Refrigeration - Evaporator vs. Holding Plate

[newhaul]

Originally Posted by Richard Kollmann

RamondR The reasons eutectic plates are not stable in maintaining box temperatures are most all of them use Glycol and water and not true eutectic solution.

.

Eutectics are alloys of inorganics (mostly hydrated salts) and/or organics. They have a single melting temperature, which is usually lower than that of any of the constitutive compounds. Eutectics form one single common crystal when crystallized (Hasnain, 1998). One of the most important characteristics of eutectics is their capability to melt/freeze congruently without phase segregation

So by that definition
Polypropylene glycol and water are a proper true eutectic solution.

[kenbo]

Originally Posted by OzeLouie

Hi Kenbo, the test referred to was a fair comparison as both systems were using capillary throttling and identical condensing units. Check the report here: https://www.ozefridge.com/trial-data (I know 'cos I was there, observing with great interest!! )

Also find it odd that you experience issues with cabinet temperature stability using eutectic. A correctly functioning eutectic system will provide a far more constant cabinet temperature and faster pull down of added warm product, but in reality very few so called 'eutectic systems are operating correctly. A eutectic system must be controlled so as to cause the refrigeration unit to run until the eutectic mass is frozen solid then remain off absorbing heat until the mass has thawed. It's simple if done right! Unless it functions this way it is NOT a eutectic system and none of the eutectic benefits, especially power efficiency, can be enjoyed.

OzeLouie,

Thanks for responding. But my question about comparing the efficiency of thin plate evaporators vs. eutectic really centered around the use of a TXV, not a critical orifice.

Secondly, I believe that you may have made my point about the swings in box temperature with eutectic systems. Assuming a steady-state heat load, the period at the bottom of the curve where the eutectic becomes thawed overwhelms the plate's ability to remove the heat for a time on either side of that thaw point. The length of that period of time will vary of course but it is in that period where the temperature of the box and its contents will rise. In my experience and only my experience, a thin plate evaporator with a variable speed compressor will hold tighter temperature control.

[OzeLouie]

Originally Posted by kenbo

OzeLouie,

Thanks for responding. But my question about comparing the efficiency of thin plate evaporators vs. eutectic really centered around the use of a TXV, not a critical orifice.

Secondly, I believe that you may have made my point about the swings in box temperature with eutectic systems. Assuming a steady-state heat load, the period at the bottom of the curve where the eutectic becomes thawed overwhelms the plate's ability to remove the heat for a time on either side of that thaw point. The length of that period of time will vary of course but it is in that period where the temperature of the box and its contents will rise. In my experience and only my experience, a thin plate evaporator with a variable speed compressor will hold tighter temperature control.

No problem Kenbo, it's a pleasure to respond to posts with positive content.
New to this, I had asked my dad how to respond and he said: “just like you’re talking to someone at the bar, be nice or get ready to duck”

We like 99% of the rest of the worlds small system manufacturers use capillary as a throttling device for simplicity, efficiency and reliability. TEV systems have their place especially with a eutectic system where the return gas velocity is higher (working harder) and there is a need to address variations in environment etc.

Rich’s products (Technautic) very successfully employ a TEV. to address those issues. Our approach is to thermostatically control condensing temperature with twin, two speed condenser fans and water cool assist if required. Either will provide similar performance outcomes.

Using a TEV on these small aluminium cyclic plates has no merit whatsoever and that is why they virtually don’t exit, as some who tried that don’t either!
Regards cabinet temperature maintenance, there is little difference provided the systems are correctly engineered so as to be ‘fit for purpose’ and are controlled correctly.

The eutectic system must be set to cut off the unit once the eutectic mass is frozen and on again as soon as the mass thaws. In reality the restart should occur just before thawing is complete. (Still some ice slurry evident) The systems control probe must read the eutectic mass as in what we call ‘Sure Thaw’.

Below is a test that indicates the similarity of cabinet temp maintenance of each system type… As can be seen the empty cabinet temperatures are similar. However adding warm / hot product would see the eutectic lower product temperature much faster.
Regards OzeLouie

Regards the idea of running the cyclic system constantly at reduced speed, I can’t comment but it ‘goes against the grain’ for me and there appears to be no trial data or evidence that supports the claimed efficiency of the concept. Perhaps someone has some data to report, would be great to see.

[RaymondR]

What is illustrated is that sufficient eutectic capacity must be installed to carry the heat absorption period through the period when solar power is not available (We should remain mindful that the primary aim is the reduction in battery cycling)

However since the period on batteries is relatively short, in the eutectic tank instance in comparison to the constant cycling evaporator plate system, the pull down of the batteries is far less harmful to battery life than a full overnight pull down. The benefit of the eutectic system is still realized but just not as extensively.

From a viewpoint of tight temperature control the situation would appear to be as follows.

if an electronic thermostat which allows tight control of the switching hysteresis is employed to control compressor start up, the compressor will start when the eutectic fluid temperature has risen slightly. If sufficient refrigeration capacity is available to exceed the heat bleed in to the box the system will then draw the eutectic fluid down to its freeze temperature and the eutectic will then freeze at a constant temperature. The eutectic fluid is just absorbing excess refrigeration effect over that required to maintain the box temperature constant. The thermal inertia effect of the presence of the heat absorbing mass of the eutectic fluid has kicked in.

Let me illustrate this with a little narrative.

Decades ago the CSIRO research establishment built a machine to scribe difraction gratings. The results were below expectations. The problem was found to be poor temperature control in the work space was causing thermal expansion and contraction of the machine. The machine was then moved down into an underground bank vault where the large masses of surrounding stone provided a high degree of thermal inertia to hold the work space temperature very constant.

The tank full of eutectic fluid in the refrigerator box tends to do the same thing.

[Richard Kollmann]

The game playing on this thread needs to end. RaymondR I am sorry you were unable to get the facts you were looking for in the 398 posts covering TXV versus Capillary tubes and the comparison of different types of evaporator performance. I will put the facts together and PM them to you and Kenbo next week you are the only ones that seam to be looking for answers.

[kenbo]

Thanks Richard, appreciate your time and knowledge.

[wsmurdoch]

Originally Posted by RaymondR

If sufficient refrigeration capacity is available to exceed the heat bleed in to the box the system will then draw the eutectic fluid down to its freeze temperature and the eutectic will then freeze at a constant temperature.

The usual liquids (ethylene glycol or propylene glycol in water) in a holding plate do not freeze at a constant temperature at the compositions and temperatures normally used. As the temperature of the mixture is lowered, a point (the freezing point) is reached and the first bit of water ice freezes out of the solution. Now missing some water, the remaining liquid is richer in glycol and has a lower freezing point. When that lower freezing point temperature is reached, more water freezes out yielding a liquid still more rich in glycol... and on and on.

In the usual case the glycol/water mixture in a holding plate freezes a little at a time as the temperature is lowered. It begins with a few crystals of water in a lot of liquid, grows to a slush, and finally becomes a near solid something like a Popsicle or Ice Lolly.

The same thing happens in reverse when the frozen mixture thaws.

There is a perhaps better explanation here. Windborne in Puget Sound: Why does a holding plate work?

[RaymondR]

Originally Posted by Richard Kollmann

The game playing on this thread needs to end. RaymondR I am sorry you were unable to get the facts you were looking for in the 398 posts covering TXV versus Capillary tubes and the comparison of different types of evaporator performance. I will put the facts together and PM them to you and Kenbo next week you are the only ones that seam to be looking for answers.

Hi Richard,

With your valuable assistance I was able to come to a very thorough understanding of the operation of a TXV. It has two functions: the first is to restrict the flow of refrigerant from the condenser so that it has time to waste the heat from the compressed refrigeration vapor, and the second to optimize the amount of fluid in the evaporator so that it can absorb the maximum amount of heat.

The first of these functions may be readily served by a capillary tube however it requires a delicate balance in the heat being wasted in the condenser. If excessive condensing occurs then excessive liquid refrigerant will be retained in the condenser thereby starving the evaporator of liquid refrigerant.

Ozyfridge appears to have cleverly avoided this problem by varying the amount of cooling air passing through the condenser. How they measure and control the amount of liquid refrigerant in the condenser I don't know. Perhaps differential temperatures.

There is no optimization of the amount of heat absorbing liquid refrigerant in the evaporator in a capillary tube regulated system other than having the correct amount in the system as unlike the TXV controlled system there is no high side accumulator to retain refrigerant in the event it is not vaporized in the evaporator. It's a Goldilocks situation: if you don't have enough the evaporator is not utilized to it's full potential and if you have too much you risk liquid refrigerant returning to the compressor. To paraphrase Goldilocks, "Not to little, not to much but just right." is required.

I decided some time ago that the TXV controlled system would provide better performance and allow extra refrigerant to be contained in the system which would help a lot if there was a leak however the problem I discovered was getting ones hands on a TXV small enough for the limited capacity DC compressors we use in these systems.

After going through the extended familiarization required to spec a valve and finding that Parker now owns Sporlan and placing an order and waiting a couple of months I discovered that the valve required was now an OEM only item and everyone in the organization down to the coffee lady was pissed off at me for my temerity in attempting to source the valve. An explanation that I was an OEM with an intended production run of only one system didn't budge them.

I know there have been hundreds of posts on the subject of evaporator plates versus eutectic tanks but it appears to be a subject that many of us are deeply interested in and I am sure that many of the participants are appreciative of the opportunities presented to learn more about marine refrigeration. I know I certainly am.

We are not the AGW/CC crowd, being primarily technologists we are far more capable of critical and objective thinking we just need to persevere and stay cool.

[RaymondR]

Originally Posted by wsmurdoch

The usual liquids (ethylene glycol or propylene glycol in water) in a holding plate do not freeze at a constant temperature at the compositions and temperatures normally used. As the temperature of the mixture is lowered, a point (the freezing point) is reached and the first bit of water ice freezes out of the solution. Now missing some water, the remaining liquid is richer in glycol and has a lower freezing point. When that lower freezing point temperature is reached, more water freezes out yielding a liquid still more rich in glycol... and on and on.

In the usual case the glycol/water mixture in a holding plate freezes a little at a time as the temperature is lowered. It begins with a few crystals of water in a lot of liquid, grows to a slush, and finally becomes a near solid something like a Popsicle or Ice Lolly.

The same thing happens in reverse when the frozen mixture thaws.

There is a perhaps better explanation here. Windborne in Puget Sound: Why does a holding plate work?

Nigel Calder appears to put the temperature point at which the unfrozen slush phenomena dominates in at a fairly low temperature point but does not really address low concentrations of glycol.

[Richard Kollmann]

Originally Posted by RaymondR

Nigel Calder appears to put the temperature point at which the unfrozen slush phenomena dominates in at a fairly low temperature point but does not really address low concentrations of glycol.

What happens in a eutectic plate when warm is it only takes one Btu per pond to lower the liquid solution one degree in temperature. Once solution reaches its adjusted freeze point Popsicle texture ice begins forming on the evaporator coil inside plate. This coil frost continues to expand and the thicker it becomes it acts as an insulator of a heat, unless coming in contact without better heat conductor like plates surface metal or metal frame or cooling fins. The slush first begins at the bottom of plate although when liquid solution in plate is below the freezing point of water plate's exterior surface will develop frost nearest to refrigerant evaporator coil. True eutectic plates require structurally sound plates and with a small energy sours compressor the energy transference inside plate requires a special design application. Of the large number of plates I manufactured only a few were efficient enough for 12 volt compressors and those that were had evaporator coils spot welded to exterior plate's surface.

[Richard Kollmann]

RaymonR, The selection of a TXV for your own designed refrigeration is smart for several reasons;your system fits the mobile category, with a receiver and sight glass, correct refrigerant charge is no problem, cap tube on eutectic plates is too restrictive in conventional mobile use.

I am not sure why you are having a problem getting a TXV valve with a wide temperature range above and below zero and an orifice size when warm 30% larger Btu than compressor manufacturer's normal Btu operating range.

[Exile]

Originally Posted by Richard Kollmann

RaymonR, The selection of a TXV for your own designed refrigeration is smart for several reasons;your system fits the mobile category, with a receiver and sight glass, correct refrigerant charge is no problem, cap tube on eutectic plates is too restrictive in conventional mobile use.

I am not sure why you are having a problem getting a TXV valve with a wide temperature range above and below zero and an orifice size when warm 30% larger Btu than compressor manufacturer's normal Btu operating range.

Just a layman here interested in hard ice cream and cold beer, and admittedly often cannot follow the more technical posts over the engineering intricacies of modern marine refrigeration. But isn't what you're describing above a product which has long been offered by Technautics/CoolBlue? Or are your comments referencing a system with evaporative plates? If Ray's interest is instead a eutectic/holdover system, then why reinvent the wheel?

Apologies if I lost the flow of the discussion here.

[Richard Kollmann]

I do not know what RaymondR motivation is in building his own system. Thehnautics Cold Blue system is a conventional designed with excellent air cooled performance and eutectic true energy plates, Yes they could build a plate to fit Ray's box but not for the price he will spend on building his own. Many boats require custom built refrigeration.

Conventional refrigeration is what you want unless boat is located close to its manufacturer. Refrigeration techs are not inclined to solve problems on unconventional refrigeration.

[newhaul]

Originally Posted by Richard Kollmann

I do not know what RaymondR motivation is in building his own system. Thehnautics Cold Blue system is a conventional designed with excellent air cooled performance and eutectic true energy plates, Yes they could build a plate to fit Ray's box but not for the price he will spend on building his own. Many boats require custom built refrigeration.

Conventional refrigeration is what you want unless boat is located close to its manufacturer. Refrigeration techs are not inclined to solve problems on unconventional refrigeration.

Just a note here agree with having common refrigeration systems for ease of finding a tech if ever needed . Remember that anywhere there are larger commercial fishing vessels
There are technicians that are well versed in working on eutectic holding plate systems .
They use it because of the fairly constant temperature control when heat loading product.

[Richard Kollmann]

Originally Posted by newhaul

Just a note here agree with having common refrigeration systems for ease of finding a tech if ever needed . Remember that anywhere there are larger commercial fishing vessels
There are technicians that are well versed in working on eutectic holding plate systems .
They use it because of the fairly constant temperature control when heat loading product.

The difference with qualified fishing boat eutectic refrigeration is their compressors are either engine driven or AC electrical greater than 1/2 HP.
Pleasure boat refrigeration compressors are generally smaller the 1/3 HP and the most common Danfoss BD compressors are less than 1/6 HP. Twelve volt refrigeration condensing units rarely need a refer tech to repair them as problems are most likely electrical. A good eutectic plate requires no maintenance.