View Full Version : fractional hp rotary compressor system sizing

28-04-2016, 10:13 PM
hey everyone.. "long time listener, first time caller".. I have a project and i've sourced some compressors from a company in china, but would like to have someone a bit more in-the-know review what i am looking to accomplish before i order any components. If something isn't right here, i will need a bit of help in tracking down the correct technical details/terms, as i am at least 10 years out of practice/formal training with HVAC. I'm eager to study, but sometimes I cant recall the exact term or name for the behavior i want to track down, its too easy to get lost in the forum search or google search. Given the tight requirements of low starting amps and low running current, i'm thinking i should find a reliable compressor and work from there. This is all for personal at-home use, not for a customer.

I've a need for some low-BTU 115v and 48v air-conditioners/dehumidifiers roughly in the 1500-2000BTU range. I've found the smallest units I can buy for a reasonable price are 5kBTU, with LRA peaking 20A+ , running amps 3-3.5A. These tend to cycle on and off too quickly and yes they can maintain temperature, however the short run-time provides poor humidity control. I am aware there are self-contained 2500BTU units, such as "pet-cool", this is exactly what i am looking for but i am not pleased with its weight or efficiency, quite high power draw for watts pumped/moved. Not sure how a 2500BTU air-conditioner draws nearly as much as my 8000BTU R22 window-shaker, but long story short it looks like i'll either have to spend a lot more than i want to for a low-volume-production mini chiller/AC unit, or devalue my spare time and do it myself, perhaps put my hvac tools back to use.

bit of info about me:
I have experience in working with R12 and R134a in automotive applications and have the equipment for this. Gauges, vacuum pump, adapters, fittings etc. as well as limited experience in replacing compressors on home hermetic air-conditioning and refrigeration.

Some of this is for vehicle-mounted equipment cooling, which i am looking toward DC/inverter rotary-compressors as well as low-hp rotary compressors. Seems the key-word to look for here is "high back pressure" pumps, with the expectation being evap temp to be no less than 45F called out in spec-sheets of most any hermetic compressor labled as HBP, when the high-BTU figures are called out. target cooling temp for the environment these are used in is roughly 70-75F with exterior temperatures approaching 85-90F.

115v compressors can be driven by an inexpensive inverter with roughly a 10% loss/cost to conversion losses, have been running a 1/10hp tecumseh compressor for quite some time on a small sinewave inverter. Catch is, some compressors have low inductance for the motor + start winding, such as my home-refrigerator wont start on anything less than a 1000W inverter. Looking to keep this small, simple and not lock myself into having to design two separate systems for the 115v fixed application, and mobile/vehicle application. Power/watts are cheap with mains/grid power, a penny per hour to run a standard 1/6hp pump. Not so much with DC + batteries.

Just want to put together something that does the job and duplicate the effort for 2-3 units if this project works out favorably. These will be used in battery-powered situations, so the difference between a unit consuming 150W and 300W has a profound effect on total cost for runtime. cost of more batteries and solar adds a significant multiplication factor to the overall cost of motor-horsepower.

115v compressor:
QX36HB HBP 110V/60Hz R134a rotary dehumidifier compressor
Model: QX36HB
Power Supply: 115-120/60V/Hz
Displacement: 3.6cm2
Motor Type: split capacitor
Cooling Capacity: 440W
Cooling Capacity: 1500BTU/h
Current: 1.1A
Noise: 48dB(A)
Vibration: 20m/s2

HBP ASHRAETest condition:
Evaporation Temperature: 7.2 C
Condensing Temperature: 54.4 C
Ambient Temperature: 35 C
Subcooling Temperature: 46.1 C
Suction Temperature: 35 C

I have most of this worked out, found a 2kBTU braze-in expansion valve. I have some inexpensive digital thermostats with cool / vs / heat mode selectable, a time-delay relay (once compressor turns off, must wait x many seconds before allowing re-energizing compressor), plenty of 1/4" and 3/8" leftover copper tubing. Have some spare condensers, roughly 10x8" copper w/aluminum finned (refrigerator condensers), and DC blower fans of varying sizes/CFMs.

Just need to sort out the compressor and align my expectations of its behavior in this situation accordingly, as efficiency is key and i've only a handful of BTUs to work with, i cant be wasting them with poor design or improper component selection. If this pump really can move 1500BTU with the above temperature parameters, i want to be certain i am on the right path to reach that figure/goal, before i start cutting tubing.

Questions i have:
-Given that i expect evaporator load to vary wildly, quite extreme on initial startup (110-120F),would it be wise of me to select an expansion-valve, instead of following the fixed-capillary orifice tradition with small AC units? I'm quite used to these in automotive applications, my understanding is this is helps keep the load presented to the compressor at a somewhat constant level until its time to cycle the pump on/off once target temp is reached, although i do need to study more as i'm rather lost on sizing these, many are called out in BTUs, and i am thinking I need to familiarize myself with the math formulas and size based off expected coolant flow rate/displacement. IIRC expansion valves are mostly just important due to the varying pump-rpm, not so much the evap load?

-life expectancy/cooling of rotary compressor .. i would imagine much like a reciprocating pump, the rotary pump will be short-lived if the evaporator return is hot. If i anticipate a lengthy start-up period where the air hitting the evap will be unreasonably hot, will an expansion valve offer any help here or should i be controlling the heat-load to the evap, such as throttling down the DC blower fan in the evap, by tracking the outlet temp heading to the pump? i have some simple thermal switches that can select a low-fan RPM if the evap outlet tube is hot, or just use a timer and some guess-and-check adjusting at install time (startup with slow evap fan, fast condensor fan, after x many minutes or once x temp is reached, resume to full fan speed)

-multiple / split condensers .. the small condensers i have on hand are too small to just use one without a gusty powerful watt-hungry loud fan, will need at least 2 or more of this size (10x8") for the condenser and can probably get away with just one on the evap side. is it advisable/acceptable to connect these in series for the condenser, and put on a standard dryer/receiver, or are there other considerations that may not be apparent beyond surface-area/CFM/watts-removal? I've never worked with a system this tiny, the compressor is slightly larger than a can of soda, so i'm a bit anxious of this project being overly-sensitive to design flaws, lack of coolant, too much coolant, etc.

-can a rotary pump such as the one above function as a heat-pump, given that i design for the use of the correct valving? Exterior temperatures of 45 to 55F condensing humidity, with a goal of interior temp of 70F is roughly the situation this would be used in. I assume i can use it as a heat-pump by reversing flow and leave everything else the same, but i am unsure of finding accurate, clear information to further my self-study on whether this is worth the bother of the $100 in extra valving parts. Mostly just want to plot out a somewhat-accurate estimate of potential watts-pumping capacity vs temp vs airflow CFM and am getting lost in finding references / examples of the forumlas. Compressor draws 1.1A for 130W heat, if it could also pump 200W from the outside environment at these temperatures i would certainly pursue this, much less watts however and id loose interest.

thanks for any input, looking forward to testing out these little baby compressors and see how they behave :)