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Old 31-10-2015, 14:28   #1
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TXV Refrigeration 101: What is wrong with this photo?

It's time for a little holding plate refrigeration 101.
This photo was sent to me by a client with dual holding plates in his freezer box as part of a little routine Saturday trouble shooting. As soon as I opened the photo, I saw the problem....do you?



What you are looking at is the Thermally Adjusting Expansion valve (TXV) attached to the inlet of holding plate No 1. What's wrong is that the TXV sensing bulb is just floating in air and not firmly strapped to the copper tube outlet line of holding plate No 2. The sensing bulb is filled with R134a gas and transmits it "reading" back to the TXV via the capillary tube. The TXV attempts to maintain a 10-deg temperature differential between the TXV inlet and holding plate outlet, so with the sensing bulb floating in air, the TXV is just wide open trying to get down to the 10-deg differential. (screaming like Scotty...I gotta have more Cooling Captain...I can't hold her together much longer) This causes the low pressure side to basically flood with refrigerant all the way back to the compressor inlet. No low pressure on low pressure side and no cooling. In extreme instances, the compressor body will even feel cold as the flooded refrigerant gas is expanding and removing heat in the compressor....not good.

Critical orifice systems like the standard white thin rolled aluminum evaporators, don't use a TXV, but just a fixed orifice, as the name "critical orifice"system implies. In those systems the rate of refrigerant injection into the evaporator is constant, or varies by the variable speed of the compressor.

Any questions....I'm happy to answer.

Well back to finishing up the last of my 21 through hull fittings here in the boat yard with high hopes of being able to splash this floating condo back in the water soon! My wife is already joking that after living on land in the boat yard now for over 3 months, she could be happy staying on land....aHHHHHHHH.
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Old 31-10-2015, 15:29   #2
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Re: TXV Refrigeration 101: What is wrong with this photo?

Here is the Fix.....
The sensing bulb attached to the outlet line of holding plate No2.

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Old 31-10-2015, 15:50   #3
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Re: TXV Refrigeration 101: What is wrong with this photo?

Quote:
Originally Posted by SV THIRD DAY View Post
It's time for a little holding plate refrigeration 101.
This photo was sent to me by a client with dual holding plates in his freezer box as part of a little routine Saturday trouble shooting. As soon as I opened the photo, I saw the problem....do you?



What you are looking at is the Thermally Adjusting Expansion valve (TXV) attached to the inlet of holding plate No 1. What's wrong is that the TXV sensing bulb is just floating in air and not firmly strapped to the copper tube outlet line of holding plate No 2. The sensing bulb is filled with R134a gas and transmits it "reading" back to the TXV via the capillary tube. The TXV attempts to maintain a 10-deg temperature differential between the TXV inlet and holding plate outlet, so with the sensing bulb floating in air, the TXV is just wide open trying to get down to the 10-deg differential. (screaming like Scotty...I gotta have more Cooling Captain...I can't hold her together much longer) This causes the low pressure side to basically flood with refrigerant all the way back to the compressor inlet. No low pressure on low pressure side and no cooling. In extreme instances, the compressor body will even feel cold as the flooded refrigerant gas is expanding and removing heat in the compressor....not good.

Critical orifice systems like the standard white thin rolled aluminum evaporators, don't use a TXV, but just a fixed orifice, as the name "critical orifice"system implies. In those systems the rate of refrigerant injection into the evaporator is constant, or varies by the variable speed of the compressor.

Any questions....I'm happy to answer.

Well back to finishing up the last of my 21 through hull fittings here in the boat yard with high hopes of being able to splash this floating condo back in the water soon! My wife is already joking that after living on land in the boat yard now for over 3 months, she could be happy staying on land....aHHHHHHHH.
Hurry up and relaunch she's starting to grow land roots and soon you woun t be able to replant back on the boat
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Old 31-10-2015, 18:08   #4
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Re: TXV Refrigeration 101: What is wrong with this photo?

Doesn't look attached to me, looks like it's just sitting there. Where is the clamp?

How common are dual plate systems?


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Old 01-11-2015, 05:25   #5
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Re: TXV Refrigeration 101: What is wrong with this photo?

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Originally Posted by a64pilot View Post
Doesn't look attached to me, looks like it's just sitting there. Where is the clamp?

How common are dual plate systems?


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From memory the tx valve bulb should be installed at the 4 or 8 o'clock vertical position unless the manufacturer specifies differently.

Also, it should be clamped with a copper clamp with brass nuts/bolts so no dissimilar metals.

Geeze it's been a lot of years since i worked on tx valves...When i get my boat I might have to dust off my old tools...
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Old 01-11-2015, 08:13   #6
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Re: TXV Refrigeration 101: What is wrong with this photo?

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Originally Posted by a64pilot View Post
Doesn't look attached to me, looks like it's just sitting there. Where is the clamp?

How common are dual plate systems?


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Yep...it is just sitting in place now waiting for the copper strap to be installed.

Technically you want to make sure you measure the gas temp in the tube without being influenced by the compressor oil that flows in the bottom of the tube. So as long as the sensing bulb is installed in the upper half of the tube, you are good.

It is not the norm to have a dual plate freezer system (cost being a driver here) but on larger boxes or on boxes with too small of side walls for a reasonable size plate to fit, then you go with two. The advantage is an increased plate surface area allowing for faster and more heat removal from the box and then you have a larger volume of eutectic solution inside the box allowing for longer compressor off cycles.

Here is a photo of a dual freezer holding plate system on a Beneteau 423 we did recently.
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Old 01-11-2015, 12:22   #7
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Re: TXV Refrigeration 101: What is wrong with this photo?

Tread very carefully, Third Day. She may not be joking! ��
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Old 01-11-2015, 12:55   #8
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Re: TXV Refrigeration 101: What is wrong with this photo?

Hey Rich,

Have a question. Have a Seafrost holding plate freezer and the temp sensor is inside the box - coil attached near the top of the holding plate. No access to the plumbing as it is all below the freezer or punching directly to the holding plate from the adjoining compartment (with the compressor). Sound like a problem not being attached to the line, or is this probably a non-TXV system?

Bill
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Old 01-11-2015, 13:18   #9
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Re: TXV Refrigeration 101: What is wrong with this photo?

Hey good question Bill.

There are two types of Expansion Valves:
1) Thermally adjusting (like the ones we use), meaning they adjust the amount of refrigerant being injected into the holding plate to maintain temperature differential, so they need the sensing bulb.
OR
2) Fixed or Static (like the ones used by SeaFrost), meaning it is manually adjustable to match the holding plate at the factory, but does not adjust the refrigerant based on the heat load of the plate/box.

Now don't confuse the expansion valve with the Thermostat capillary. One controls the refrigerant flow on a thermally adjusting expansion valve, while the other is turning the compressor on/off based on the temp set points.

I hate to speak for Seafrost or for systems I haven't seen because sometimes things get tweaked far from "Normal" , but generally for holding plate systems, you want the thermostat to be measuring the holding plate temp and not the Box air temp. So you want the thermostat to be in contact with the holding plate. Now if you have a programmable electronic thermostat then perhaps you have reset the cut on/off temps to be the actual Box temps and not the holding plate temps. But for freezer plates the cut on/off temps are targeting the holding plate temps that are (again generally based on the box size and insulation value) about 10-15-degs F colder than the box temp. So if you use a mechanical thermostat set up for a freezer and let the sensing tip of the thermostat flop around in the air, then it will read a much warmer temperature than the plate and tell your compressor "Keep running...I ain't cold enough yet". This is a common cause for the symptom when people call with the "my unit runs forever, won't turn off, but the box is super cold" trouble shooting question. The thermostat can get bumped or pulled out of the holding plate mounting tube.

But back to the programmable thermostat.
If the cut on/off temps have been changed to Box temps and not plate temps, then it could be totally fine for the thermostat to be flopping around in the air of the box, preferable 1/2 way up in the box. I don't personally like this approach because the temperature of the holding plate is much more stable than the air temp so getting your temperature from the plate and not the air in the box can help cut down on excessive cycling when you open the box. One more point, a eutectic holding plate system operates under the theory that the eutectic solution is not allowed to defrost and phase change form solid to liquid. So I like to have the temp control right on the holding plate to ensure the most optimal running of the compressor. I know people like to see the real box temp rather than the colder plate temp, so for them I recommend just adding a second thermometer even with a digital display for the box temp, and just let the thermostat control off of the plate temp.

Ok...way to much info I'm sure, but I can't help it because the alternative is to put on the White Tyvex suit and put the last coat of bottom paint on...
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Old 01-11-2015, 13:33   #10
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Re: TXV Refrigeration 101: What is wrong with this photo?

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Originally Posted by a64pilot View Post
How common are dual plate systems?
I have dual plates in my freezer because a larger plate won't fit through the top lid opening. I have dual plates in my reefer because two smaller plates were easier to deal with.

Not sure how common it is but I have seen other boats with dual plates also.
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Old 01-11-2015, 13:59   #11
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Re: TXV Refrigeration 101: What is wrong with this photo?

Dual plate systems were also very popular in engine driven, 120v AC driven, and high amp 12v (like 40A) DC systems where the logic was you wanted to store as much "Cold BTUs" in your holding plates as possible because you only wanted to run your system at most twice per day. Of course if you don't have room or access for one large plate...two smaller ran in series is a good solution.

With the invent of the efficient 12v DC compressors, the trend has been to save cost on the dual plate set-up because now you don't need to run your big diesel or engine to pull down the plates. The 12v compressor just turns on from the thermostat when ever it needed. That lets you reasonably store less "Cold BTUs" in your plate and puts it in your batteries as electrical amp hours. Holding plate systems get a big efficiency boost when you can freeze them down (store cold) when you have excess power available that otherwise would have been dumped by your solar or alternator controller. Store energy in your battery or holding plate is how you can think about it.

A dual plate system WILL give you a colder and more even freezer temp and the common layout would be to put them on opposite walls, like the SeaFrost system advertises. We can add a second holding plate to our standard system for $705 and we are doing one for a Catalina 42 at the moment, it's just not as common I think due to budget constraints. But the good news is that the second plate can be easily added to the single plate set-up, it's just a plug and play addition for that true deep freeze and less compressor cycles. (the compressor will run for a longer time when it run, but then will stay off longer...no free lunch folks)
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Old 01-11-2015, 14:25   #12
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TXV Refrigeration 101: What is wrong with this photo?

Full disclosure, I have a dual plate system, I just didn't know how common they were.
You can get some of the "thermal flywheel' effect by filling your fridge with mass, I've found Beer to be an effectively flywheel, just tell the wife you have to have that much beer, it keeps the fridge colder


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Old 01-11-2015, 15:45   #13
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Re: TXV Refrigeration 101: What is wrong with this photo?

Might as well keep the roll going and talk about how to squeeze more efficiency out of your holding plate system, since its now too hot to get in the white Tyvex suit and paint the bottom. I'll do that this evening now.

If you can store up "Cold BTUs" in your holding plate when you have excess 12v DC power then you can use those BTUs later when your charging source go away (alternator/wind/solar). So what are some ways to do that?

1. Old Fashioned Bigger Is Better
Bone simple takes us back to the early days of holding plate refrigeration where bigger was better in terms of holding plates, but at an increased cost and space in the box. If you increase your holding plate eutectic solution volume and are able to freeze it down during the day when you have solar/wind/alternator power to spare, then you will have less compressor cycling at night. No controls...nothing really special to do once you have installed the larger or extra holding plate volume.

2. The El Cheapo
I use the phrase fondly because I fall into this category myself. I like cheap and I like easy, but this does put my memory to the test and some times it fails and I freeze my lettuce.

Here's how it works.
The thermostat affixed to the holding plate turns the compressor on/off based on the temperature set points and is just a simple switch that is either open and the compressor is off or closed and the compressor turns on. So you can install a bypass switch around the thermostat leads so that when you turn on the switch the compressor controller sees the "yes turn on signal" and starts up. So any time I am motoring or when my solar controller is floating my batteries and dumping available power I turn the switch to ON and force on my compressor. This will freeze down the plate to a lower temp than "normal" and I will be able to store more "Cold BTUs" in my holding plate. So when the sun sets or alternator turns off it will be a longer time for the 12v compressor to kick on and use battery power to pull out heat.

The negatives of this El Cheapo Approach are:
A) if you forget to turn off the switch it can run all night because the damn things so quite you will forget it is on. (ok put a timer switch in, to prevent that, but I'm too lazy for that)
B) You run the risk of freezing your refrigerator if using a spill over approach because you will see lower temps in your freezer spilling over and if your unit is just a refrigerator plate, it will get much colder than the "normal" 25-degs F


3. The Tech Nerd
Come on, admit it, you know who you are and we love you but at the same time hate you for wiring up gizmos that we can't figure out then telling us how easy it is on the chat rooms.

Here's how it works.
Take the same concept from the El Cheapo, but rather than relying on yourself to turn a simple switch on and off, set up a voltage sensing relay to a nice full battery charge voltage...say 13.9v...and then when your battery gets to that voltage from either wind/solar/alternator the relay will close a switch and boom...the unit kicks on automatically. You would have the same negative as the El Cheapo in that on long motors or too sunny of days or on shore power you unit won't turn off and you can freeze your beer. But a true tech nerd would also put a timer into that relay circuit to only close the switch for an hour at a time and then wait 30 minutes again before cycling on if the voltage is high enough. Heck get real cool and add an OFF function when it senses being on shore power or make it cut off at a low temp setpoint.

4. The Money Fixes Everything Guy
For those of us that are too lazy for an on/off switch and not electrically smart enough to play electronic engineer, there of course is the "money solves all problems" approach. SCAD makes what's called a SensiStat to optimize performance of holding plate refrigerator systems.
SensiStat | Technologies LLC
(I'm not affiliated with this in any way...but I might one day carry it as a dealer for them if I ever get around to it)

It follows the same logic as above, by turning on the unit when you have a preset and adjustable battery voltage BUT it doesn't just force the unit on and freeze your beer. It shrinks the hysteresis down to 1-deg but adds in Box and Holding plate monitoring into the equation. Plus lots of additional cool features for the money. These babies list for $295 and from the testing I've done they do work and save you some battery power, just how much can depend on your boats energy profile. Obviously if your batteries are never at some high full charge set-point then forget it...spend the $295 on insulation, better hatch seals, of the famous "Beer Flywheel Affect"...ha ha

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Old 01-11-2015, 16:04   #14
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Re: TXV Refrigeration 101: What is wrong with this photo?

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............
You can get some of the "thermal flywheel' effect by filling your fridge with mass, I've found Beer to be an effectively flywheel, just tell the wife you have to have that much beer, it keeps the fridge colder
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...spend the $295 on insulation, better hatch seals, of the famous "Beer Flywheel Affect"...ha ha
Lovely, just simply lovely.

I do this with my simple A/B non-holding plate system. My son has patiently explained to me, over-and-over again, that keeping the quantity of beer or ale sustained is as difficult as maintaining the quality of said liquid. The darned brat still hasn't worked out the kegerator. Lazy dolt!
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Old 01-11-2015, 16:59   #15
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Re: TXV Refrigeration 101: What is wrong with this photo?

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I like cheap and I like easy, but this does put my memory to the test and some times it fails and I freeze my lettuce.
Man!!

I feel for yah.....!
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