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Best compressor setup for small Displacement Engine?

Runboy on Thu July 03, 2008 10:02 AM User is offline

Is it best to run a small compressor at higher speed or a larger compressor "pulley'd" down?
Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

CCWKen on Thu July 03, 2008 9:51 PM User is offlineView users profile

Refrigerant pressure is about the same among compressors so the variable is volume. If you have a small system, use a small compressor. Judging from your title you're using an engine that won't develop it's rated horsepower until you hit 3600rpm. But... few, if any, OEM small engines are run at their peak rpm. The governors are usually set in the 3200rpm range and anything above that voids the warranty. What this means is you're getting less than rated HP. The torque on most small engines is hardly enough to get past the compressor's break away torque so some details of your engine would help.

I've got plans on the drawing board to build an air conditioned grass cutter. Similar to a Grasshopper (ZTR). I've got most of the parts accumulated just looking for time. I chose a 3-cylinder diesel engine driving a 5-segment hydraulic pump to handle all the drive and control systems. I was planning on running the compressor straight off the engine. The diesel (Kubota) has a huge flywheel and high torque so the compressor clutch cycling shouldn't even burp the engine. That's not the case with a small gas engine.

-------------------------
Ken Kopsky

Custom Car Works
"Rules are for the guidance of wise men and the obedience of fools."

webbch on Fri July 04, 2008 1:54 AM User is offlineView users profile

I didn't seen any PM function on this site - I've just been wondering - are you the same CCWKen from the homeshopmachinist forum that built your own grader a couple years back? Nice work there. Any plans to add a cab and a/c to that setup as well? :-)

Chad

bohica2xo on Fri July 04, 2008 3:55 AM User is offline

Runboy:

You might do better here if you simply laid out what it is you are planning on doing. You have started a couple of threads with very general questions. I have no idea what "small displacment" means to you. CCW guesses you are thinking Briggs & Stratton, but to many people under 3L is "small"...

Depending on the capacity you need, and the drive you have there are many options. A scroll type compressor is rather effecient, but can have crappy performance at lower speeds.

A variable displacment compressor does well with MVAC applications, since it can reduce load if it is not needed. This is great if you are building a car unit with variable loads & engine speeds. Useless if you are building a large refrigerator with a fixed speed engine...

Many piston pumps lose some effeciency as speed increases. Take a look at the picture below:



If you look at the graph, power absorbed is closely related to pump speed. Capacity is relative to speed as well, but effeciency drops as speed increases. If you only needed a little cooling, that V7 running about 1500 rpm is fairly effecient.

It all depends on what your goals are. With more information, thre are several people that can help you here.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Runboy on Fri July 04, 2008 7:33 PM User is offline

Thanks to everyone for the input!
Ok, here is the bizarr idea I am tossing around - I am trying to come up with a system that would turn the inside of a 1959 VW panel bus into an ice box. After successfully putting together a system on a air cooled beetle I am thinking that if there where some alternative way to drive the compressor it would work out much better. The little Nippondenso 10PO8E I have on the beetle puts a pretty good load against such a small engine and every last pony available is needed to move a bus.

I was thinking to maybe remote mount an electric start briggs and stratton type engine somewhere either in the engine compartment or underneath. There is a lot of room underneath the bus for condensers etc.

I was thinking maybe using something like a couple of the rear system evaporators out of a suburban or similar mounted to the inside of the roof since there is no underdash space. Any other ideas?

Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

CCWKen on Sat July 05, 2008 12:00 AM User is offlineView users profile

Quote
Originally posted by: webbch
I didn't seen any PM function on this site - I've just been wondering - are you the same CCWKen from the homeshopmachinist forum that built your own grader a couple years back? Nice work there. Any plans to add a cab and a/c to that setup as well? :-)

Chad

Yep, that be me. Thanks for the flowers. I thought about adding a cab and air to the "Kenbota" but the little 2-cylinder diesel is a bit small to drive a compressor too. I've had to settle for a top and just shade.



-------------------------
Ken Kopsky

Custom Car Works
"Rules are for the guidance of wise men and the obedience of fools."

CCWKen on Sat July 05, 2008 12:18 AM User is offlineView users profile

Just so we keep the thread on track--

Mike, that's a lot of space to make into an "icebox". The BTU's required, with good insulation, is going to pose the biggest problem. The engine alone will probably need to be in the 18-20hp range unless you plan to run a 10-12hp engine wide open all the time. It's not my project but I think I'd be looking at gutting an old freezer and running it off a small generator or using an inverter. How long and for what purpose will the vehicle have to be an "ice box"?

-------------------------
Ken Kopsky

Custom Car Works
"Rules are for the guidance of wise men and the obedience of fools."

Runboy on Sat July 05, 2008 1:57 AM User is offline

Ken, thank you for your interest and input in this post.
I don't literally mean an "Ice box". Just a good cooling a/c system that will cool it down on a 100+ degree day.
Any thoughts?
Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

bohica2xo on Sat July 05, 2008 5:00 AM User is offline

Mike:

For a box the size of a microbus, you will need something the size of that V7 to get the job done. The 9kw on that graph is just a taste over 12 horsepower.

By the time you buy a decent small engine that will produce 18 to 20 horsepower, deal with the mounting / vibration issues, etc. - you would probably be way ahead of the game reworking the VW engine for another 20 hp or so. Aircooled VW parts are still a bargain, and even a 90.5x69 engine should have more power than the original with the A/C on.

Don't forget the electrical loads you will have with the A/c system. The condensor(s) will need fans as well as the evaporator(s). You will want a decent internal regulator alternator on the belt drive with the compressor. 3G, 4G, 6G Ford units, or AD244, CS144 GM units would be good choices. Any of those alternators max out @ 2kw, so plan on 4 horsepower to drive them at full output.

An 18 hp B&S or Honda twin is about 1200 bucks. I can build a lot of VW engine for that much money.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Edited: Sat July 05, 2008 at 5:01 AM by bohica2xo

Runboy on Sat July 05, 2008 10:32 AM User is offline

bohica2xo, thank you so much for the information. Can you give me an understanding of "compressor capacity" in relationship to "evaporator capacity/sizing" in simple terms. I am just not understanding the effect of changing the size of a compressor in any given system. If it can provide equivalent pressure to the one it replaced, wouldn't performance be about the same?
Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

bohica2xo on Sat July 05, 2008 1:35 PM User is offline

Mike:

Evaporator capacity is related to both airflow & refrigerant flow. Pressure is dependent on temperature as well as volume in a system. You must also consider condensor capacity within the system.

If you were using a fixed displacment compressor that was too large for the evaporator, the low side would drop below freezing very quickly. In a CCOT system this would mean rapid cycling. An evaporator that is too big for the compressor capacity will transfer heat just fine, but it will not feel "cold" unless you reduce the airflow.

One of the things that makes the V5 / V7 compressors attractive for this sort of application is the variable displacment based on evaporator pressure. A V7 is used by GM in vehicles as small as the S10 - even though it has huge capacity.

You should be sure you have enough condensor capacity. In MVAC, there is no way to have "too much" condensor. If you can find space for a big condensor, and provide enough airflow you should be able to cool all of that space. Make sure you use a TXV on any evaporator(s).

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

Runboy on Sun July 06, 2008 1:54 AM User is offline

One ? on TXV Systems. Are you supposed to have a temp switch on the evaporator to cycle the compressor off if it freezes up or will the TXV completely control that? The cheap underdash system I have in the bug has an adjustable thermostat but I believe it senses cabin temp and not evaporator temp.
Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

bohica2xo on Sun July 06, 2008 1:35 PM User is offline

Mike:

Evaporator temp control is usually handled in one of the following ways:

Temperature sensing. Using a probe of some sort to shut the compressor down if the evaporator drops below freezing.

Pressure sensing. using a pressure switch to extrapolate the evaporator temperature from a known refrigerant's P/T chart. The switch shuts down the compressor.

Evaporator pressure control. These systems are constant run. Using an absolute pressure reference, they control the evaporator pressure to hold it above freezing. Older GM systems did this with a seperate valve called a POA or STV. Chrysler did it with a valve in the compressor inlet. All of those systems simplt throttle down the compressor suction, letting the compressor run at full speed.

The V5 / V7 type compressors use a control valve to de-stroke the pump for evaporator temperature control. This gives you a reduction in absorbed power when lightly loaded, while providing evaporator pressure control. No need for added switches or sensors.

You appear to be a serial KrautCan owner. Been there, done that. I had A/C in a 1966 type III. With the 6 volt system you had a choice - headlights or A/C.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Sun July 06, 2008 2:01 PM User is offline

Undersized coils either evaporator or condenser will lower system efficiency. This is the main reason that car A/C is so much less efficient than home A/C, there just isn't enough space to install adequate coils and fans. The evaporator does need to get cold enough to condense water out of the air, but beyond that overcooling air to a 40 degree vent temp then mixing it with the hot air already in the car is a waste of energy. Ideally the air flow would be much greater and the vent temp higher.

For example from the curves for the V7 at 3500 rpm you get about 13 kW capacity (42,000 btu/hr or 3.5 tons), at 7.8 kW input. Driving the compressor with a perfectly efficient electric motor (and also neglecting power required for fans) this is an EER rating of 5.4. Almost any ordinary window unit is at least 10 EER these days and central systems are 13 and higher.

Runboy on Sun July 06, 2008 7:20 PM User is offline

Thanks for the info guys.

lol - no, no "serial KrautCan owner". Starting my driving career in a vw and have just come back to playing with them after going on to mainly old american stuff and a diesel pickup. Just always have loved tinkering with stuff and the simplicity, low parts cost/high availability make a vw a fun one to play with. Not to mention current fuel prices!

One question on the V7, aren't they used with orifice tubes in the systems GM incorporates them on? Can you give me a logical explanation as to why they would use an orifice tube over a txv? It seems that ford does on all of their full sized vehicles as well. It can't be the overly used and simplified "the automaker is trying to save a buck" speech. After all, isn't the V7 a rather pricey piece of hardware?

Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

mk378 on Sun July 06, 2008 8:17 PM User is offline

You can use a V7 with either orifice tube or TXV. There are a few GM models with V5s or V7s running TXV systems just for confusion. A SUV with dual air may have both going at once. The orifice tube is used to save a buck. The car maker has to buy a compressor either way, but can cut costs on other parts of the system. The variable compressor is nice that it doesn't cycle on and off, contributing to a smoother ride that might help to sell the car.

Runboy on Tue July 08, 2008 12:11 AM User is offline

One other question on the V7 Compressor - can they be set up with a single groove pulley?
Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

bohica2xo on Tue July 08, 2008 11:09 AM User is offline

The clutches on the V5/V7 are interchangable. There are V5's that have a single groove pulley.

There are also a couple of companies making serpentine drives for type 1 VW engines. BMD is one of them. You might also look at adding a second pulley for the compressor. Some S10's had a secondary pulley bolted to the damper that works well for that type of job.

A variable displacment is a great match with a TXV. And yes, they really did run orifice tubes to save 15 bucks per car. Ford put tubes in Explorers, and TXV's in the Lincoln version - they know a TXV is a better performing system.

B.

-------------------------
"Among the many misdeeds of the British rule in India, history will look upon the act of depriving a whole nation of arms, as the blackest."
~ Mahatma Gandhi, Gandhi, An Autobiography, M. K. Gandhi, page 446.

mk378 on Tue July 08, 2008 11:47 AM User is offline

A fully loaded V7 will be pushing the horsepower limit of a single v-belt. It would be better to upgrade the engine to a serpentine belt. You will also need to fit a modern alternator to power all this stuff.

Runboy on Fri July 11, 2008 1:45 AM User is offline

A little off topic of the post heading but fits in with this post and my curiosities:

(1) What is the largest capacity system used in an oem application that parts could be scrounged from?

(2) Would a parallel flow condenser work as an evaporator if a fan with shroud was fitted to pull air through the whole surface area?

Mike

-------------------------
89' F-350 Crew Cab 7.3L Banks Turbo

2000 E-series Condenser, R134 Converted with Orange Orifice Tube
[IMG]http://members.aol.com/runboy7426/runboycaricaturesmall.gif[/IMG]

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