Sanden Compressor Puzzle

[Richard Kollmann]

Pressures look great at 25 and 35 minutes into the run for that system in tropical waters. At some point around 45 minutes plates should have been frozen solid were they? Running compressor any longer would not achieve much.

If you believe increasing high side pressure by flooding condenser actually improves refrigerant flow then there must be a problem with super heat. Sense TXV adjustment is not possible and no one has adjusted it valve inlet screen must be restricted. I prefer to believe this is a tropical problem with this two or more plate designs in tropical conditions.

High refrigerant pressure of 125 psi dropping to 118 will not increase load on generator so its bogging down must be something else increasing load.

[dkall]

The plates seem frozen but the plate temps are not what they were. I usually shut the Refer side down when the temp in the box is 40-45 and the plate temp is 18-22 F. I usually shut the Freezer side down and thus the system off when the plate temp (measured at the top outboard of the first plate in the freezer series) is under 10 F. Frosting is on the first freezer plate in the series and and before I do much else I guess it's time to defrost. Refer plate is ok frost wise.

[dkall]

Oh, and one more thing. Since the conclusion seems to be that it's not the load that could be bogging down the compressor, what's the conscience on being at the boarder line with oil?

If I can get one of the pressure oil thingies to add say an ounce or half ounce would that have any adverse effect.

After defrosting and seeing what happens I'm thinking of running again at 3k and then doing a temp check on the compressor every 5 min. Where would be the best place to check the temp at?

I do appreciate the quality of responses I've received. Thanks all.

[DeniseO30]

Dave. I just can't help but feel the turning off plates, oversize (or high rpms) compressor, borderline condenser, and tropical waters may all be the problems. Richard certainly has allot more knowledge of your system then me so I don't have much to offer.

[sfnz]

adding oil is relatively straight forward
1/ pump system down as if changing drier(minus 10" or so and watch for gauge rise on turning off, can take a few cycles to acheive zero pressure on gauge)
2/ disconnect LP(blue) hose from gauge set
3/ centre top of compressor is a plug/ bolt........remove to add oil
4/ on odd occassion one gets lucky here and there's enough flexhose length to dismount compressor and invert to firstly drain oil otherwise just add extra, the suction return hose can be removed to help here but DONT loosen the discharge hose, with this all the refrigerant is trapped between the discharge valve plate in the comp and the shutoff valve on the receiver leaving the rest of the system, TX valve included " DEAD" so it's also a great time to remove and clean the TX strainer/ filter, and change system drier
5/ either a quick purge to system ( crack receiver valve and allow to exit on gaugeset briefly before tightening) or use the vac pump
6/ open receiver valve fully and away you go.

For comp RPM change the only way to do so is a change in engine pulley, 2 1/2" engine pulley being about as small as you'll get with regard to belt longevity, easy enough to have made, 1/4" plate with a pulley spigotted and contersunk grubscrewed for behind, for this 2 belts are essential,

polyloester oil/ lubricant is hydroscopic, it absorbs moisture which is, just to make your day, acidic and doesnt always show as a TX valve blockage on these open drive systems

[dkall]

Looks like the pully change is out of the question. The generator also drives my HP watermaker pump and that is a serpintine belt on the outside of the crankshaft. It I start here then I keep on going.

What I don't understand is why after 4 years it would be different now vs earlier and why does Sanden say their compressor speed is fine at 3k and can be constant at I think they say 6k rpms.

Will look at the last post closer but I'm thinking something is different in my setup and to do it they way you suggest I need to evacuate the system completely, add oil and then recharge.

Sincerely,
Dave

[DeniseO30]

How long have you been in tropical waters Dave?

[Richard Kollmann]

Sfnz, Are you familiar with this type refrigeration system it does not have a king valve on receiver? Also how would you gain access to expansion valve inlet screen it is inside the SPU. Why and how much oil would you recommend be added to this system?

[dkall]

I've been in the tropics for 3 years.

I defrosted the freezer and times are much better on the running and pull down. My guess is the compressor still will bog down at 3k rpm. Plan on running it once there and doing temp checks. Does anyone know what temp the compressor should be.

And yes I can't really pull it down and isolate the system to add oil.

Maybe Richard can tell me exactly what the SPU does. That is where I connect the hoses to and that is where the HP switch is. Also Cleave informs me that the SPU lets some refrigerant through to cool the compressor. I do not have freezing anywhere near the SPU but do have frost on the hoses running from the SPU to the compressor. The SPU is approx 3-4' of tubing / hose length from the compressor.

Again, thanks for all the input. Funny, after 4 years of using this system I'm still learning about it.

Sincerely,

[Richard Kollmann]

All of the engine driven compressors commonly used for pleasure boat refrigeration like yours were designed for high back pressure Air Conditioning (AC) not low back pressure refrigeration. Temperature of compressor is determined by pressure and refrigerant gas vapor volume. The only cooling this compressor receives is by the super cooled volume of low pressure gas vapor and oil returning from evaporator. When this compressor is used for AC its high pressure runs as high as 200 psi with 134a refrigerant on a very hot day. For it to produce cool evaporator air evaporator’s low pressure will be 35 to 45 psi. If you check a pressure temperature chart for 134a at evaporator pressure of 34.5 psi evaporator temperature could be 40 degrees F (4.4C).

Now take the same automobile Sanden compressor without any modification and install it in a low back pressure boat refrigeration application with holding plates. Refrigeration where energy is stored in eutectic holding will not operate with the same steady evaporator temperature/pressure range as conventional evaporators. This style of compressor as an AC unit might be able to make one half ton of ice in one hour if system components were designed for max output of compressor. There is a big difference in freezing water to 32 degree ice and freezing 10 degree freezer holding plate eutectic solution solid. To freeze holding plate solution solid in a short period of time it takes an internal evaporator coil temperature 20 degrees F colder than eutectic solution’s freeze point.

Most all boat refrigeration systems of more than 1/6 horse power use seawater as a condenser cooling medium. It is how efficient condenser’s cooling is in removing waste heat from condenser that controls refrigerant high pressure. In a perfect system with holding plates having an aggregate of less than ten gallons of solution high refrigerant pressure seems to be best in a range of 105 to 125 psi.

Suction low pressure side of refrigerant system is controlled by controlling evaporator’s superheat. On your system there is no good way for you to change evaporator superheat because all refrigerant flow control is sealed up inside Suction Pressure Unit (SPU). There is a cut away picture in my DIY third addition book of what is inside a Seafrost’s refrigerant flow control unit; Parker Expansion valve, High pressure switch, and a return gas thermostat to control compressor clutch.

Starting compressor with warm 70 degree or warmer plates expansion valve is full open to ½ ton capacity almost to air conditioning evaporator temperature/pressure range of 15 plus psi. With in minutes control valve senses return suction line temperature and begins to restrict valve orifice opening narrowing superheat and evaporator coil’s temperature. As long as compressor runs suction pressure and evaporator coil temperature will drop. The rate of evaporator temperature change will depend on evaporator pressure change. If 3400 Btu was removed at a suction pressure of 8 psi and then pressure is reduced to 2 psi by control valve actual cooling done may be below 1000 Btu. When holding plate solution is changing to ice around 144 btu per pound of solution is stored in plate. Ounce plate is frozen solid only ½ of a Btu per pound is stored. So running compressor after plates are frozen solid is not going to store hardly any usable energy.


If generator engine bogs down and high refrigerant pressure does not increase I have no idea why. A ratio of 125 to 6 psi and plates freeze solid in less than one hour makes me believe refrigerant flow is good. A change in box temperature after three years in tropics might be due to moisture in insulation.

[dkall]

R,
It's not the change in the box temp that was / is happening. It's the change in the pull down time. Even now pulling the freezer down to 10 degrees F or less takes approx 20 minutes longer.

Thus I don't think no matter what is happening to my insulation and because of openings underneath and blue board and ultra U panels I can't see the insulation being saturated.

But, reading your comments I see that the refrigerant getting back to the compressor is what is cooling the compressor. Since the SPU on the Seafrost (Not the Seafrost TVX) is within 4' of the compressor and from what Cleave at Seafrost indicated that the SPU is to let a specific amt of refrigerant back to the Compressor to cool it; from what you indicate I've not enough cooling going going back to the compressor to cool the compressor.

Today I'm running the temps on the compressor to see if what they are when I have a full run, then most likely tomorrow I'll run it at 3k again and see what they are when it bogs down. If as you indicate and I'm suspecting I'm now low on what the SPU sends back there are two options, add more refrigerant till I get frost back onto the elbow (?) and possible evacuate and remove the SPU flush and put back in service. The last option will only happen when we become marina bound in about 6 weeks.

I did see the cutaway you have in the book on page 6 but that is of the Control Valve not the Suction Pressure Unit. I don't see the SPU and not entirely sure what it's made up of. If there is a screen there that may need to be cleaned. I can't imagine that anything entered the system but hey; I'm only human.

Thanks for your thoughtful response as I keep learning all the time.

[Richard Kollmann]

As far as I know SeaFrost’s engine drive Refrigerant flow control device is the same as box they now called SPU on your system. If your system is different than all of the SeaFrost systems I have seen I would need to see pictures of all its components.

The first day when refrigerator is turned on box heat load is not stable so any testing done should be done any day thereafter. If you want to test insulation's resistance to heat loss check its exterior temperature against cabin ambient air. Many types of insulation do not live up to their advertising when installed in a marine live aboard boat. Box temperature profile will define a holding plate refrigeration systems performance. When plate is frozen solid and compressor stops plate temperature will be near or below eutectic frozen solution temperature. Box temperature will be 10 to 20 degrees warmer. One hour later if insulation is adequate box temperature should continue to be colder if insulation is good. A correctly matched holding plate refrigeration system will require only one hour of engine running time per day to maintain safe product temperatures in each box. During compressor run time low pressure’s profile in evaporator will define heat energy removed and at what temperature. Lower the suction pressure the less heat energy (Btu) is removed because of less refrigerant flow. To freeze a zero degree F freezer holding plate in one hour low pressure in its evaporator coil must slowly decrease to reach ½ psi, hopefully at a rate of one degree at a time. With 134a refrigerant evaporator suction pressure at ½ pound positive pressure, temperature in evaporator coil in theory is -20 degrees F below zero.

The problem with relying on a suction gauge to compare evaporator pressure to temperature is your gauge is not connected direct to evaporator pressure so you can expect errors in your readings. Importance of suction gauge readings to me on holding plate systems is to see high suction pressure and good flow when plates are warm and a slow reduction to a very low pressure when plate is frozen.

Professional HVACR people measure evaporator super heat to determine if refrigerant phase change from high pressure liquid to low pressure gas is completed while in evaporator. If phase change is not complete in evaporator performance energy is wasted outside refrigerator. On a conventional evaporator superheat can be measured by comparing evaporator’s inlet temperature against its outlet temperature and then expansion valve adjusted to desired superheat.

Adjusting refrigerant control devices on holding plate systems is not easy because there are no two engine driven icebox boat conversions the same. Length of lines, 90 degree turns, number of evaporator coils and compressor rpm are just a few of refrigerant flow differences from boat to boat. And without access to super heat control changes to superheat are not possible such as on your unit.

[dkall]

Here is a picture of what I believe the cutaway in your book is. The second picture is of their SPU.

[Richard Kollmann]

Top TEV is valve only. Bottom one I have not seen before unless it also has valve inside. Is there only one of each units in system? The big one on freezer and small one on refrigerator tubing.

[dkall]

R,

There are two TEV's top picture and one SPU Bottom picture. The freezer has one TEV and the refrigerator has one TEV.

The SPU is the one I've often refered to when we talk about frost up to the Compressor. I've no frost on the side away from the compressor but I get frost from the Valve towards the compressor In Seafrost's instructions this valve setup is to be less then 4' hose length from the compressor and this is what Cleave told me meters out refrigerant through to the compressor for cooling. It is also where I connect my gauges to for getting the readings. On the Sanden I have there are no fittings for connections.