Why does capacity decrease in high velocity scenarios?

alwayslearning

I was just on the AHRI website.  Why does the total capacity of a condenser matched with a "high velocity" evaporator drop 15% or more than a nominal tonnage number?  For example, why does a 3 ton match system only have 28k-30k capacity?

Techman

Hi-V

Hi-V systems operate closer to 32* evap temp than convential AC systems and sometimes closer to 30* evap temp. So across the board, the lower the system operates in evap temps the lower the "efficency" ,or the lower the evap temp the more it costs per BTU. 1 ton of AC = 1H.P.= 12,000 BTU and 1 ton of freezer =2H.P.=12,000 BTU .That is double! That is an older rule -of-thumb  but easy to follow.

alwayslearning

32° or 42° ?

According to the charging instuctions for one of the brands (ESP) of high velocity DX coils, one would charge to a 42° suction temperature line.  Then adjust the TXV to 4° superheat.  No consideration is noted for subcooling?????  Seems strange to me.... only concerned about the evaporator.

Techman

Hi-V

Are you talking 42*  suction line temp @ the compressor? One of the "advantages" of hi-v is its "better" dehumidification ability which is a result of the much colder evap temps.Hi-V's also have a factory built-in freezestat because of very low evap temps that they run at and frosting of the evap is KindaSorta "normal".As evap temp/pressure drop ,so does the "ammount" of freon in a cubic inch drop.Flapping jaws here,there is 100 particales of freon per cu. in. in an AC operating @ 42* evap and only 90 particals (cu.in.) in the Hi-V system operating closer to 32* evap and only 75 particals(cu.in.) in a refrigerator operating w/ a 20* evap temp and only 50 particals (cu.in.)in a freezer w/ a 0* evap and just 25 particals in a freezer w/ a -20* evap.

alwayslearning

Now, I'm confused?

I appreciate the knowledge, information, and response to my question.  I also do not wish to beat a dead horse.  However, according to the manufacturer's installation manual, one would charge to 42°F, not 32° for a suction line temperature (see attached file). 

I can understand that the lower the suction temp/pressure, the lower the "efficiency" of the cooling operation.  I can also grasp the dehumidification point made earlier.  Isn't all of that sort of null and void if the manufacturer isn't calling for a 32° suction line temp?  Now I'm back to my original question....?

Techman

Capacity

Always do as the mfrg says.The mfrg is giving those specs at startup when its real hot outside and inside.Wait until the house has reached its stabilized conditions when the suction press/temp is a lot lower, then that FACTORY MANDATED and FACTORY INSTALLED "freezestat" will come into play,maybe.Just like the % of oxygen in the air gets less(thinner) as you go from sea level  upto a mountain top. The pressure gets less the higher up you go & the density of a cubic foot gets less also. As the refrig press drops its density also drops ,less freon per cu.ft. =less capacity.Does the AHRI capacity #s, give a corresponding evap temp to those capacity ratings ? A regular AC 3t system has more capacity @ 45* evap than it does @ 42* evap temp,and so on down it goes.

RDSTEAM

correct me if i'm wrong

wouldn't charging to 32 degree suction temp mean that your coil is going to freeze?? or is that only on a normal flow system (not Hi-V) don't really know anything about Hi-V systems

Techman

Capacity

Not 32* suction line temp,but 32* staurated freon temp inside the evap coil,then add the TXV super heat,then add the S.H. of the suction line all the way to the comp.That # should be close to 42* suction line @ the comp. The evap coil is supposed to freeze! If EVERYTHING is proper, then you will have frost,then the freezestat will function . The freezestat is not a "safety" like a hi/lo press safety that should never actually operate . The freezestat is a normal operating control KindaSorta like the system t'stat.Or go backwards from the factory spec of 42* @ the comp,backwards to the TXV thermal bulb( minus that suction line  S.H. ) and then backup a little more and go inside the evap coil and deduct the TXV S.H..Now you a very close to 32* evap saturated freon temps which is very close to freezing. A big point here is that "suction line temp " is not "evap saturated  temp".

alwayslearning

please comment on my settings

I returned to my job site and "adjusted" the systems to what I believe is comparable to our discussions.  Please comment, criticize, etc. any or all of my current settings or readings.  Thank you.

6/25/2012Outside air temp--86°Fnorth unitmiddle unitsouth unitliquid pressure/temp295psi/96°F285psi/93°F305psi/96°Fsuction pressure/temp100psi/30°F105psi/36°F100psi/30°Fevaporative superheat5°3°4°subcooling9°8°7°supply air temp36°37°33°return air temp69°69°69°cfm770770770

alwayslearning

lets try again

Please comment on my current settings.

Thank you.

Techman

Readings

A couple of things .The hi side readings are for r-22 and the low side readings are for r-410a, the hi press readings are hi by about 35-40 psi.How close are these units to each other?

John Mills_5

Haven't looked lately

But the Unicos we put in a while back had subcool specs in the instructions. Seems to me that's the way to charge a TXV system. 

alwayslearning

response

I am sorry, but both the high and low sides are pressure and saturated temperature for 410a.

The condensing units on the north side of the building for north unit and middle unit are 15' or 20' apart.  The condensing unit on the south side of building for south unit is the only piece of equipment there.  All the units are 2' off the ground and at least 20" away from the building.  The three evaporators/fan coils are in different mechanical rooms separated by distance and walls.

alwayslearning

subcooling

I would agree whole heartedly.  Charging by subcooling with a TXV, at least to get started.  However, my learning curve is leaning backwards on my on this project.  The manufacturer calls for 4°F evaporative superheat and/or 40°F suction temperature line.  The previous posts have certainly opened my eyes to things outside of the installation manuals and how I have approached refrigeration on conventional equipment.  High velocity is a different ball game.

Maybe Unico documents all we are discussing here, or you learned it in a design suite..??  I am working with ESP's Hi-Velocity on this application.

Techman

Press

What were the 410a hi side readings ? With an 86* ambient the head press should be around 350-360 psi or so w a saturated  temp of about 108* or so. There is nothing special about the freon P/T relationship for Hi-V's.The lo side is just a little lower than "regular AC" but higher than the next step down which is hi temp refrigeration.And the S.C. is a reflection of whichever cond unit is put on the system not a reflection of the Hi-V air handler, so ESP( or Unico) has little to say about the sub cooling.And the supply air temp is awful close to the saturated freon temp.How close to the evap coil was your temp sensor?

alwayslearning

high side readings

High T/P readings were taken from the liquid line schrader on the evaporator.

North--295psi/96°F

Middle--285psi/93°F

South--305psi/97°F

Condensing units are 3 ton Goodman 13 SEER devices.  Why is my head pressure so low?

Supply air temps were taken with a probe thermistor through a 1/4" hole in the air handler cabinet.