Jacket Around Pipe Having High Pressure Ammonia

Can you make a sketch of your system? It could be possible to use an approach without any welding since the power is small. One possibility is to use clamps to transfer heat from a hot oil pipe to the evaporator. If well designed the over all thermal resistance is small and you get your 400 W where you need.

I was thinking the same thing but....I was more focused on how he's going to throttle the oil so that he's delivering close to a constant 400 watts?

There are different types of valves which will allow it based on for instance the temperature difference of ammonia between in and outlet. An other possibility is to use a variable thermal resistance as for instance a variable air gap. There are possibilities which can be implemented.

I do not understand why my comment appears as anonymus ! Can any body explain it ?

Earlier it was happening if you do not log-in to CR4. But now it is mandatary. So while replying if you klick for bieing anonymous in your post.

Yes Dear,

I am bit late.Here is the sketch of Generator of Ammonia system and proposed jacket is shown in red. The original electrical heating element (400 Wattts) was in side the coil.

My Fear: The moment we try to weld the jacket with pressurized ammonia pipe the pipe will brust and may cause an accident apart from leaking/ release of Ammonia for which I have no vendor not even an radius of 400 Kms.

- Regards

-Dr N P Singh

Nick name's approach makes sense. Extending the idea a bit, you could surround the ammonia pipe with 6 oil pipes of equal diameter, using thermal mastic to improve the heat transfer.

A bit fancier, you might fabricate two semicircular jackets with inner surfaces matched to the o.d. of the ammonia pipe.

The ammonia pipe in question is the regenerator, rather than the evaporator.

Thanks dear for sparing time to think and reply.

I am bit late. Here is the sketch of Generator of Ammonia system and proposed jacket is shown in red. The original electrical heating element (400 Wattts) was in side the coil.

My Fear: The moment we try to weld the jacket with pressurized ammonia pipe the pipe will brust and may cause an accident apart from leaking/ release of Ammonia for which I have no vendor not even an radius of 400 Kms.

- Regards

-Dr N P Singh

Dear Narinder, Before answering I need some clarification on your following statement:

"a ammonia refrigerator which runs on an electrical heater (400 watts). I wish to run this machine with waste heat from our foundry. In order to supply heat to its evaporator I wish to install a metal jacket around the pipes of evaporator.

1. Evaporator is for removing heat, through cooling load. Why you want to supply heat at evaporator? What is your cooling load (purpose of refrigerator)?

2. You say ammonia at high pressure, but evaporator is usually at the lowest pressure in the cycle. Low pressure ammonia from evaporator is mixed with lean solution (of ammonia & water) to make rich solution and pumped at high pressure to generator. Am I right?

3. 400 kW heater may be there (I guess) at generator to boil the rich solution, please clarify. What is the temperature, flow rate and specific heat of hot oil. Whether it is sufficient to replace electric heater?

4. Normally in VARS, high pressure rich solution is preheated by out comming hot lean solution from generator through a heat exchanger to improve COP. Do you have this arrangement?

Dear Sir,

Thanks for sparing time to reply.

Point wise explanation:

1. Let me correrect my self : The electrical heater is supplying heat to the generator ( not evaporator as inadvertently mentioned by me in my statement)

2. The whole system is sealed and Ammonia + water is filled at high pressure, I will check up at the plate attached to the system to precisely tell you the pressure in side the pipes. As far as the distribution of pressure is concerned, to my understanding, as long as the system is idle, inside pressure at all the components is same, what ever aditional pressure difference is generated that is because of heating at generator, where the absorbed Ammonia is released and expands. There is no pump in the system, the placement/arrangement of all the components is such that the released Ammonia moves upward and water having absorbed Ammonia moves down due to gravity and thus drives the cycle without the aid of any pump.

3. Yes there is a 400 watts electrical heating element for supplying heat to the generator. I have measured the temperature. It varies in the range of 150 to 170 degrees cels. As I said the system is sealed there is no possibility to measure the flow rate. The specific heat capacity of oil ( to be used as heat transfer liquid) is 1.8 compared to water with Sp. Heat capacity of water being 4.187 kJ/kgK.

4. The above described heater is the one that preheats the rich Ammonia solution, I wish to replace the heat supply from heating element to the waste heat using engine oil as working fluid.

Regards- Dr N P singh

1. Cooling load is not yet given by you. It should either be expressed in TR or kW (1TR=3.5kW).

2. Just alone heating will increase the whole system pressure and will not cause pressure differential (as you have mentioned), which is required for expansion. No pump means the pressure difference between condenser and evaporator is exerted just by head of liquid column (very interesting).

3. I asked temperature and flow rate of waste hot oil to see whether it is sufficient in place of heater. Specific heat capacity of oil is fine. As you need rich solution to be heated up to 170 degC, waste hot oil temp should be at least 180 degC.

4. Once you get temp and flow rate of oil, check whether it is enough for your requirement. Then think of how to connect. I think clamping with high temp sealing will work. Oil flow direction opposite to shown in figgure will be better through special arrangement.

Dear Sir,

Thank for the reply.

1. Basically the cooling load can be treated as that of a 165 litre Fridge, which can be equated to ½ TR approx.

2. No pump does not means the pressure difference between condenser and evaporator ( generator for VARS systems) is exerted just by head of liquid column. As mentioned earlier the system is VARS -Vapour Absorption Refrigeration System, where pressure difference is created by the love of Ammonia for water. Ammonia has got so much affinity for water that at low temp. Ammonia gets dissolved/absorbed in water and hence drop in pressure and reverse action takes place at high temp in generator, where dissolved/absorbed ammonia get released and hence rise in pressure. The whole refrigeration cycle runs only because of heat. Does not it sounds very interesting.

3. As far temp of hot oil is concerned we have options to control the temp. As of now I am planning to keep the temp in the range of 220-230 deg Cels. As far flow rate of hot oil is concerned it will be 5-7 liter per min.

4. My only worry is how to connect. For temp & flow rate we have controls. But no idea how to connect, the whole project is stand still for this reason only. Once connected leak proof, we can easily control the temp & flow rate. You have suggested that high temp sealing will work- pl. tell me some brand name(s) for high temp seals.

5. Oil flow direction opposite to what is shown in fig will not suit as the system will be attached to a thermo-syphon cycle. I do not want any additional pump to be installed. However I do not entirely rule out the possibility of your suggestion, once the jacket is made leak proof reversing oil flow direction should not be a problem.

Regards- Dr N P Singh

1. ½ TR is a very small load. What for you are using?

2. "VARS-where pressure difference is created by the love of Ammonia for water."

It sounds to me very interesting. Pressure at evaporator is less than generator. So if there is no pump in between, liquid from generator will flow back to evaporator instead of going to condenser. Can you post the diagram of the complete refrigeration cycle?

3,4&5. You have plenty of oil at 220-230 deg C and control also. I think you can try sprinkling hot oil from top portion of coil and collect it at bottom. It may serve your purpose and no sealing is required.

1. ½ TR is a very small load. What for you are using?

2. "VARS-where pressure difference is created by the love of Ammonia for water."

It sounds to me very interesting. Pressure at evaporator is less than generator. So if there is no pump in between, liquid from generator will flow back to evaporator instead of going to condenser. Can you post the diagram of the complete refrigeration cycle?

3,4&5. You have plenty of oil at 220-230 deg C and control also. I think you can try sprinkling hot oil from top portion of coil and collect it at bottom. It may serve your purpose and no sealing is required.

Heat conducting epoxy. Better than welding any day.

Is this an adsorption refrigeration system?

If it is, than you need to heat the generator with the hot oil. This is the place where the electric heater is.

Can you coil tubing around the generator that uses the hot oil, and leave the heater in place as a secondary source of heat?

If you can then use thermal conductive paste or epoxy.

You will need to control the oil flow or it will run as long as the generator is hot.

Yes Dear,

I am bit late.

It infact is a VARS ( Vapour Absorption Refrigeration system) using Ammonia-Water system. A sketch is attached for referenec.

My fear: The moment we try to weld the pressurized Ammonia pipe with Jacket, the pipe will brust and may cause an accident apart from leaking/ release of Ammonia for which I do not have any vendor to recharge ammonia even in the radius of 400 kms.

Regards-

Dr N P Singh

Can you wrap the ammonia line around the hot oil barrel or just place the coil in the oil? This would mean that the oil system would be an open system with the coil sitting in the barrel.

Yes dear,

As I said while starting the discussion that. it is not possible to change high pressure Ammonia pipes neither they can be cut, welded or moulded. No vendor is ready to refill ammonia in the system. I need to weld a metal jacket around the generator so that hot oil can be circulated in the jacket.

My question remains: How to weld (leakproof) /seal a metal jacket around generator coil hving high pressure ammonia so that it remains leak proof in the operating temp renge~ 220 deg cels. Coil will be immersed inside hot oil but oil circulation will not be a open system. i have already posted proposed size and shape of the jacket. Again my specific question how to weld/ seal the two point where Ammonia pipes are commining in contact with the proposed metal jacket-nps

Option #2

Can you wrap an heated oil line around the ammonia line?

Yes Dear,

Kindly see the attached sketch of the high pressure ammonia pipe (Black spiral pipe) and decide for yourself whether you can suggest me, to wrap heated oil line of very small diameter around ammonia line and consequently very little heat transfer. For the sake of simplicity in sketch it is shown that there is some space between two pipes, actually there is hardly any space between two rounds of pipe in the spiral and still if you feel your suggestion has merits please send me the logic behind your suggestion. -nps

Suggestions are welcome, provided there is possibility to implement them meaningfully

If hot oil after use can flow by gravity to the sump, then you can design an open system (without pressure). For this take a 3" thin pipe about 500-600 mm long, insert it inside the coil, weld a thin flange (3" ID, 8" OD) at bottom such a that inlet ammonia pipe is just resting on this flange. Now place a 6" thin pipe over this flange. Make 1" notch on it for allowing 1/2" ammonia inlet pipe to pass through and weld the circumference. Inlet oil pipe should be connected to open top (just put a lid to cover). Guide hot oil outlet from the notch at bottom to the sump. Sealing will not be a problem at below 500 mmwc pressure. Regulate inlet hot oil flow such that it maintains a constant level. Also provide a small over flow pipe from 50 mm blow the top edge of outer pipe and connect it to outlet line. This cross flow arrangement will also provide better heating.

Dear Pritam,

Thanks for sparing time to reply.

Your post seems quite acceptable, but there are certain points that I would like to mention here.

I am quoting from your answer:

Make 1" notch on it for allowing 1/2" ammonia inlet pipe to pass through and weld the circumference.

My question is: weld with what ( Solder, gas welding or arc welding) and at what temperature. That is exactly my fear, that once we try to weld a high pressure ammonia pipe at high temperature,it may brust, and may cause an accident.

I have tried solder it started leaking.

I have tried M-Seal, on soldered joint it also leaked.

As stated very correctly by you, for sealing sustaining pressure ~ 500 mm WC is not a problem. It is the temperature cycle i.e. daily- oil & jacket and any joint will be heated to ~ 220 deg cels and in night it will cool down to ambient temp ~ 35 deg cels.

As the welding/ sealing material and Mild steel (Ammonia pipes) are having different expansion factors with temperature, hence with cyclic heating & cooling the joint starts leaking.

Yes dear I am in a fix and not able to move even a single step ahead. Hoping that more and more skilled people will come forward to find a workable solution.

-npsingh

All welding joints I have suggested in my comment are either gas or arc welding suiting to inner/outer pipe material & thickness without touching ammonia line. First inner pipe to be welded with flange end iserted to position from bottom. Outer pipe shall be first cut into two parts longitudinally and after positioning weld two longitudinal joints (can be tried with one cut also if you can stretch it enough to pass 1/2" ammonia outlet line). Next is to weld outer pipe with flange (inner edge of flange is welded in first step). Making 1" notch is for avoiding touching of ammonia inlet line during welding. This notch (with ammonia inlet line passing at the centre) will serve the purpose of outlet for used hot oil. Here you can restrict the opening by some means to regulate the flow rate. Hope its clear Dr. Singh.

This idea does not require much sealing (only between ammonia inlet pipe and oil oulet pipe). Even you can think of clamping over a PTFE tape. Get some idea.

I consider welding as a VERY dangerous solution. This is the reason I suggested a heat transfer via solids by conduction and contact. It does NOT affect in any way the ammonia pipe and is for the low power an optimal solution. Of course to make pleasure to PRITAM it is possible to turn shells and weld on them pipes BUT NOT on the ammonia pipe ! I shall send the sketch for the shell solution in a short time.

I have not suggested any dangerous welding. It is clearly mentioned that all welding joints will be without touching ammonia line. Only while welding near the notch 'ammonia inlet pipe' is to be protected by shielding with heat resistant material like asbestos cloth. So there is no point in "make pleasure to PRITAM". It is for the pleasure of Dr N P Singh, who can save few kilowatts of energy by waste heat utilisation.

I made a suggestion which as far as I see was not understood so in order to explain how you could really solve your problem here are some principle sketches you should now transform in a real design:

You can manufacture two series of contact pads as in above pictures and connect the internal axial ducts with your hot oil supply. The pads have to be in intimate contact with the ammonia tube and not fastened to each other so that temperature variations will not induce strain in the coil. thsi means that you should have a possibility at assembly to press on the tube pads pairs internal + external but let the pairs free to move one from the other. Free means with a small stiffness ot the bonds.

You can the exterior of course cover with a thermal insulating material so that the outer teperature will not be an accident source for personnel. I let you see which fasteners are the best. The contact pads can be any metal you can find around since the contact area will be big and temperature drops small. The groove radius has to be about 5% bigger than the coil tube radius so that the contact will be in the grrove middle. You amy use one of the recommended solutions to fill the air gap and increase the heat transfer capacity. Any way your power being quite small and the metall conduction high (even with steel but better with aluminum or cooper) I doubt that you could have any problem.

I estimate that the higher thermal resistance will be at the wall in the oil circuit so that may be you should check the convection at this level.

As you see in the external pad there is possible to have more ducts than in the internal which allows a higher heat transfer area.

This way you avoid TOTALLY your welding problem and all related risk.

If you have any question please ask.

Dear Guru jee,

Your sketch really looks great and I have taken a print of sketch. I am consulting local workshops so that they can try to febricate exactly as you have suggested.

Let me get the reply from the work shop, I will get back to you with the feed back.

-nps

People say: 1 kilowatt saved is 1.5 kilowatt generated (considering transmission loses and pilferage of power)- I thing they are correct.