Change the coil but not the condensor/compressor

[HighPotential]

I have a Carrier 38HD A030C nominal 30,000 Btuh compressor/cond unit and a Carrier CE3AXA0300000ABAA 30,000 Btuh coil. My heater and blower unit is rated at 1450CFM but my coil is only rated to 1000 CFM nominal, 1250CFM max. This will kill my extra 200CFM which I really need for my 3300 sq-ft house. I was wondering how I can tell if I could simply change out the coil for the 38HD A042C coil rated at 54,000 Btuh and 1450 CFM nominal. Since it is a bigger coil, will my compressor be able to keep up? I have all of the data, just having trouble figuring it out.

Thanks!!!

 

[mdonner]

1. Are you sure 30,000 btu/hr is enough for 330 sq ft house? (it depends on design conditions which you do not specify).

2. Too large evaporator may increase superheat to a value that will dammage the compressor (too high discharge temp.)

3. With design conditions and the known capacity and air flow, you can use the psychrometric and the refrigerant LogP-I charts to find what will happen.

 

[Acies]

Installing a large evaportater coil should not be a problem here. The lower rated coil brings a much worse problem that of face velocity of the coil. Manufactures often allow a maximum face velocity of 550 feet per minute (fpm) to prevent the water on the coil from blowing off and into the ductwork. The coil rated for the correct airflow would correct this problem. Whether this is a problem depends on what your weather is like. In humid climates this is a bigger problem. In dry climates it is possible you will have a dry coil. Increased superheat is not a problem unless you start to go two coil sizes larger than what matches the condensing unit size. Since you are going from an 030 unit to a 038 coil this will be fine.

Simliar to the previous post the question of capacity vs. load can't be addressed based on what you have provided.

 

[HighPotential]

Thanks for the info. Acutally, Acies, I'm proposing to go to a 42 coil from a 30 coil. My compressor is a Carrier 38HD030C Product Data here:

http://www.commercial.carrier.com/wcs/prod_physicaldata/0,1180,CLI1_DIV12_ETI440_PRD100,00.html

I have an 030 coil and want ot go to a 042 coil. The coils are as listed above with product data here:

http://www.xpedio.carrier.com/idc/groups/public/documents/techlit/ce3a-2pd.pdf

Hope this helps.

Thanks again...

 

[lilliput1]

I live in New Jersy and my 1600 SF house require 2.5 ton (30,000 Btu/hr). So your unit seems small. Where is your house? Typically residential AC units are 350 to 400 cfm/ton. So 875 CFM to 1000 CFM should be the airflow for cooling. If you force in more airflow, the coil will have increased sensible capacity but reduced latent capacity. Also you will get moisture carryover if above 500 FPM face velocity. The blower & heater seems to be oversized for this cooling unit. Usually the correctly matched heater will require less CFM than the cooling CFM & the fan is run at the slow speed tap when heating. This is to minimize the feeling of draft from the supply air in winter.

 

[HighPotential]

Your assumptions are correct. I live in Los Angeles so the air usually dry. 3 weeks a year are humid if that. Summers gets over 100 sometimes but most days are in the low to mid 90's. My house is 3300sq-ft. It will be well insulated and I have using radiant reflective roof sheathing which has made a big difference. Still, at 330 sq-ftr, I need the full 1450 CFM my heater/blower can deliver for 4 changes an hour. If my coil cuts me down to 1250, this hurts my overall house flow. That's why I was checking to see if I could simply get a bigger coil, say a 42 and it would work just as well, with my 30 compressor.

 

[mdonner]

I want to add some observations, based on the assumptions the calculated heat load is 30,000 Btu/hr and the calculated airflow is 1450 cfm:

Thumb rule figures:
400 cfm/Tr: yours is 1450/2.5=580 (+45%)
350-700 sq ft/TR: yours 3300/2.5=1250 (>+78%).

030 evaporator at 67F WB RA and 40F evaporator with 1250 cfm yields 48,000 Btu/hr.
042 with 1,400 cfm yield 68,000 Btu/hr, which means 30% of heat transfer area will be used by the system for refrigerant vapor (additional) super heating.
This is just an indication: the actual working point will be balanced by actual performance curves of fan coil and condensing unit at prevailing ambient conditions.

I believe the required figures will be somewhere between your figures and the rules of thumb 3300/(<700) -> (>4.7)TR and (>4.7) x400 = >1880 cfm

 

[HighPotential]

Thanks for all the info guys. But, being an electrical engineer, I got lost towards the end.

Can I get a consensus from the panel?

Based on the info going from 030 to 042 in the coil will work fine with my 030 compressor?