Selecting an Accumulator

[Sgrios]

After looking through 14 pages of threads last night I was not able to find the definitive answer on selecting an accumulator for a single stage unit.

I plan to use a danfoss NF11FX and r-507. A guess, 9 feet of .031 cap tube to start.

I know the accumulator size is selected based upon the maximum volume of refrigerant being held and the maximum operating pressure of the system. There are also Vertical, Horizontal and Stand pipe variations.

This being said, can anyone point me in the right direction for selecting an accumulator, based off of the information above?

IE. Type, Overall length, internal volume, inlet and outlet sizing.


Side question in regards to Accumulators:
Speaking in general design terms, how is the orifice to meter oil return to the compressor connected back to the compressor? A diagram would be great.

I think I need to pick up some more books. Any recommendations?

Lots of questions... Thank you all for your help. I'm hoping this can help others also.

:)

[killermiller]

You mean you actually want to engineer your system instead of throwing a bunch or random parts together?

[Sgrios]

I'm not a engineer by trade, but I'd like to do a bit better than using random parts and slapping things together in hopes it will all work out.

It's been almost three years of lurking and posting. Some more research would not hurt things. However I'm not exactly sure what size accumulator I should use, considering the small refrigerant charge and system size of these units. I plan to design a unit, but it would not hurt to have a idea of what size accumulator I would need in case I experience flood back to the compressor.

Side note:
I own Modern Refrigeration and Air Conditioning 2004 18TH Edition and other recommendations would be great.
I know walt listed off some other books in the past, but I cannot find the thread.

[killermiller]

The smallest one you could find would probably be ok. The smallest one I found at Johnstone Supply held 35oz. That is way more refrigerant in an entire SS. Rotary compressors have one built in.

[sdumper]

I have had best results so far with an 8inch long copper pipe 1 inch in diameter and stuffed with copper pad

[wdrzal]

Actually it's in the chapter with desuperheaters for single stage units :rolleyes:.

You don't need one. Any liquid refrigerant leaving the evaporator does no good at all. You really never have a 0 heat load. I would bet almost all people with "flood back" with a evaporator/system this small have it over charged/=too low superheat or the cap tube is sized wrong..

A cap tube system when designed with the correct cap tube and correct charge is self regulating. As the heat load drops,so does the discharge temperature/pressure and thus the mass flow through the cap tube is reduced.

Now a disclaimer since I haven't built one yet:

I would say if you kept 2 to 4 degrees sub-cooling and 12 to 14 degrees superheat things will work fine.

If someone is getting flood back, installing a suction line filter/dryer would act some what like a accumulator (in function).

[sdumper]

Walt on the SS units you have built did you not notice a difference when using an accumulator?

[wdrzal]

I never built units for pc's (yet)and just about everything I designed and work on are much larger.But with that said I think the forum as a whole took a step in the wrong direction when everyone started using HPB compressors designed for AC instead of LBP that are designed for low temperature refrigeration. Instead of using the properties of the refrigerants ,everyone started using deep vacuums with extremely high mass flows from high horsepower compressors...not only the lowest temp on a P/T chart matters but how much heat, kj/kg or btu/ Lb a refrigerant can move... Some of you guys are using compressor so big you actually need to flood the suction line with liquid refrigerant to keep the compressor from overheating,then that just means you need to reject more heat from the condenser. Remember the "heat of compression" the bigger the compressor the more heat you add to the system,so the more heat needs rejected so just not the condensers but the entire system grows in size and cost. To move 250 watts you don't need gallons of refrigerant,but the charge should be in ounces, jeez keep in mind the evaporator is only 1.5 inches. Seems like the COP went out the window,when window unit compressors started being used.

Giant compressors,deep vacuums & high mass flows is the wrong means to the end. Sure you can make it work,but far from ideally.

"IMO" <rant off>

[wdrzal]

Walt on the SS units you have built did you not notice a difference when using an accumulator?

What difference are you asking about ?:shrug:

[one_servant]

Well phrased question Sgrios. This is a topic that I have always felt that I have had holes in my knowledge base. My original assumptions were that a small accumulator would work just fine even though the smallest one usually finds are overkill for our applications. Regardless...better to have that safety feature than to just "risk it" if you have the space for the accumulator in the first place.

[ultralo1]

I...not only the lowest temp on a P/T chart matters but how much heat, kj/kg or btu/ Lb a refrigerant can move...


To move 250 watts you don't need gallons of refrigerant,but the charge should be in ounces, jeez keep in mind the evaporator is only 1.5 inches. Seems like the COP went out the window,when window unit compressors started being used.

<rant off>

Nice rant and I understand and agree with you.
I also understand these guys using what is economical for them.
No disrespect towards anybody.

wdrzal, Given the size limits of the evaps when are they going to reach the limits of being able to remove heat with that watt density? After all that is a small area and that limits the amount of flow. I guess that you could increase the flow of refrigerant but still you are going to reach a wall at some point arent you?

[teyber]

My evap at 1.5" diameter and ~ 1" tall-
http://www.blazingpc.com/forum/showthread.php/i7_bencher_build-16471/index.html?t=16471
holding over 300w at ease, with a very reasonable delta. thanks to sdumpers amazing tuning abilities. Just saying it isn't so much about mass but a blend of surface area/ and WHERE the mass is.

[sjg0]

wdrzal, Given the size limits of the evaps when are they going to reach the limits of being able to remove heat with that watt density? After all that is a small area and that limits the amount of flow. I guess that you could increase the flow of refrigerant but still you are going to reach a wall at some point arent you?

Understand that they will never develop a CPU which the load is so great not even a phase-change will be able to handle it. Keep in mind CPU's are designed for air. Also the trend seems to be to go "green" ie: Chips will become more energy efficient and thus waste less energy in the form of heat.

[wdrzal]

wdrzal, Given the size limits of the evaps when are they going to reach the limits of being able to remove heat with that watt density? After all that is a small area and that limits the amount of flow. I guess that you could increase the flow of refrigerant but still you are going to reach a wall at some point aren't you?

Since practically ,evaps are size limited the simplest way is a larger Delta T,so the heat flows faster. There are sure small gains to be made in evap design but personally I feel (cause there's no real data) from the best to worst design is only a few degrees. Thinking 5 maybe up to 10.

No one has actually measured in a accurate way evaporator performance. DetroitAC built a calorimeter :up: for testing evaps but the evaps makers didn't want to pay for the data, I'm just guessing . :(

Fluid dynamics software is pretty accurate but it's expensive and even cad models take quite a bit of time to build and run flow tests.

I understand about Ebay,that's ok if your building a one off for your own use. You spend your time searching the parts for your unit.

Impossible to run a business building from Ebay parts.

But this is what ruined most builders, you have to buy parts at wholesale,build a unit that you can repeat and make a profit. You can't do that from Ebay. Ebay is not a reliable source for parts,and if you have to redesign each unit,it's cost prohibitive...... I hope future builders learned from past lessons. :up:

[Sgrios]

Well phrased question Sgrios. This is a topic that I have always felt that I have had holes in my knowledge base. My original assumptions were that a small accumulator would work just fine even though the smallest one usually finds are overkill for our applications. Regardless...better to have that safety feature than to just "risk it" if you have the space for the accumulator in the first place.

Thanks one_servant. My knowledge on this subject is very limited also. However, I have no issues reading and I personally love furthering my knowledge, but the few books that I own do not cover system design very well.

Any suggestions on a book(s) that deals with system design would be great.

Thank you for all your help so far guys. :up:

[Jack]

I don't know much about accumulators either. Most people here use trail & error, I guess.

Also I've only seen 2 books recommended here:
1. Modern Refrigeration and Air Conditioning.
2. TECH Method (https://www.merchantamerica.com/tmethod/index.php?ba=view_category&category=3332), by Gary Lloyd

[mytekcontrols]

I absolutely love the information that Walt presented, and find myself in agreement. However I can see some situations that can cause flood back to occur even in the best designed cap tube fed systems (especially where no accumulator is being used).

Since we are talking relatively high loads when the CPU is active (200-300 watts). Having a semi fixed flow design utilizing a cap tube to handle this load could be problematic when the load is not present. I also am not a PC cooler user or overclocker, so I can not speak from experience in this area. But if there are times when the cooler is being run without a load, you could see some serious flood back issues with a cap tube fed evaporator. Having an accumulator would certainly be good insurance in these situations.

[Sgrios]

I understand about Ebay,that's ok if your building a one off for your own use. You spend your time searching the parts for your unit.

Impossible to run a business building from Ebay parts.

But this is what ruined most builders, you have to buy parts at wholesale,build a unit that you can repeat and make a profit. You can't do that from Ebay. Ebay is not a reliable source for parts,and if you have to redesign each unit,it's cost prohibitive...... I hope future builders learned from past lessons. :up:

Some how I missed this last night, really good lesson indeed.


I absolutely love the information that Walt presented, and find myself in agreement. However I can see some situations that can cause flood back to occur even in the best designed cap tube fed systems (especially where no accumulator is being used).

Since we are talking relatively high loads when the CPU is active (200-300 watts). Having a semi fixed flow design utilizing a cap tube to handle this load could be problematic when the load is not present. I also am not a PC cooler user or overclocker, so I can not speak from experience in this area. But if there are times when the cooler is being run without a load, you could see some serious flood back issues with a cap tube fed evaporator. Having an accumulator would certainly be good insurance in these situations.

That was some good information mytek.

Thanks everyone for your input thus far.

[one_servant]

Having an accumulator would certainly be good insurance in these situations.

That's essentially my thinking with regard to an accumulator....they are extra work and a little extra money but it's like buying a small insurance policy. However, it would be nice find an accumulator that would be more appropriate for systems that are as small as most of ours are.