Shannon 38 - New Refrigeration

[obxmedic]

Hi All!

I am replacing the original 1980 refrigeration on my Shannon 38 cutter ketch. I plan on doing alot of long distance cruising. I have saved my pennies, and know its going to cost a bunch! I have ample AH in my batteries, So>>>>> Whats the Best refrigeration system I can put in my boat? Solar panels and wind generator will refuel the batteries. I don't mind having ice either, as I will have a watermaker. OBX MEDIC
Do the things you like! Enjoy the people you met.

[mesquaukee]

Since you will be generating your power from the wind and solar stay away from any system that has a cold holdover plate which uses a eutectic (or any liquid) solution to store cold. They are energy hogs using as much as 2 times as much as other systems

You do not want one that uses a capillary tube for metering the refrigerant as once again they are energy hogs ( use 20-50% or more ampHrs)

You want one with a TX valve (thermostatic expansion valve); they use the least energy (AmpHrs/btu and therefore the least ampHrs/day). This type of metering system "thinks".
The evaporator should if possible totally surround your frozen food compartment. You do not want it only on two sides. The frozen food on the far side will not be as cold as it should be due to the insulating effect of the food. To compensate for this you may have to reduce the temperature, this uses much more energy (10-30% or more)

There are a few do it yourself systems that roughly fit the above criteria. They all make some sort of compromise.

You may want to have a custom designed system that more suites your space requirements. They would use less energy as you can place the compressor and condenser in cooler better ventilated spaces. You can split the condensor and evaporator, it is better for cooling. If you do not want to go with a keel cooler insist on installing an oversized condenser. You want to reduce the condensing temperature (pressure) as much as possible to save energy.

[Richard Kollmann]

WOW mesquaukee, How can you possibly quantify technically these strange opinions?

Stay away from any system that has a cold holdover plate which uses a eutectic (or any liquid) solution to store cold. They are energy hogs using as much as 2 times as much as other systems

You do not want one that uses a capillary tube for metering the refrigerant as once again they are energy hogs ( use 20-50% or more ampHrs)

You want one with a TX valve (thermostatic expansion valve); they use the least energy (AmpHrs/btu and therefore the least ampHrs/day). This type of metering system "thinks"

[mesquaukee]

As a chemical engineer by training I was motivated to make a study of marine refrigeration systems for several reasons. I was dissatisfied by the performance of the 2 systems I had. In speaking with a few people about what to do I was dissatisfied by their answers. I was also annoyed by claims made by people who had no training in or understanding of the refrigeration process.

I decided to study, design, build and rebuild refrigeration systems. I had an advantage, I spent a few years when I was young working in refrigeration so I was familiar with the equipment and tools.

If you are using solar panels or a wind generator to charge your batteries it makes no sense to have a large compressor to freeze cold holdover plates.
For the same maximum freezer temperature you have to have a much lower evaporator pressure. The lower the evaporator pressure the more energy you use to move the same amount of heat. If the evaporator plate totally surrounds the food the food can be no warmer than the cut in pressure.
The problem with cold holdover plates is you need;
1.) a temperature differential between the refrigerant and the solution in the cold holdover plates
2.) a temperature differential between the food and the frozen solution in the plates
3.) there is the temperature differential through the food to the side without a cold plate.
Add that all up the evaporator pressure is quite a bit lower then is nescessary as outlined in the table below. I am using a temperature of 14F which will keep frozen food for up 2 months. A lower temperature is required if you want to keep food for 6 months or more.
................Evap T.... PSIA .......................BD50....................... . TFS4FK
....................F ...................BTU ..amps... btu per.. % Inc ..BTU.. amps.. BTU per.. % inc
.................................................. ..............ampHr ...energy .....................ampHr.. energy

Evaporator ...14 ......29.1.... 271.. 3.9...... 69................. 1015.... 11.5 ...88
Cold holdover 0 ......21.2... 178.. .3.2...... 55........ 25% ...810.... 11.4 ...71... 42%
Plate ...........-10 ......16.6... 126.. 2.74..... 45........ 51% ...650..... 11.2... 58 ..92%
....................-20 .......12.9.... 88 ...2.32.... 37........ 83%.... 530 ....11.1.. 48.... 165%
Table contains observed readings (amps) and values from engineering tables (BTU), etc

As can be seen there is a large difference, the difference is dependent on having 1 or 2 or more cold holdover plates.
I have disassembled cold holdover plate systems using the BD35 and BD50 compressors, hand built evaporators totally surrounding the food, installed it with a TX valve and ran it in the same box. The drop in ampHrs per day was in the range of the above table.
I have also removed large cold holdover plate systems using a third hp compressor and installed an evaporator surrounding the food and with a small 110 volt compressor running off an invertor. The drop in energy used was larger than the BD50, in the fractions.

Capillary tube systems are difficult to design and are finicky in operation.
The capillary tube has to have the same rate of flow of refrigerant as the compressors’ rate of flow.
If the evaporation temperature is raised the gaseous refrigerant becomes denser. The compressors’ rate of compressing refrigerant increases since the compressor is a constant volume pump.
For the capillary tube to increase its rate of refrigerant flow, the pressure difference across it has to be increased. Consequently the pressure in the condenser has to increase thereby raising the condensation temperature.
The reverse is true. If the condensation temperature is increased the evaporation temperature has to increase.
The opposite is also true. If the condensation temperature is decreased the evaporation temperature has to decrease.
In general a rise in the condensation temperature of 10ºF degrees results in a rise of 6ºF degrees in the evaporation temperature.
This creates a problem in design. It has to work in a wide variety of situations. To do this compromises have to be made which all result in more ampHrs per day energy use as the system is designed to be inefficient.
I have taken apart a few capillary tube systems installed a new evaporator which totally surrounded the food and reduced the ampHrs used by 30-60 %.

The TX valve is without doubt the best way to meter the flow of refrigerant, coupled with a low side pressure control there is nothing better to use in an environment which is difficult to predict.

Having an oversized condenser is also better, if you can sub-cool the condensate before it enters the box you can move more heat.

I hope all this answers your questions. I apologise for being so lengthy.

[Richard Kollmann]

mesquaukee, I hope you will at least admit that some your main opinions are contrary to all the mobile refrigeration industries standard design methods.

obxmedic, I am sorry I hijacked your question but there are over one hundred boat refrigeration options available to select from. The important things to consider are: the boats power grid that will supply power to all electrical equipment on board, daily consumption of power affected by cruising area climates, desired box’s temperatures and finally matching Btu capacity of all refrigeration unit's components to your installation application.

[mesquaukee]

Originally Posted by Richard Kollmann

mesquaukee, I hope you will at least admit that some your main opinions are contrary to all the mobile refrigeration industries standard design methods.

Obviously they are.

What I have done was to very simply:


1.) compare the energy efficiency of 2 ways of metering the refrigerant (capiliary tube and TX valve) using the exact same compressor, box and environmental conditions


2.) compare the energy efficiency of a cold holdover plate to a evaporator and TX valve using the exact same compressor, box and environmental conditions.

The results that I found, which I have duplicated, speak for themselves.

[obxmedic]

Thanks for the replies. Alot of technical stuff to take in. I am starting to wonder whether I'm am able to do this by myself. I was trying to find someone who has the same size boat and the same cruising disires that I have. That has recently replaced their system. Bob

[Richard Kollmann]

Bob, you are not required to understand all the technical opinion minutiae to select the right refrigeration unit for your boat. I have a Slide Show on my web site that will guide you through the steps of how to select a refrigeration system that you will be comfortable with. Ounce you have a general conceptual idea of what system will work for you start shopping through the several reliable companies the sell ice box conversion units. After you think you have found what you need ask for opinions on this forum and SSCA forum about your selection. Your budget and size of box to be refrigerated will have a major impact on how you equip the complete boat. The next item to evaluate is the use of alternative energy to support the boat’s DC power grid do not include this in your questions about refrigeration treat it as a separate upgrade. Deal with responsible companies that warranty their products like Technautics, Seafrost, Frigoboat, Adler Barbour, Grunert and Nova Kool. I have listed the major brands even though their may have been acquired by other companies because their products perform well in all climates.

[Badsanta]

mesquaukee, where can I buy a system you describe?

Richard, always a great and clear response.

Bob, as a retired medic I hope to meet you on the water. If you ever head up the Chesapeake bay give us a shout.

[Starbuck]

Richard's site is here.

[mesquaukee]

Originally Posted by Badsanta

mesquaukee, where can I buy a system you describe?

I do not know of an off the shelf install it yourself system like that.

After saying that you can come very close. You will need to do your homework. From the list of the manufactures suggested by Kollmann in a post above select one that uses a TX valve and uses bendable evaporator plates. Tell them what you want. They will tell you what they can do.
What I am trying to do is point out what things to look for in a system if you desire to maximize electrical efficiency.
What I know is the thermodynamics of a refrigeration system and how to use a mollier diagram to determine what are the desired criteria of the ideal system (most efficient system possible). In plain English you want a COP as large as possible. COP (coefficient of performance) is the amount of heat moved divided by the energy used to move it. Compressor manufacturers have tables showing the COP for a range of evaporator and compressor temperatures (or pressures). Examining them you can see immediately what makes an efficient system. Using a Mollier diagram just fine tunes the efficiency.
Knowing what I wanted I disassembled and built a number of systems attempting to model the ideal. Doing that in the middle of nowhere had its problems.

I am in the process of writing a e-book for cruisers whose aim is to not to rely on a generator or running your engine for charging your batteries. It is both an explanation of the refrigeration system and an argument for not using Cold Holdover Plates. It is yours if you want it, just PM me and I will send it to you. Do what you want with it, e-mail it to friends or what ever. I have no interest in making any money from it, nor do I ever want to see or work on another system. I would enjoy hearing your comments however. There are a few small errors in it (inverter size), it is a work in progress.
I am working on a 2nd part on how to build a system in a third world country entirely of readily available local parts or modify the system you have to increase its efficiency. How to build your own evaporator that actually fits the shape of your box in order to maximize space usage. Once again it would be free, do what ever you want with it

My desire is just as any cruiser before me, pass on the knowledge. The one thing I know of cruisers is they hate paying for anything. Being out for years on a fixed budget takes its toll.

Knowing how all the systems work and how they relate to each other is crucial to reducing energy use. Then one can make an informed selection of the equipment and where and how it should be installed.

How a person can select a system and have it installed without knowing how the system works is beyond me. Perhaps they are the ones with their diesel engines chugging away for 2 to 4 hours once or twice a day. Somehow they always seem to be upwind of me.

[obxmedic]

thanks mesquaukee,

what do you mean by pm? I would like th read the rest of your info. Bob

[mesquaukee]

Originally Posted by obxmedic

thanks mesquaukee,

what do you mean by pm? I would like th read the rest of your info. Bob

PM means private message. Left click on Mesquaukee and select private message. Send me an e-mail address that I can send an attachment to.
It is fairly large, about 3.4 MBytes. Tom

[Richard Kollmann]

mesquaukee, You are correct the real breakthrough in boat refrigeration is in the ability to adjust compressor speeds to balance system capacities for a greater SCOP. I assume your 110 volt compressor is fixed speed, what is rated capacity and COP of your compressor? If this compressor is driven by a 12 volt inverter, what is the inverter’s COP while driving compressor? How have you calculated btu capacity of condenser and evaporator to insure adequate return super cooled gas for compressor cooling?

[gbanker]

We have had good luck with dual (freezer and refrigerator) Frigoboat systems with the keel cooler option.