Printable View
Hi.
I am going to design big cold storage room with Ammonia and automatic hot gas defrost.
I am wondering if we can put the automatic hot gas defrost valve inside the cold room or just above the roof of the cold storage. The cold storages will be built by PU panels inside room.
I saw some people put all the valves inside the machine room but that is not automatic hot gas defrost.
Thanks:cool:
My preference is to put them on the roof above the unit so any leaks go directly to atmosphere instead of accumulating in the coldspace.
Hi
I agree totally with Brian_ , keep anything leakable outside the enclosure. Remember the myth about hot gas defrost, it is actually cool gas by the time it gets the evaporator, insulate everything.
Is system liquid over feed or direct expansion, multi-evaps and cyclic defrosts, shuttered fan /evap enclosures etc..
I would be adding water defrost as a plan B., backup option.
Hi Magoo, I am going to use liquid overfeed. There will be many evaporators of the system. No shuttered fan enclosures. It is a -18C cold room. If I put valves above the roofs, the valves will still be inside the room and I am afraid there will still be NH3 accumulation problem. Can we just put all the valve in the machine room?
Hi Jerrycool.
with liquid overfeed system you will have to pumpout coil first, other wise the coil acts as a condenser and will fill coil with liquid. The suction regvalve will need to be fairly close to coil /coils as well, along with liquid line solenoid valve.
Hi,
Typical coldstore pipework is a main pipe run (pump liquid, wet return & hot gas etc) past each evaporator group with valve stations for each group. Valve stations outside the cold space are normally easier to work on and pose less risk to stored product.
Valve station in plant room seems ok, but I'd expect significantly more pipework to run each evaporator group back to said plant room.
Be mindful of defrost grouping, rule of thumb is to hot gas defrost maximum of one third of total evaporator capacity, so as remaining evaps can provide load for plant to generate the hot gas.
Hello Magoo and HVACRsaurusQuote:
Originally Posted by HVACRsaurus
It is correct to have more pipe work if all the valve stations are located inside plant room.
But please see the attached drawing. The PU panel roofs are about 1.5 m lower than the room roof. If I put the valves station between the room roof and the PU roof, the ammonia will be still accumulate inside the room except I add ventilation to the space.
Attachment 6741
Hi again Jerrycool.
seriously rethink defrost system, and use water defrost with big drain trays/heated and drains/heat traced. You will still need suctionstop regulators as well plus PRV.
Many design gurus have lost their shirt on cool gas defrost systems. Sorry, so-called hot gas defrost systems.
1.5m roof space is not much for valve station access. I've worked on similarly confined valve stations before and it's pretty cumbersome. However, valve stations within coldstore are also difficult to service, and it's easier to ventilate a roofspace of ammonia than a cold room. A cold room typically has low air change and ammonia hangs in the air for a long time. Granted, best option for service is valvestations in plant room, especially if they are well laid out and have good access. One method of improving the temperature of hot gas is to use a liquid float/liquid drainer near the valvestation/s or at the end of the hot gas pipe run. The idea is that any unwanted condensed liquid in the hot gas line is rejected (to the interstage or low accumulator) and only "hot gas" enters the evaporator.
I would have to agree with Magoo and go for water defrost. Depending on how far away your valve stations will be if you go hot gas you will increase the risk of liquid hammer from condensed liquid. From what I have read about hot gas defrost design......closer the better.
Based on your conditions, I would mount the valves in the room. The 1.5m space is not an area I want some to have to service an ammonia valve.
Mounting the control groups in the machinery room on the third floor would be rather expensive, not only in material, but also in the amount of non-revenue generating space that will be created. Facility managers want spaces that make money, whether it is production floor or warehouse space for rent.
Since your are planning a liquid overfeed system each suction line would require an accelerator loop or a double riser to handle the vertical lift needed to return the refrigerant to the engine room.
In this senerio I would use a liquid drainer on the hot gas defrost, a drainer must be mounted below the coil it is draining, so another reason for the valves in the room.
As far as water defrost, I suppose if the water is free and there are no charges for sewage I might consider that route, but it is not my first choice for cold storage rooms.
Ventilation-you will have to have a properly sized emergency ventilation system regardless of where the valves are located in this set of conditions.
Have you considered using a secondary refrigerant system? Have a heat exchanger in the engine room chilled with ammonia and pump a glycol brine to the air units. Defrost could be either electric or a warm brine.
If you have a serious leak in that false ceiling space, it can take hours before you can reach the problem, even if it's well ventilated.
Inside the freezer: what a bout the goods when you have a leak? How will you remove NH3 outside this space in case of a failure/repair?
Is outside the building on the side not an option?
Why then not going to the roof above the roof, outside the building like pictures attached
Hello Magoo,Quote:
Originally Posted by Magoo
If I use hot gas defrost, I will use Hansen HA4A as defrost drainer. It will keep the inside of the evaporators at a proper pressure. I did this kind of system very successfully with R22. I think NH3 system should be the same.
Water defrost will waste too much water. And I did see some water defrost cold room have iced floor. I really don't want to experience that.
Hello McFranklin,Quote:
Originally Posted by McFranklin
I reallized that it will cause service difficulty if ammonia valves installed in the 1.5m confined space.
What's meaning of accelerator loop? ( Ammonia pump?)
I think the room temperature is too low to use brine. It will cause higher power consumption. Hot brine is a problem. Is there any successful example for that kind of system? Owner may choose small condensing unit other than central plant if power consumption is high.
Hi Peter1,Quote:
Originally Posted by Peter_1
If possible, I would do the job in your way. I checked the drawing. the 2 sides of the cold room are for production. If I really want to put valves outside the room, we may put the valves just outside the loading dock. There will be far away from the cold room and there are always trucks in and out. It seems there is no perfect plan for this kind of case.
An accelerator loop is a piping technique. You reduce and put traps in the suction piping, this forces the vapor flow to increase. The accelerated flow carries the liquid ammonia and any oil up the vertical rise. There is a permanent penalty associated with this technique due to the line restriction. You can also achieve this by using a double riser system, one riser is sized for 1/3 the load, the other is sized for 2/3 of the load.
Sorry about the two posts, I'm having computer issues.
The use of secondary refrigerants (brines) is well established. If you will look up calcium chloride or a proplyene glycol (Dowfrost) you will find they are rated well below your target temperature. A 50% by volume solution of the Dowfrost has a freeze point of -34 degrees C. (I am sure other brands have similar qualities). A CaCl solution 29% by weight has a freezepoint of -49.4 degrees F.
Most ice rinks in the US and Canada use this system, Nestle's uses it in a number of their facilities, you minimize the ammonia charge, and locate it in one small area of the facility.
The warm brine is not a problem, a second smaller heat exhanger uses compressor discharge gas to warm the brine for the defrost cycle.
In terms of power consumption, brine systems are less efficent. In terms of cost, they are more expensive. But in certain situations they make sense.
In one of your follow on posts you stated that the owner would use split systems if the power consumption was too high with the central system.
This leads me to believe that the owner is going to use the lowest inital cost option. And if that is the case, trying to use a brine/glycol system is dead before you even start.
Thanks you McFranklin, I may check the cost of your plan but I really want to use overfeed system with hot gas defrost if I can solve all the problem will happen.
Hi Jerrycool,
bare in mind the hot gas temps for ammonia are lower than for R22, water defrost does not have to go to waste, can be re-generated from a holding tank and pre heated with discharge HX, or from oil coolers.
The systems we're servicing all use hotgas defrost without any problem.
Magoo, are you sure about the lower NH3 discharge temperatures versus those seen with R22? ;)