. At
relatively low ambient conditions (below 70F or 21C)the coils can
be switched off and the system will operate in theeconomizer cycle
mode, further reducing the cooling energyand eliminating
supplementary reheat. In milder climates, thesystem can be used
with reduced effectiveness. In this applica-tion, a lower supply
air duct relative humidity is achieved, thusreducing the
possibility of mold or mildew build-up inside theduct.The system
has proven its reliability with hundreds of in-stallations in the
UAE. As an example, one 5200 cfm unit (with-out supplementary
reheat) was delivering a leaving air tempera-ture of DB/WB =
71.5/64F (21.9/17.8C) when ambient tem-perature was 93F (34C) and
46% RH. After the runaround coilcirculating pump was switched off
and the system was left tostabilize, the leaving air condition was
determined to be 60.5/60F (15.8/15.6C).e d i s t u O e d i s t u O
e d i s t u O e d i s t u O e d i s t u O e d i s t u O e d i s t u
O e d i s t u O e d i s t u O e d i s t u O l a u n n A l a u n n A
l a u n n A l a u n n A l a u n n A e r u t s i o M e r u t s i o M
e r u t s i o M e r u t s i o M e r u t s i o M l i o C g n i l o o
C l i o C g n i l o o C l i o C g n i l o o C l i o C g n i l o o C
l i o C g n i l o o C l i o C t a e H - e R l i o C t a e H - e R l
i o C t a e H - e R l i o C t a e H - e R l i o C t a e H - e Rb l
u B y r D b l u B y r D b l u B y r D b l u B y r D b l u B y r D b
l u B t e W b l u B t e W b l u B t e W b l u B t e W b l u B t e W
n i B n i B n i B n i B n i B t n e t n o C t n e t n o C t n e t n
o C t n e t n o C t n e t n o C l i o C f f O l i o C f f O l i o C
f f O l i o C f f O l i o C f f O l i o C f f O l i o C f f O l i o
C f f O l i o C f f O l i o C f f O t a e H e l b i s n e S t a e H
e l b i s n e S t a e H e l b i s n e S t a e H e l b i s n e S t a
e H e l b i s n e S t a e H l a t o T t a e H l a t o T t a e H l a
t o T t a e H l a t o T t a e H l a t o T . p m e T B D . p m e T B
D . p m e T B D . p m e T B D . p m e T B D t a e H l a t o T t a e
H l a t o T t a e H l a t o T t a e H l a t o T t a e H l a t o T.
p m e T b l u B y r D . p m e T b l u B y r D . p m e T b l u B y r
D . p m e T b l u B y r D . p m e T b l u B y r D W WWWWF F F F F F
F F F F s r u o H s r u o H s r u o H s r u o H s r u o H b l / b l
b l / b l b l / b l b l / b l b l / b l F F F F F b l / b l b l / b
l b l / b l b l / b l b l / b l h / u t B h / u t B h / u t B h / u
t B h / u t B h / u t B h / u t B h / u t B h / u t B h / u t B F F
F F F h / u t B h / u t B h / u t B h / u t B h / u t B2 0 1 7 7 6
2 4 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0 6 5 , 3 9 4 0 8 2 , 7 1 7 0 7 0 6
9 , 7 4 17 9 8 7 4 7 3 6 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0 6 5 , 9 3 4
0 4 2 , 3 6 7 0 7 0 6 9 , 7 4 12 9 6 7 3 9 2 6 5 1 0 . 0 3 . 6 5 5
9 0 0 . 0 0 6 5 , 5 8 3 0 2 9 , 5 7 6 0 7 0 6 9 , 7 4 17 8 4 7 6 9
4 1 5 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0 6 5 , 1 3 3 0 2 1 , 8 9 5 0 7 0
6 9 , 7 4 12 8 2 7 7 3 6 6 4 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0 6 5 , 7
7 2 0 2 3 , 0 2 5 0 7 0 6 9 , 7 4 17 7 0 7 3 0 9 1 4 1 0 . 0 3 . 6
5 5 9 0 0 . 0 0 6 5 , 3 2 2 0 2 5 , 2 4 4 0 7 0 6 9 , 7 4 12 7 9 6
3 5 1 , 1 5 4 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0 6 5 , 9 6 1 0 6 5 , 7 0
4 0 7 0 6 9 , 7 4 17 6 5 6 2 3 9 7 2 1 0 . 0 3 . 6 5 5 9 0 0 . 0 0
6 5 , 5 1 1 0 8 8 , 7 6 2 0 7 0 6 9 , 7 4 12 6 8 5 8 1 8 3 9 0 0 .
0 3 . 6 5 3 9 0 0 . 0 0 6 5 , 1 6 0 6 5 , 1 6 0 7 0 6 9 , 7 4 1L A
T O T L A T O T L A T O T L A T O T L A T O T 2 1 3 , 5 2 1 3 , 5 2
1 3 , 5 2 1 3 , 5 2 1 3 , 5 0 2 7 , 2 5 1 , 5 4 0 , 1 0 2 7 , 2 5 1
, 5 4 0 , 1 0 2 7 , 2 5 1 , 5 4 0 , 1 0 2 7 , 2 5 1 , 5 4 0 , 1 0 2
7 , 2 5 1 , 5 4 0 , 1 0 4 8 , 1 7 4 , 6 5 0 , 2 0 4 8 , 1 7 4 , 6 5
0 , 2 0 4 8 , 1 7 4 , 6 5 0 , 2 0 4 8 , 1 7 4 , 6 5 0 , 2 0 4 8 , 1
7 4 , 6 5 0 , 2 0 2 5 , 3 6 9 , 5 8 7 0 2 5 , 3 6 9 , 5 8 7 0 2 5 ,
3 6 9 , 5 8 7 0 2 5 , 3 6 9 , 5 8 7 0 2 5 , 3 6 9 , 5 8 7Table 2:
Energy analysis of a 10,000 cfm fresh air AHU summer cooling
requirement in Augusta, Ga.7 0 ASHRAEJournal October 1 9 9 8Cost
Effectiveness of CoilsRunaround coils can achieve substantial
energy savings inhot and humid climates. The hotter and more humid
the
climate,themoreeffectivethesystem.Thesystemsdesigncanbeflexible.
The runaround coils recovery system is relatively inex-pensive. The
added cost for installing a 10,000-cfm (4719 L/s)AHU is estimated
at approximately US$8,000. This is offset bythe reduction of
cooling plant capacity by about 15 tons (53kW) for Augusta and 20
tons (70kW) for Abu Dhabi, with
anFigure2:ThepsychometricprocessofthefreshairAHUwith runaround
recovery coils.Please circle the appropriate number on the Reader
ServiceCard at the back of the publication.Extremely Helpful
..................................................... 466Helpful
...................................................................
467Somewhat Helpful
................................................... 468Not Helpful
.............................................................
469installedcostofaroundUS$800perton.Substantialenergysavings are
gained mostly during peak summer demand.The runaround recovery
system described in this article pro-vides a real opportunity for
capital cost savings, as well as oper-ating cost savings. It is a
reliable and flexible system that can beused with other system
components to further save on energy.Bibliography1982. Stoecker, W.
F. and J. W. Jones. Refrigeration& Air Condition-ing, Second
Edition. New York: McGraw-Hill.ASHRAE Standard 90.1-1989, Energy
Efficient Design of New Build-ings Except Low-Rise Residential
Buildings.1992 ASHRAE Handbook, HVAC Systems & Equipment.1995.
Desiccant Technology Transfer Workshop Manual, TechnologyTransfer
Workshop: Desiccant Cooling Systems. Advertisement in the print
edition formerly in this space.
