A/C that runs off a battery...

[Jimmyt]

In 3a and 3b is where I had the question about low versus high pressure. This is the reading on the low pressure side. He's talking about refrigeration at this point and I'm trying to rationalize how to adjust this reasoning for the pressure needed for an A/C unit. I'm trying to extrapolate this 3rd video information for an A/C unit. As the evaporator isn't in an ice box, it probably won't frost in an A/C unit. So how do you know you're filled properly when charging up the A/C system? Will it be a higher or lower pressure than 11.7 psi and what do you do as a "you're done" gauge?

What refrigerant are you going to use? They have different pressure vs temperature properties.

Also, different refrigerants use different oils, so if you scavenge a used compressor, try to make sure you use a compatible refrigerant and oil to the original application.

[Inquisitor]

Are you referring to a swamp cooler? Will not work in high humidity.. only in VERY dry locations (ie: not on the water ..).
Will also make it very humid in the boat.

Agree. I was referring to locations such as Inquisitor referenced with 15% RH ambient. Even using the swamp cooler he said his house interior only got up to 30-40%... I'd still be reaching for the chap stick.

Oh yeah! Between the altitude and dryness... we had some serious adjustment phase.

[Jimmyt]

In 3a and 3b is where I had the question about low versus high pressure. This is the reading on the low pressure side. He's talking about refrigeration at this point and I'm trying to rationalize how to adjust this reasoning for the pressure needed for an A/C unit. I'm trying to extrapolate this 3rd video information for an A/C unit. As the evaporator isn't in an ice box, it probably won't frost in an A/C unit. So how do you know you're filled properly when charging up the A/C system? Will it be a higher or lower pressure than 11.7 psi and what do you do as a "you're done" gauge?

Well, shame on me for not just answering your specific question (which required watching enough of his video so I could get your context).

The lower pressure is called evaporator or suction pressure. The higher pressure is condenser or head pressure.

He's using 134a refrigerant which is user friendly from a pressure standpoint. So, if you were to use 134a, I can give you approximate numbers to get you in the ballpark of a reasonable charge.

When you have airflow over the evaporator and water or airflow over the condenser, you won't have to wait so long for the gauge to settle out as he has to on his freezer. But, you need to follow his procedure of bleeding some refrigerant in, closing the valve and getting the gauge reading, then repeat until you reach the target pressure.

Expansion device is critical: expansion valve or cap tube has to be appropriate for the pressures/temperatures you're shooting for.

His target pressure of 11.6 psi corresponds to a suction temp of 9 deg F. Using a 10 degree approach, that should give him a freezer temp around 19-20 deg F.

That wouldn't do for air conditioning, as you surmise, so a target suction pressure of 40 psi would correspond to an evaporator refrigerant temp of 45 deg F and gives you a 10 degree approach to your 55 degree target air temp. Similarly, if you're going water-cooled on the condenser, and your water temp is around 83 deg, use a 10 degree approach and shoot for a condenser refrigerant temp around 93 deg. So, when charging, the head pressure will be around 110 psi. If you're running air-cooled condenser, the approach may be 20-30 degrees, so the head pressure could go to 170 psi on a 90 deg day.

I use the suction pressure as the charge guide, while watching the head pressure to look for trouble.

The right way is to evacuate the system and weigh in the appropriate charge. However, when you backyard engineer the system, there is no documentation of what the charge should be. It can be calculated, but the juice probably isn't worth the squeeze. Charging with the gauges should give you a fair result.

Sorry I didn't answer you correctly the first time.

Also note that this answer is only true for refrigerant 134a.

[Inquisitor]

In 3a and 3b is where I had the question about low versus high pressure. This is the reading on the low pressure side. He's talking about refrigeration at this point and I'm trying to rationalize how to adjust this reasoning for the pressure needed for an A/C unit. I'm trying to extrapolate this 3rd video information for an A/C unit. As the evaporator isn't in an ice box, it probably won't frost in an A/C unit. So how do you know you're filled properly when charging up the A/C system? Will it be a higher or lower pressure than 11.7 psi and what do you do as a "you're done" gauge?

Well, shame on me for not just answering your specific question (which required watching enough of his video so I could get your context).

First off... you're a complete scholar and a gentleman! But... I don't "expect" you to dig into my cesspool, to understand where I'm coming from. But... in some way, I knew you're kind of like me and dares and challenges tend to bring out your best. I do appreciate that you were willing to study his video to understand that I was asking a more directed question than just high/low pressure in a Carnot cycle. Not many people could/would have made that leap.

I should add here... I'm far more willing to spend $200-300 on an experiment that may, or may not work... and learn something than I'm willing to spend $1000 to $2000 on a product that will work. I think I was just screwed together different than most people.

The lower pressure is called evaporator or suction pressure. The higher pressure is condenser or head pressure.

He's using 134a refrigerant which is user friendly from a pressure standpoint. So, if you were to use 134a, I can give you approximate numbers to get you in the ballpark of a reasonable charge.

I'm way out of my league... I know decades ago, we all started switching from some bad fluro-sh~t-carbon to some not-so-fluro-sh~t-carbon. I'm not sure what it is I can just go to the local Pep Boys and get... or whether it's compatible with these Chinese refrigerator parts.

When you have airflow over the evaporator and water or airflow over the condenser, you won't have to wait so long for the gauge to settle out as he has to on his freezer. But, you need to follow his procedure of bleeding some refrigerant in, closing the valve and getting the gauge reading, then repeat until you reach the target pressure.

Expansion device is critical: expansion valve or cap tube has to be appropriate for the pressures/temperatures you're shooting for.

Here you've introduced a totally new concept... Internet didn't mention this. He doesn't mention this. In fact he seems to be cobbling together (as you so aptly noted) common, automobile evaporator parts.

His target pressure of 11.6 psi corresponds to a suction temp of 9 deg F. Using a 10 degree approach, that should give him a freezer temp around 19-20 deg F.

That wouldn't do for air conditioning, as you surmise, so a target suction pressure of 40 psi would correspond to an evaporator refrigerant temp of 45 deg F and gives you a 10 degree approach to your 55 degree target air temp. Similarly, if you're going water-cooled on the condenser, and your water temp is around 83 deg, use a 10 degree approach and shoot for a condenser refrigerant temp around 93 deg. So, when charging, the head pressure will be around 110 psi. If you're running air-cooled condenser, the approach may be 20-30 degrees, so the head pressure could go to 170 psi on a 90 deg day.

I use the suction pressure as the charge guide, while watching the head pressure to look for trouble.

Brain Surgery. Entropy and Thermo-dam-namics was my brick wall.

The right way is to evacuate the system and weigh in the appropriate charge. However, when you backyard engineer the system, there is no documentation of what the charge should be. It can be calculated, but the juice probably isn't worth the squeeze. Charging with the gauges should give you a fair result.

"The juice probably isn't worth the squeeze." Priceless!

Tomorrow... I'll re-study your last two paragraphs and start web surfing.

[Massey]

I have put some thought into an AC system for my boat. Where I live and plan to sail the water temps don’t get over 50 degrees even in the summer. There are pockets yes that may get over 50 but all and all 50ish all year long. So if I were to pump seawater up from outside, into an automotive heater core and have a fan blow air through that heater core, I would get cooling. Humidity control isn’t that big of a deal where I live, we don’t really have a humidity issue here. The downfall to my idea is the salt water would clog up the core after a while, and the possibility of sucking up seaweed and other flotsam would cause a nightmare of maintenance. Some of this could be remedied with a holding/filter tank, but that adds weight and complexity not to mention more space that I don’t really have. But it would work…

[Jimmyt]

I have put some thought into an AC system for my boat. Where I live and plan to sail the water temps don’t get over 50 degrees even in the summer. There are pockets yes that may get over 50 but all and all 50ish all year long. So if I were to pump seawater up from outside, into an automotive heater core and have a fan blow air through that heater core, I would get cooling. Humidity control isn’t that big of a deal where I live, we don’t really have a humidity issue here. The downfall to my idea is the salt water would clog up the core after a while, and the possibility of sucking up seaweed and other flotsam would cause a nightmare of maintenance. Some of this could be remedied with a holding/filter tank, but that adds weight and complexity not to mention more space that I don’t really have. But it would work…

50 degree water is excellent. We condition commercial buildings routinely with 45 degree water, including controlling humidity. Coil selection will be the key to making it work, but 50 deg water will get it done if you get the rest of it right. You will have a very efficient setup.

[Jimmyt]

I know decades ago, we all started switching from some bad fluro-sh~t-carbon to some not-so-fluro-sh~t-carbon. I'm not sure what it is I can just go to the local Pep Boys and get... or whether it's compatible with these Chinese refrigerator parts.

134a is used in automotive and you can buy it at your auto parts store. It's a good choice from that standpoint, in addition to having friendly pressures. Should be plenty of parts to choose from.

It ain't complicated. It's just different.

See if this clears things up. https://www.forane.com/export/shared/.c ... -chart.pdf

For simplicity, just think saturated state for now. We'll leave superheat and subcooling for beer and pizza night.

[Jimmyt]

Here you've introduced a totally new concept... Internet didn't mention this. He doesn't mention this. In fact he seems to be cobbling together (as you so aptly noted) common, automobile evaporator parts.

I checked around a bit and you can get an adjustable 134a thermostatic expansion valve in the size you're contemplating. So, you can dial in the condition you want.

[Inquisitor]

I have put some thought into an AC system for my boat. Where I live and plan to sail the water temps don’t get over 50 degrees even in the summer. There are pockets yes that may get over 50 but all and all 50ish all year long. So if I were to pump seawater up from outside, into an automotive heater core and have a fan blow air through that heater core, I would get cooling. Humidity control isn’t that big of a deal where I live, we don’t really have a humidity issue here. The downfall to my idea is the salt water would clog up the core after a while, and the possibility of sucking up seaweed and other flotsam would cause a nightmare of maintenance. Some of this could be remedied with a holding/filter tank, but that adds weight and complexity not to mention more space that I don’t really have. But it would work…

I'm considering this concept to add some free-lunch cooling for our house. We have a small mountain creek running next to it. I can draw high enough above to not even need a pump. The only electricity is to run a fan over a car radiator. The major problem (like yours) will be to filter and deposits. In my case silt, leaves, twigs and hard water versus flotsam and salt deposits.

[Inquisitor]

I checked around a bit and you can get an adjustable 134a thermostatic expansion valve in the size you're contemplating. So, you can dial in the condition you want.

So far I haven't found how this is used. There was no Wikipedia type explanation or flow chart. As the name expansion suggests, it goes just before the evaporator??? Clark just talks about a spray can nozzle. I did see in one datasheet https://sporlanonline.com/literature/10 ... Series.pdf the phrase superheat that you previously mentioned. So far, I'm not fathoming what the thermostatic variability provides. I don't recall seeing one of these in the last student fridge I dissected to be used as a vacuum pump. Maybe it was there... but I just needed the compressor.

134a is used in automotive and you can buy it at your auto parts store. It's a good choice from that standpoint, in addition to having friendly pressures. Should be plenty of parts to choose from.

It ain't complicated. It's just different.

See if this clears things up. https://www.forane.com/export/shared/.c ... -chart.pdf

For simplicity, just think saturated state for now. We'll leave superheat and subcooling for beer and pizza night.

Hmmm... considering "See if this clears things up" you seem to have faith I can decipher this chart to some constructive end without brain surgery techniques. Remember... in Thermodynamics class I was probably day dreaming about a girl.

Purely by seat of the pants... Paraphrasing what I've learned so far from you above, a good goal is 55F air from the return and that likely takes 45F fluid/coils/plates to achieve. Using the chart this correlates to 40.1 psig for the low side pressure. So this is the pressure I need to achieve when filling the 134a while at a steady state of usage. How would one determine where to pick a steady state? When starting to cool down the 90F room or once it reaches the desired temperature/humidity??? I'm guessing (again) that which ever is picked, it'll be the most efficient in that condition.

From here, I'm purely guessing now, but I would think I would need the pressure differential a compressor can provide. 40.1 + X = Y. That would be high pressure value and in the chart would correlate the temperature of the fluid coming into the condenser that needs to get rejected to sea water. Unfortunately, the compressors don't seem to show such information. https://www.danfoss.com/en/products/dcs ... b-overview And I'm betting the smaller Chinese compressors in small fridges are going to be even harder to find such data.

[Inquisitor]

JimmyT,

Maybe you're leading this horse to water and let me discover how this free lunch doesn't exist. The 40.1 psig being almost 4x the pressure of Clark's fridge example and fridge compressors design parameters means it will require that much stronger a compressor (can't use a fridge compressor in a A/C unit) and/or use 4x more energy just to achieve the same pressure/temperature differential???

[Jimmyt]

You got the suction (evaporator) pressure logic correct. The head pressure, similarly, comes from the temperature of the medium into which you are rejecting the heat. If you are rejecting it into water that's typically 83 deg, add ten degrees to get a 93 deg condenser refrigerant temp. Go to the chart and get the pressure that corresponds to 93 deg and get a pressure around 110 psi.

The expansion device creates the pressure differential. The compressor has to create adequate flow to move the required amount of heat around and keep up with the flow demand of the expansion device.

And, if I understand you, yes. Generally speaking, the more flow and pressure you create, the more energy you use. But, you have it sideways. Compressor cooling capacity is a tricky question. When you are looking at a rating at 9 deg in the evaporator, and thinking about what a rating might be at 45 deg; and you are rejecting heat at the same condenser temp in both cases; the system will have more capacity at the higher evaporator temperature. The hill is smaller. 110psi - 11 psi = 99psi vs 110psi - 40psi=70psi. The compressor isn't working as hard at the higher evaporator temp - if the flow is about the same.

[Be Free]

I'm considering this concept to add some free-lunch cooling for our house. We have a small mountain creek running next to it. I can draw high enough above to not even need a pump. The only electricity is to run a fan over a car radiator. The major problem (like yours) will be to filter and deposits. In my case silt, leaves, twigs and hard water versus flotsam and salt deposits.

Don't forget the crawdads and salamanders

[Be Free]

More on-topic...
Jimmy,

I generally lump all of the "free lunch" type cooling videos into two categories: works, but not where I live (swamp cooler and water/air) and "does not really understand refrigeration". Is that reasonable?

I'm like Inquisitor in that I really like to know how things work and am totally OK with building something to test a theory, but I've not found anything going down any of the refrigeration/AC rabbit holes that does not lead to one or the other of those two categories. You really appear to understand the intricacies of refrigeration so "Help me Obi Wan, you are (my) only hope".

[Jimmyt]

More on-topic...
Jimmy,

I generally lump all of the "free lunch" type cooling videos into two categories: works, but not where I live (swamp cooler and water/air) and "does not really understand refrigeration". Is that reasonable?

I'm like Inquisitor in that I really like to know how things work and am totally OK with building something to test a theory, but I've not found anything going down any of the refrigeration/AC rabbit holes that does not lead to one or the other of those two categories. You really appear to understand the intricacies of refrigeration so "Help me Obi Wan, you are (my) only hope".

Yes Bill. Where you and I live, there is no free lunch cooling unless it's December through February- and sometimes not even then.

I have an AC project on my future to-do list, but I'll probably take RIS' approach and just use it when I have shore power (or possibly gen set). My current concept is to cannibalize a small window unit so that most of it is matched up already - and the starting point is relatively cheap. The down side is, window units run 410 refrigerant which has higher operating pressures than 134a. So, you have to be more careful with putting it together and charging it. Not terribly difficult to fool with (virtually all residential systems run 410), but a casual DIY needs to know that you're dealing with higher pressures. For example, at 93 deg condenser refrigerant temperature, you're looking at 287psi. At 120 deg F, for air-cooled, you could be looking at 418 psi (probably a worst case).

I like Inquisitor's approach of using solar and his mega-battery to do an off grid AC. Plus, rolling your own, using 134a lets you use lower pressure assembly techniques. All I really need is a cool dry place to sleep - to keep the Admiral from making me walk the plank. So, his idea of using a unit that has about 1/4 to 1/3 of the capacity required for the whole boat (just cooling the vberth) should be a workable concept. Matching up the components is going to be the tricky part. However, as the Emily and Clarke video indicates, you can put together a system that functions to some degree without really having a full understanding of the process minutiae. The concepts have been around a long time and were working pretty good long before computer simulations, material optimizations, energy codes, and EPA restrictions turned it into what it is today.

I'm no Obi Wan, but I do have a fair understanding, and have been trained as a refrigerant tech. As a design engineer, I had a desire to know how to repair/verify the systems I was designing.