Open Flash Economiser for NH3 plants :

[Zulu]

I need to design/size a open flash economiser for a R717 plant, but I do not know. Anybody could give my guidelines of tell me where to find guidelines or examples

[sandybapat]

Zulu

Is it an economiser for screw compressor or interstage cooler for a two stage reciprocating or screw compressor? What are the evap/ condensing temperatures, capacity etc.

[Magoo]

Exactly, is it an intercooler or an economizer. Two totally different peices of equipment. By the way I have a huge dislike for open flash economizers, prefer closed flash for controlled liquid temps and flow rates.

[Zulu]

Exactly, is it an intercooler or an economizer. Two totally different peices of equipment. By the way I have a huge dislike for open flash economizers, prefer closed flash for controlled liquid temps and flow rates.

It is single stage system with screw compressor. Intermediate temp is (-) 5.6 deg C. Evaporating temp is (-) 19 deg C and condensing is (+) 41 deg C. Capacity of plant is 880 kW,whereas for economiser it is 141.2 kW.

[Magoo]

Hi Zulu,
what compressor brand selection are you planning to apply to the design. One compressor or two or three. With total system capacity at 880 Kw I would be selecting multi compressors for stand-by capacity and percentage redundancy back up for continuous capability

[Zulu]

Hello Magoo,

I plan to install 1working+1standby. Compressor brand can be either Frick or Grasso. Do you recommend mono screw or twin screw for this application. I think mono screw is having better COP. But right now I need to know the economiser sizing as this will decide my layout nad accordingly specs can be released.

[Magoo]

Hi Zulu,
won 't work with +41'C SDT, plus 880kw refrig at -19'C SST is a massive compressor, and a massive drive motor, add economizer will struggle during pull down. You may require 3.3 Kv drive motor.
You need to rethink your approach, and select multi compressors, as suggested earlier.

[RANGER1]

Hi Zulu,
won 't work with +41'C SDT, plus 880kw refrig at -19'C SST is a massive compressor, and a massive drive motor, add economizer will struggle during pull down. You may require 3.3 Kv drive motor.
You need to rethink your approach, and select multi compressors, as suggested earlier.

Magoo, for the above kw it would only be a Mycom 250VSD 300KW or 250VMD 400kw absorbed power economized.
If Zulu could drop SCT down it would make a big difference, but must be for a reason!

[Magoo]

Hi Ranger1.
I totally agree, 400 odd Kw drive motor at design. We do not know the process requirements and pull down period.

[sandybapat]

For flash type economiser design you only need to know the economiser capacity, total plant capacity and what is the condensing, evaporating and economiser port pressure at operating conditions. Once you select economiser for operating condtions, the same should be checked for adequacy for pulled down conditions. But that does not make much difference, since during pulled down the compressor will operate on partial load to keep the discharge pressure with in limit.

No. of compressors will not have any effect on sizing of economiser. The control system will be different depending upon No . of compressors.

With flash type economiser, liquid feed to evaporator has to be designed with care other wise the eceonomiser pressure will not be sufficient to feed the evaporator. For your temperature conditions the difference between economiser pressure and evaporator pressure is just 1.6 Bar and if economiser is in plant room and the evaporator is far away, then this 1.6 bar pressure difference may not be sufficient to reach evaporator. As Magoo rightly said, one should use either shell & tube type or PHE type or shell & coil type economiser, so that the subcooled liquid will be more or less at condensing pressure and feeding to distant evaporator can be easily achieved.

[Zulu]

The complete chiller will be skid mounted. Flooded chiller is very close to compressor. Suction line is hardly 3-4 meter long. I need to know how to size the open flash economiser. 400 kW motor is correct. it is suitbale for VFD operation.

Please help on open flash eco sizing or provide guidelines.

[sandybapat]

The liquid from HP receiver will be flashed in to economiser. The flash gas will be going to the economiser port of compressor. the velocity in the economiser vessel should be so maintained that liquid will not be carried to compressor. ASHRAE Refrigeration hand book - chapter on Ammonia refrigeration gives the guide lines for surge vessel design the same are applicable for flash economiser.

[Magoo]

Hi Zulu,
with a close coupled compressor skid arrangement you will definitely need a suction surge vessel between flooded chiller and compressor, and I seriously recommend a closed flash economizer as well, a shell and coil arrangement with level master or similar danfoss level controller, econo capacity is surface area and mass weight of coils and temp differences and HX factor. Get coils rolled up and heat treated to stress releive before installing in econo vessel

[Nico]

Magoo, what's your problem with open flash economizer. They get you maximum efficiency, are cheap and simple almost maintenance free design (just oil drain).
Also very easy to design, basicaly just droplet size (can be controlled by demister), gas density (evaporating pressure and type refrigerant) and upward flow speed (vertical separation) or horizontal flow speed and separation time (horizontal separation).
Key in the design is that the gas flow in the separating volume must be uniform (same speed, no turbulence).
If you follow "the book" (ASHRAE) without considering the droplet size or without having uniform speed in the vessel than you are in trouble.

When I design a skid like this (-20/+40) I would sertainly select flooded evaporator with separator, open flash economizer and high pressure float valve between condenser and economizer and between economizer and separator.
The float valves can be replaced by level transmitters and expansion valves (e.g Siemens MVS, Danfoss AKVA or similar).
This requires a little larger volume of the separator, but very small refrigerant content and no high pressure liquid tank.

I like the closed flash economizer just for the ability to be used as water separator, but than you need to be able to pump low pressure liquid to the vessel....

Any other case I would select a DX shell and tube for eco feed for reason of low investment and less refrigerant charge.

Zulu, your condensing temperature is very high, shouldn't you consider evaporative condenser or cooling tower?

Cheers, Nico.