Oversizing A coil for Dehumidification !
Paul Sobieck
provide more dehumidification ?
Contractors have told me that if you oversize the Evap coil you will get
better dehumidification in a house.
In Wisconsin typically, contractors size a new construction home by taking
the square footage and divide by 700. Example: 1400 sq foot house / 700
= 2 ton AC.
Would one want to put a 2.5 ton Evap coil and a 2 ton condenser in this
house? Any drawbacks to this operation? Advantages ?
Any feedback is helpful. Thanks
Iove doII
>From: "Paul Sobieck" psob...@execpc.com
>Date: 12/14/00 5:40 AM Pacific Standard Time
>Message-id: <3a38ccc1$0$90270$272e...@news.execpc.com>
ton, as long as the air distribution system (blower, motor, duct system) has
the adequate capacity. Pay the extra money and get a coil with an expansion
valve versus a fixed restrictor. The expansion valve will meter the correct
amount of freon thru the coil based on the actual load conditions.
Larry Kapigian
LibertyHVAC
In refrigeration, if you oversize the coil you will increase the humidity of
the room. This is because you slow the air down across a larger medium and the
refrigerant isn't getting down to lower densities which cause a colder coil,
thus the humidity isn't drawn out.
On residential units I would believe it is the same. If you wish to de-humidify
I think you would use a smaller coil, thus the compressor pulls the pressure
down lower than a matched system and the coil gets colder. Of course the air
speed has to be lower too so water won't blow off.
Any thoughts?
Rich
J. S. Nunes
in mind that this is according to the factory system match specs
RGPL-05EBMKR 2 Stage, variable speed gas furnace
RANB-030JAZ 2 1/2 ton 13 seer condenser
RCBA-4882GG21I 4 ton Multiflex coil
RXCT-BCG Expansion valve
This their componant combination that will actually give 13 SEER......Is a 4
ton coil oversized for a 2 1/2 ton system?? aparently not for this
particular system but it seems like this is more the exception than the
rule.
However, I figure that their design engineers are a lot smarter than I am or
I would be doing their job and making a LOT more money than I am now,
without having to go out in the worst weather to fix broken systems.
It appears to me that the "oversized" coil in this instance is for much
colder air and better dehumidification due to the lower airflow with the TXV
to regulate the refrigerent pressure/temperature to keep the coil surface
temp just above the freeze point.
I dunno.. maybe I am out in left field on this one... what do you folks
think??
"Larry Kapigian" <sub...@superior-air.com> wrote in message
news:3A38DB88...@superior-air.com...
Sonofdawra
>de-humidify
>I think you would use a smaller coil, thus the compressor pulls the pressure
>down lower than a matched system and the coil gets colder. Of course the air
>speed has to be lower too so water won't blow off.
>
>Any thoughts?
>Rich
>
temperature goes slightly warmer as the suction pressure rises slightly. This
will then work backwards of what the original poster was wanting. To increase
dehumidification, you want to increase the coil TD. Going to a larger coil
will decrease the coil TD and not give as much dehumidification.
TD = EAT - BP where EAT is coil entering air temp and BP is the boiling point
temperature of the refrigerant.
As Rich says, going to a size smaller evaporator coil will drop the suction
pressure some which will lower the refrigerant boiling point slightly which
will increase coil TD. By dropping the suction pressure somewhat, the
compression ratio goes up and the net result is a lowering of the SEER some.
For the original poster, I would first want to make sure that the correct size
unit is selected for the house. Therefore, if a good heat gain calculation
hasn't been done yet, that is the first place to start. Then make sure there
isn't some fudge factor added by the contractor to where the unit is oversized.
Installing oversized equipment is usually the number one cause of poor
humidity control in residential construction.
Have a nice day, Ron
TURTLE
--
This is turtle.
Bigger coil on smaller condenser will give you more effency but less
dehumitification.
TURTLE
Everyday is my first day in the business.
Got my e-mail address back as above .
Sent via Deja.com
http://www.deja.com/
Devon Ferns
<sonof...@aol.com> wrote:
> >
> I agree with you Rich. If the coil is increased in size, then the evaporator
> temperature goes slightly warmer as the suction pressure rises slightly. This
> will then work backwards of what the original poster was wanting. To increase
> dehumidification, you want to increase the coil TD. Going to a larger coil
> will decrease the coil TD and not give as much dehumidification.
>
> TD = EAT - BP where EAT is coil entering air temp and BP is the boiling
> point
> temperature of the refrigerant.
>
> As Rich says, going to a size smaller evaporator coil will drop the suction
> pressure some which will lower the refrigerant boiling point slightly which
> will increase coil TD. By dropping the suction pressure somewhat, the
> compression ratio goes up and the net result is a lowering of the SEER some.
>
Coil capacities are related to many variables, including surface area,
fin spacing, pipe diameter, refrigerant temperature, approach
temperature, refrigerant flow, air flow, air velocity, incoming air
temp and humidity, etc.
All coils have a Sensible/Total, (ST) performance ratio, specified at a
given set of conditions. Changing any one of the conditions will
change the performance characteristics of the coil.
At 800 cfm. the face velocity of a 2 ton coil would be 400 fpm.
assuming 1 sq/ft. of primary sarface area per ton.
At 800 cfm. the face velocity of a 2.5 ton coil would be 420 fpm.
assuming 1 sq/ft. of primary sarface area per ton.
This would result in a longer dwell time for the air flowing inside the
coil, allowing time for more moisture to be removed at essentially the
same suction temp.
I think this scenario would result in a smaller ST ratio, thus an
increase in latent capacity that would be greater than any slight
increase in evaporator pressure.
Some manufacturers may add a row of tubes to gain capacity from 2 to
2.5 tons. In effect making the coil thicker. Once again the air would
be iside the coil longer although not at a reduced velocity.
Dehumidification would increase in this case as well.
Gerry Ferns
LibertyHVAC
Darn you, after reading your post it made me have to blow the dust off my
product data! First I had to find them!
Using American Standard for an example, it is hard to determine exactly what is
going on since they use different cfms when you mis-match the equipment. But
here goes
5 ton with 5 ton TXV coil @ 1700 CFM:
54500 Total
36400 Sensible
17,600 latent
5 Ton with 4.5 ton TXV coil is exactly the same!
5 ton condenser with 6 ton coil @ 2135 CFM's:
57,000 Total
41,700 Sensible
15,300 Latent.
Now, it is true that the cfm's increased, and I don't have time to fiqure this
out with math, but according to A/S the six ton coil on a five ton unit has
2,500 more total BTU;s BUT it also has 2,300 LESS latent BTU's,
Therefore, it seems that what you gained in total btu's came from your latent
heat and the SEER went up from 11.10 to a 12.0.
Now, if these were calculated at the same CFM's, I don't know what would
happen. I will tell you what I personally experienced in the field with
American Standard, the bigger coil over the condensing units produces
complaints from customers with higher humidity.
In an old house I try to always use the next smaller size coil and it seems to
take the problems of higher humidity away.
The real problem I personally have, (That's if my post above is correct) is
what cfm should I design at, the coil size or the condensing size? I know
there isn't that much difference, but I size at .1 inch, and I'm thinking about
starting to use .12 or higher. It is hard to compete with the other companies
that use the rule of thumb method and put in half the metal.
Please let me know what you think about the above post. As Turtle correctly
states, EVERY day is myfirst day in this field. It's amazing how much I don't
know!
Rich d
LibertyHVAC
would definatly draw out the moisture.
Those high pressure systems really draw out the H20 because they are so thick.
The one Uni-Pak had a coil up to 9" in thickness!! That was the dryest house I
have ever worked on, but the price is terrible.
Rich
Buck153
(Sonofdawra) writes:
snip
>>Any thoughts?
>>Rich
>>
>I agree with you Rich. If the coil is increased in size, then the evaporator
I agree with both of you guys. The larger coil increases efficency, a TEV
controls refrigerant by superheat, and keeps the coil fully used and yet
maintain superheat.
REMEMBER THE KEY TO THE ABOVE IS QUANTITY OF AIR.
If your moving 800 cfm of air thru a 4 ton coil, this large coil will be closer
to the air temperature and thus higher humidity. This large coil is only
making that 2 ton compressor work at it's higher suction pressure thus higher
capacity, but there's only two tons of air flow load.
If you want a lower humidity, you have the smallest coil possible, and keep it
hovering just above freezing.
If your really serious you have a coil designed for your latent load. This
coil will be much deeper, to allow the air going thru to be in contact much
longer.
Custom designed coils for a job requires much ability to do the job.
Buck
John Mills
>
> I just quoted a 13 SEER Rheem system with the following componants....Keep
> in mind that this is according to the factory system match specs
>
> RGPL-05EBMKR 2 Stage, variable speed gas furnace
> RANB-030JAZ 2 1/2 ton 13 seer condenser
> RCBA-4882GG21I 4 ton Multiflex coil
> RXCT-BCG Expansion valve
>
> This their componant combination that will actually give 13 SEER......Is a 4
> ton coil oversized for a 2 1/2 ton system?? aparently not for this
> particular system but it seems like this is more the exception than the
> rule.
> However, I figure that their design engineers are a lot smarter than I am or
> I would be doing their job and making a LOT more money than I am now,
> without having to go out in the worst weather to fix broken systems.
You gotta look at Rheem differently. By putting a TXV on the RCBA-48 you
are creating a RCHJ-36 which is only "1/2 ton" oversized. Or since Rheem
doesn't make 1/2 ton coils, no biggie. So the model # of the coil really
tells little anymore. A 3 ton coil for a 10 SEER outdoor unit won't cut
it with a 12 SEER outdoor unit. Rheem is one of the brands that uses
TXVs over 10 SEER. I still can't figure out how Janitrol can get 14 SEER
from a piston outdoor unit and a fixed meter coil. Back to coil sizing,
Trane rates its matches with all kinds of indoor coils. I use the same
size indoor coil as outdoor unit. But looking at the chart, putting a
TXA-048 coil on a 7A0036 gives a hair more sensible capacity, but NO
difference in latent and probably will run a warmer discharge.
> It appears to me that the "oversized" coil in this instance is for much
> colder air and better dehumidification due to the lower airflow with the TXV
> to regulate the refrigerent pressure/temperature to keep the coil surface
> temp just above the freeze point.
I am a strong fan of TXVs. With a piston, in some cases, cool out, warm
& muggy inside, proper superheat could be as high as 30 degrees. Well,
if you have 40 degree refrigerant entering the coil, it leaves at 70
degrees. So 1/2 way through the coil, it is above the dewpoint and doing
nothing to suck out moisure. Same coil with a TXV typically set to 15
degrees, the warmest the coil gets is 55 which should be below a typical
summer dewpoint in humid country.
--
HVAC Advice, Pictures, Links...
http://www.geocities.com/~johnmills
http://www.appelheat.com
alt.hvac Charter, FAQ, Links...
http://home.att.net/~alt.hvac/
LibertyHVAC
Have you in the past had special coils make? If so, please tell us the
procedure and who/where you had it. What was involved in costs and did they
come with cases? What did you use for static loss criteria?
Thanks
Rich
Rich
TURTLE
Marc O'Brien
"Paul Sobieck" <psob...@execpc.com> wrote in message
news:3a38ccc1$0$90270$272e...@news.execpc.com...
> On a typical residential furnace is it typical to oversize the A coil to
> provide more dehumidification ?
By my understanding, after conversations with Mike Hardy, Abby Above Average
and Danmurphy, is that the indoor unit sensible heat ratio (SHR) depends
greatly on the difference between mixed return air temperature (RA) and coil
temperature (ADP).
The higher your RA and the lower your ADP the less your SHR will be which
amounts to increased proportion moisture removal.
So to remove moisture then you'd very likely need to affect two things, both
which are mutually related. You'd want the unit to run longer affected by
reducing effective capacity, and you'd also want a colder coil temperature.
Two notable changes to the indoor unit can be made to affect this, a
reduction in air flow and a reduction in coil size. Even an increase in coil
depth can assist too.
Reducing the air flow or reducing the coil size has two major effects:
1) that is reduced overall effective system capacity = longer run time.
2) reduced SHR i.e. increased moisture removal.
BTW, Mike Hardy emailed to say hi and to appologise to everyone he's not
posting. I think that's Mike's way of reminding us he's having a great time
:-)
Mike Hardy
Buck153 wrote in message <20001214183853...@nso-ca.aol.com>...
>In article <20001214102817...@ng-fm1.aol.com>,
sonof...@aol.com
>(Sonofdawra) writes:
>snip
>>>Any thoughts?
>>>Rich
>>>
>>I agree with you Rich. If the coil is increased in size, then the
evaporator
>>temperature goes slightly warmer as the suction pressure rises slightly.
snip
>
>
>I agree with both of you guys. The larger coil increases efficency, a TEV
>controls refrigerant by superheat, and keeps the coil fully used and yet
>maintain superheat.
Hello All,
Yes, I agree as well, I think it is likely the proposal by Paul's contractor
will have the opposite effect to what he wants. If I want a higher sensible
to total ratio using Sanyo equipment I use the next size down (smaller)
outdoor unit (not that Sanyo would approve!) i.e. the now larger indoor
evaporator coil will take out less moisture. (Hope that does not sound too
back to front :-)
As I carry out many high sensible applications I do this fairly regularly.
I think using mass produced equipment it is all a bit hit and miss though.
I would consult the manufacturer to see if they have 'mis - matched' their
equipment like this before. Some don't like you to do it (like Sanyo in my
case but never had a problem to date!), and it is best to do it with their
blessing otherwise guarantees may be invalidated. They may also highlight
any problems they have had by mis-matching i.e. there is a danger that the
sensible to total ratio may reach unity and no moisture would be taken out
at all. (There is a manufacturer in this country that actually publishes the
different sensible to total ratios achieved by 'mis - matching' their
equipment).
If they can't help, ask them which contractor buys large volumes of similar
equipment and ask that contractor if they have done this before; failing
this it becomes rather empirical I am afraid, i.e. suck it and see - that is
my experience anyway.
If a bespoke evaporator coil were being used it become less hit and miss,
because the coil cross section, depth, fins per inch etc. could be
manufactured to suit the condensing unit characteristics and a balance point
found to achieve exactly the sensible to total ratio required.
(I think Carrier would actually carry out the balance point calculation for
'mis - matched' equipment for you, even with their mass produced equipment,
but would very much depend on the size of the equipment)
But this sounds miles over the top for Paul's application.
Regards, Mike
Ambthair Services, UK - http://www.ambthair.com/
johnl45
If you perform a heat load you won't need to oversize the coil.
He problaly has some old 2.5 ton coils laying around.
Paul Sobieck wrote in message <3a38ccc1$0$90270$272e...@news.execpc.com>...
LibertyHVAC
equipment. If there is a problem with moisture use a two speed or two units. I
have had too many problems in the past using smaller coils with heat pumps. Not
that they don't run well, but the Utilities give me a fight over it.
Besides, your messing around with the COP's on the heating side when you mis
match. American Standard Product Data shows what coils can be used, but it
seems the Utility in my area doesn't care, they want a three ton with a three
ton, etc etc. or no rebates.
Rich
Marc O'Brien
"Mike Hardy" <co...@ambthair.com> wrote in message
news:91d4dm$jm$2...@soap.pipex.net...
>
> If a bespoke evaporator coil were being used it become less hit and miss,
> because the coil cross section, depth, fins per inch etc. could be
> manufactured to suit the condensing unit characteristics and a balance
point
> found to achieve exactly the sensible to total ratio required.
So, for a given RA condition and ADP temp we may further increase the
apparatus's effective SHR by playing with depth, fins etc. ??
By what proportion do you think the SHR can be adjusted with such an
approach Mike.
Buck153
(LibertyHVAC) writes:
>Buck, that is an interesting post,
>
>Have you in the past had special coils make?
No, but been on jobs where it was done.
<If so, please tell us the procedure and who/where you had it. What was
involved in costs and did they come with cases?
Not residential.
>What did you use for static loss criteria?
There again, it wasn't residential, one job I'm thinking of had a 2 horsepower
evaporator fan motor, on a 5 ton machine. Because of the high static pressure
of coil and filter.
Buck
Mike Hardy
Marc O'Brien wrote in message <91ge1b$lkg$1...@newsg4.svr.pol.co.uk>...
Hello Marc,
Maybe this is the best way to look at it, as you know DX coils differ from
chilled water coils in that the balance point has to be looked at, in
relationship to the condensing unit, whereas with chilled water this is not
the case.
W.P. Jones quotes in his book a typical performance for a DX coil, (for a
constant fins per inch) - sorry this is in SI units which he is writing in;
Given entering air = 23.9 ºC/16.7ºC wet bulb
For an evaporating temp. 4.4ºC
For a 4 row coil velocity of 2 m per sec. leaving condition 10.7/9.8, for a
6 row coil will be 8.2/7.9
For a 4 row coil velocity of 2.5 m per sec. leaving condition 11.8/10.7 and
for 6 row will be 9.2/8.7
For an evaporating temp. 7.2ºC
For a 4 row coil velocity of 2 m per sec. leaving condition 12.2/11.3, for a
6 row coil will be 10.2/9.9
For a 4 row coil velocity of 2.5 m per sec. leaving condition 13/11.9 and
for 6 row will be 10.9/10.5
So the lower the evaporating temperature the less moisture in the air and
the more rows added the more the evaporating temperature will be approached
with also the reduced moisture content. (reduced velocity as well).
So it is these factors that effect the sensible to total ratio and I guess,
in theory, if you could add an infinite amount of rows you would arrive at
the evaporating temperature and this is the lowerst sensible to total
gradient you would get. To remove even more moisture then the evaporating
temperature would have to go even lower.
(A disadvantage to this is that you would need an infinitely size fan
because of the high static pressure you are likely to encounter, although
keeping the velocity low will help minimize this).
The above would need to be plotted out on a chart or an enthalpy calculation
carried out for each condition to find out exactly what the ratios are but
at a guess (and being lazy:-) I would imagine you could get down to values
such as 0.5 sensible to total.
The above, I guess is all well and good, but conveniently ignores the
balance point between the evaporator and the condensing unit.
My experience with coil manufacturers with DX is that they will use their
programmes to calculate any of the above but will always want to assume an
evaporating temperature. However with respect to them, this IMHO is one of
the most difficult things to predict and unless you can involve the likes of
Carrier are left to do this yourself.
W.P. Jones shows a method as does the Carrier manual, and this involves
plotting co-ordinates of the performance of the evaporator on the same
co-ordinates as the condensing unit. From this may be predicted the
evaporating temperature (none of these problems with chilled water!), so
this prediction will also significantly effect the sensible to total ratio
of the equipment.
Once the coil is installed, in my experience there is not a lot you can do
with regard to evaporating temperature except vary the air volume over the
DX coil. This is why (as you know) I am always fiddling around looking at
hot gas bypass or other devices to give me more options, if you install all
this equipment i.e. a bespoke DX coil and standard condensing unit and find
you are evaporating too low and freezing up it all becomes a little
uncomfortable - and maybe this is understating it!
I have prattled on enough, hope this makes sense,
Abby Normal how do you get over this? - not me prattling on I don't mean:-)
David Andersen
does not get satisfied. Running longer increases the dehumidification.
This works to a point. But you can go outside the curve. Not likely though
on A/C. Yes, on refrigeration.
ftwhd
Im glad *you* said that. :)
A larger coil will work on the latent load while maintaining the
sensible load which boils down to longer run times with, less
humidity, and steady sensible temps. That's why they call it, Air
Conditioning and not Refrigeration. Refrigeration is the oppisite.
You try to maintain a higher RH with a colder sensible for product
preservation. A living human is not a fur, can of beer or slab of
meat.
Happy Holidays
Mike
UA local 370
Marc O'Brien
>
> Marc O'Brien wrote in message <91ge1b$lkg$1...@newsg4.svr.pol.co.uk>...
> >
> >"Mike Hardy" <co...@ambthair.com> wrote in message
> >news:91d4dm$jm$2...@soap.pipex.net...
> >>
> >> If a bespoke evaporator coil were being used it become less hit and
miss,
> >> because the coil cross section, depth, fins per inch etc. could be
> >> manufactured to suit the condensing unit characteristics and a balance
> >point
> >> found to achieve exactly the sensible to total ratio required.
> >
> >So, for a given RA condition and ADP temp we may further increase the
> >apparatus's effective SHR by playing with depth, fins etc. ??
> >
> >By what proportion do you think the SHR can be adjusted with such an
> >approach Mike.
> >
> >
> >Marc
> >http://fridgetech.com
> >
>
>
> Hello Marc,
>
> Maybe this is the best way to look at it, as you know DX coils differ from
> chilled water coils in that the balance point has to be looked at, in
> relationship to the condensing unit, whereas with chilled water this is
not
> the case.
Howdy Mike,
Also, it can normally be assumed that DX will have a constant temp while
chilled water coils experience a temp rise of around 4K to 8K. LMTD
calculations are a little more involved, I have the formula to make it easy
all the same :-)
I'm sitting here with my own little AC book.
There are example evap coil block selection tables.
A typical table includes:
For a given return air DB & WB...
Suction Temp
Rows (4, 5, 6 & 8)
Fins (8, 10 & 12)
versus
Air Velocity
MBH = Thousands BTU per hour per square foot coil face area
LDB = Leaving DB
LWB = Leaving WB
And says:
a) Rows of coil depth usually ranging from 2 to 8 rows. The capacity of a
coil increases with rows of depth but not in direct proportion. In a four to
six row coil the first two rows perform approx 50 to 70 percent of the total
cooling.
b) Increasing the rows lowers BPF
c) Increasing the rows increases the air pressure drop requiring greater fan
HP.
Anyway, whilst I can see increasing rows and fins per inch increases
capacity I cant see it altering the coil SHR.
For a given RA dry bulb and wet bulb condition a coil ADP may be selected
based on the required coil SHR.
Altering row depth and fins per inch will only alter BPF and hence coil
capacity per square foot face area. And so while increasing depth and fins
increases capacity this will only occur along the SHR line drawn between the
coils Mixed return air and the coils ADP temperature.
WB, DB & DP will tend towards unity with increased rows and fins but only
along the constant coil SHR line.
See, my book seems to explain it that Coil SHR is based on mixed RA and coil
ADP. Once you know your coil face area limitations you then check to see
your face velocity is acceptable and then to fine tune the coil to the
required capacity you'd play with fins and rows.
Increasing fins or rows increases latent and sensible duty proportionately
with the effect that only capacity is changed while SHR remains.
Marc O'Brien
"David Andersen" <hvac...@prodigy.net> wrote in message
news:91jb18$f8go$1...@newssvr06-en0.news.prodigy.com...
> Warming the coil (as you put it) makes the unit run longer, because the
stat
> does not get satisfied. Running longer increases the dehumidification.
Are you sure this idea will work, I used to think it might, now I don't ?
David Andersen
You can't argue with the Psychometrics.
Marc O'Brien
> Yes.
>
> You can't argue with the Psychometrics.
Try me :-)
Mike Hardy
There is something getting confused here I think;
Marc O'Brien wrote in message <91jkjt$jtb$2...@newsg2.svr.pol.co.uk>...
>
>Howdy Mike,
>
>Also, it can normally be assumed that DX will have a constant temp while
>chilled water coils experience a temp rise of around 4K to 8K. LMTD
>calculations are a little more involved, I have the formula to make it easy
>all the same :-)
>
>I'm sitting here with my own little AC book.
>
>There are example evap coil block selection tables.
>
>A typical table includes:
>
>For a given return air DB & WB...
>
>Suction Temp
>Rows (4, 5, 6 & 8)
>Fins (8, 10 & 12)
>
>versus
>
>Air Velocity
>MBH = Thousands BTU per hour per square foot coil face area
>LDB = Leaving DB
>LWB = Leaving WB
>
>And says:
>
>a) Rows of coil depth usually ranging from 2 to 8 rows. The capacity of a
>coil increases with rows of depth but not in direct proportion. In a four
to
>six row coil the first two rows perform approx 50 to 70 percent of the
total
>cooling.
Yes, because bypass factor is defined by (1-contact factor) = bypass factor
= moisture content at off coil temperature - moisture content at saturation
line/moisture content of incoming air - moisture content at saturation line.
>b) Increasing the rows lowers BPF
Yes
>c) Increasing the rows increases the air pressure drop requiring greater
fan
>HP.
Yes, unfortunately!
>Anyway, whilst I can see increasing rows and fins per inch increases
>capacity I cant see it altering the coil SHR.
The additional rows, whilst being a damned nuisance and increasing the air
pressure drop, will help take the 'air off' nearer to the evaporating
temperature, and in that way will alter the sensible to total ratio.
>For a given RA dry bulb and wet bulb condition a coil ADP may be selected
>based on the required coil SHR.
>
>Altering row depth and fins per inch will only alter BPF and hence coil
>capacity per square foot face area. And so while increasing depth and fins
>increases capacity this will only occur along the SHR line drawn between
the
>coils Mixed return air and the coils ADP temperature.
Probably the moisture removed will be on the SHR gradient line, mind you I
don't think this is a given, because it will depend on the configuration of
the manufacturer's coil. I think if you had a manufacturer select this it
may not be an exact straight line relationship - but I guess would not be
far out. So as the 'air off' is now nearer the saturation line the enthalpy
relationship has altered and therefore the sensible to total ratio.
This not to be confused with the sensible to total gradient which may not
alter.
>WB, DB & DP will tend towards unity with increased rows and fins but only
>along the constant coil SHR line.
Exactly i.e. a point on the saturation line.
>See, my book seems to explain it that Coil SHR is based on mixed RA and
coil
>ADP. Once you know your coil face area limitations you then check to see
>your face velocity is acceptable and then to fine tune the coil to the
>required capacity you'd play with fins and rows.
Yes, that is fine.
>Increasing fins or rows increases latent and sensible duty proportionately
>with the effect that only capacity is changed while SHR remains.
No, the gradient may not alter but the relationship between the sensible to
latent does alter and therefore the sensible heat ratio. The only way to
calculate this that I know of is;
Enthalpy of the incoming air at the same moisture content as the off coming
air - enthalpy of off coming air/ enthalpy of incoming air- enthalpy of off
coming air. This will give the loads and the exact sensible to total ratio.
Marc O'Brien
> Hello Marc,
>
> There is something getting confused here I think;
>
>
factor
> = moisture content at off coil temperature - moisture content at
saturation
> line/moisture content of incoming air - moisture content at saturation
line.
>
> >b) Increasing the rows lowers BPF
>
>
> Yes
>
> >c) Increasing the rows increases the air pressure drop requiring greater
> fan
> >HP.
>
>
> Yes, unfortunately!
>
> >Anyway, whilst I can see increasing rows and fins per inch increases
> >capacity I cant see it altering the coil SHR.
>
>
> The additional rows, whilst being a damned nuisance and increasing the
air
> pressure drop, will help take the 'air off' nearer to the evaporating
> temperature, and in that way will alter the sensible to total ratio.
Yes, I can see the effect now, I've overcome the laziness and I'm plotting
coil selection figures on my psychometric chart.
If I take the selection figures shown working with imperial::
Entering air condition: 84DB & 68WB with a coil temp of 35 (ADP)
With 500 fpm and 8 fins/inch
4 rows = 54.3LDB & 52.5LWB
8 rows = 43.7LDB & 43.3LWB
Drawing lines between entering condition and 4 rows leaving condition then 8
rows leaving condition...
...shows that leaving conditions do alter in the direction of decreased SHR
with increasing rows. On my selection table I find the same happens with
increased fins/inch as well as with reduced air velocities all this
occurring while coil temp is kept constant.
David Andersen
but the world will travel around the sun again this year, heat will pass
from warmer to cooler, the sun will rise tomorrow!
Marc O'Brien
EFK
Dehumidification has to do with temperature difference between the coil and
the air passing over it. The HIGHER the difference (TD), the LOWER the
humidity in the conditioned space. That's why we use large evaps with low
TD in perishable foods holding boxes and work rooms. On frozen food boxes,
the evaps can be smaller as TD doesn't matter that much concerning drying
out the product.
With air conditioning, it works the same way. If you oversize an air
conditioning unit, you will maintain temp, but have high humidity. If you
undersize a unit, generally you will have a lower humidity but may not be
able to maintain temp.
Oversizing or undersizing the evap coil alone would directly affect suction
pressure to a given horsepower unit, and I don't see how this is wise or
efficient. Evaps and compressors are matched by the manufacturer, why go by
some rule of thumb? The most important calculation is the size of the UNIT
capacity compared to the heat load, not just the size of the evap.
But for sake of argument, a smaller evap on a given condensing unit will
result in lower humidity. Say a 1.5 ton evap on a 2 ton unit will take out
more humidity ( if run in the airflow of a 2 ton unit ), but would probably
ice up. And the reason for this is the TD will be much greater between the
air and the coil.
We know that air coming straight off any cooling coil is at 100% RH, right?
------------------------------------------------------------
Tim <t...@sccoast.net> wrote in message news:3A58640E...@sccoast.net...
Paul Sobieck wrote:
On a typical residential furnace is it typical to oversize the A coil to
provide more dehumidification ?
Contractors have told me that if you oversize the Evap coil you will get
better dehumidification in a house.
In Wisconsin typically, contractors size a new construction home by taking
the square footage and divide by 700. Example: 1400 sq foot house / 700
= 2 ton AC.
Would one want to put a 2.5 ton Evap coil and a 2 ton condenser in this
house? Any drawbacks to this operation? Advantages ?
Any feedback is helpful. Thanks
Not true. A properly sized system which gets sufficient run-time is the best
for removing humidity. Here in Myrtle Beach, we see plenty of oversized
systems when we are called in for mold problems. A correct manual J load
calculation would be your best bet. The "divide by 700" method works a lot,
but doesn't account for anything but a square house with one story and
standard windows and insulation.
Don Ocean
Tim wrote:
> Paul Sobieck wrote:
>
>> On a typical residential furnace is it typical to
>> oversize the A coil to
>> provide more dehumidification ?
>
In some places, like Florida....
>>
>>
>> Contractors have told me that if you oversize the Evap
>> coil you will get
>> better dehumidification in a house.
>
Have to size Condensor unit for longer runs to go with this.
>>
>>
>> In Wisconsin typically, contractors size a new
>> construction home by taking
>> the square footage and divide by 700. Example: 1400
>> sq foot house / 700
>> = 2 ton AC.
>
Absolutely NOT! Use manual J or equivalent. Each home has
different insulation,
windows, doors, wall thicknesses, etc. ceiling heights,
roofs and on and on.
>>
>>
>> Would one want to put a 2.5 ton Evap coil and a 2 ton
>> condenser in this
>> house? Any drawbacks to this operation? Advantages ?
>
Pay to have a manual J run, before you consider any size.
Ronald J. Rhoades
operating to dehumidify.
If you oversize the coil, part of the coil is not cold enough and air
bypasses the dehumidify zone (in fact a wet coil will increase air pressure
drop and route a larger amount of air past the cold section.
Your oversized coil will provide the same amount of cooling (dehumidifying
only part of the air) and not run any longer, resulting in less lowering of
the humidity in the space.
EFK <turbo...@verizon.net> wrote in message
news:yG066.4969$8O3.1...@typhoon2.ba-dsg.net...
Buck153
at a lower dew point temperature. Thus lower humidity will result. This is a
basic truth.
Now if you use a larger evaporator coil it will be warmer, not colder, thus a
higher humidity.
What the larger evaporator coil does for you is increase the capacity of the
compressor, because you have more surface area of a coil for the air to go thru
and therefore enhance heat transfer. However the coil will be slightly warmer
and it slightly lowers latent heat removal.
Buck
Marc O'Brien
>"Tim" <t...@sccoast.net> wrote in message
news:3A58640E...@sccoast.net...
>Paul Sobieck wrote:
>On a typical residential furnace is it typical to oversize the A coil to
>provide more dehumidification ?
>Contractors have told me that if you oversize the Evap coil you will get
>better dehumidification in a house.
>In Wisconsin typically, contractors size a new construction home by taking
>the square footage and divide by 700. Example: 1400 sq foot house / 700
>= 2 ton AC.
>Would one want to put a 2.5 ton Evap coil and a 2 ton condenser in this
>house? Any drawbacks to this operation? Advantages ?
>Any feedback is helpful. Thanks
>Not true. A properly sized system which gets sufficient run-time is the
best for removing humidity. Here in >Myrtle Beach, we see plenty of
oversized systems when we are called in for mold problems. A correct manual
>J load calculation would be your best bet. The "divide by 700" method works
a lot, but doesn't account for >anything but a square house with one story
and standard windows and insulation.
If we walked up to some unit and swapped the evap coil for the next size up
leaving all the rest the same including the fan. The overall system capacity
would increase and perhaps even the amount of water going down the drain
during run time would increase but what would increase the most would be
sensible heat removal. This means the two things 1) the SHR increases and so
2) the temperature stat is satisfied prematurely, prematurely in that not
all the moisture intended for removal is removed during the modified run
time. So perhaps you just end up very quickly removing less moisture.
Marc O'Brien
http://fridgetech.com for in-depth technical discussions. ( No "which wire
goes where?" questions)
Martha White
because an oversized coil operates at a higher temperature, therefor,
condensation is reduced
Marc O'Brien <mlob...@fridgetech.fsnet.co.uk> wrote in message
news:93j0kf$lhd$1...@newsg3.svr.pol.co.uk...
Marc O'Brien
"Martha White" <mtw...@earthlink.net> wrote in message
news:ppq86.26088$U4.7...@newsread1.prod.itd.earthlink.net...
> Oversizing the coil will more than likely increase the RH in the home
> because an oversized coil operates at a higher temperature, therefor,
> condensation is reduced
So although coil SHR increases while also capacity increases you still
reckon the moisture mass removed per unit run time is reduced?
Martha White
yes, increaseing the coil size by 1/2 ton may only give you a couple hundred
more btus but reduce the coil temperature much morenews:93tcur$afl$1...@news6.svr.pol.co.uk...