Pumped liquid evaporator pressure drop

[Superfridge]

Hi all,

When designing an ammonia pumped recirculation system, what pressure drop (pumped liquid pressure vs evaporator pressure) do designers use for standard force draft evaporators and why? Also, consequences of too high/low pumped liquid pressure?

I've looked on evaporator manufacturers data sheets and ASHRAE handbooks without success. I find on most existing sites I visit the pumped liquid pressure is generally around 3 bar above suction pressure but I'm unsure as to why this figure is chosen other than ensuring enough subcooling and satisfying the load.

Thanks

[Tycho]

Over time, certain design standards and practices become commonly adopted within the industry due to their proven reliability and performance. The choice of a 3-bar pressure differential would be a result of historical experience and successful implementations.

[RANGER1]

http://www.cold.org.gr/library/downloads/Docs/Liquid%20refrigerant%20pumping%20in%20industrial%20refrigeration%20systems.pdf

maybe helpful

[Superfridge]

Over time, certain design standards and practices become commonly adopted within the industry due to their proven reliability and performance. The choice of a 3-bar pressure differential would be a result of historical experience and successful implementations.

Ah yes, the Dark Art passed down from sorcerer to apprentice.

[Superfridge]

http://www.cold.org.gr/library/downloads/Docs/Liquid%20refrigerant%20pumping%20in%20industrial%20refrigeration%20systems.pdf

maybe helpful

Thanks RANGER1, I'll have a squiz when I get a spare 10

[RANGER1]

Superfridge,
Have seen plants with 150 up to 450kpa pump pressure.
Agree 300 kpa would be most common.
Had new plant with 450 kpa where penthouses over 4 stories high.
As long as you have pressure to furtherest point so gets liquid.
If high pump pressure Reg valves set to suit.
Danfoss has best set up on Coolselector to accurately set their valves.
Just need evapoator capacity, suction pressure, valve size.
You actually find different pump pressures do not change settings to much.
You more than likely have Hansen in your part of the world.
Pump has to stay in it’s pump curve to avoid cavitation (design engineer territory).

Do you have an issue?

[GrantD]

Hi Superfridge,

The evaporator data sheet will advise the flow rate of refrigerant required (based on a 3:1 or 4:1 Recirculation Rate for "blower coils"). You require enough pressure to deliver the refrigerant to the hand expansion / flow regulating device only. The pressure should then fall away and the coil can do its job - boil the refrigerant. Too much flow rate then the coil performance will decrease (brining occurs).

There is no firm and fixed rule on what to select the pump flow rate duty pressure at. The factors are as follows: temperature of refrigerant (for density), the geodetic head that the pump must deliver to (highest blower coil from Nh3 pump) and the pipe friction loss.

NH3 (ammonia) at -10 deg. C has a density of +/- 652kg/m3 (0.65 of water) and at -40 deg. C it has a density of +/- 690kg/m3 (0.69 of water). With water 10m is 1 bar. With NH3 = 10m at -10 deg. C is 0,65 bar (65 Kpa).

I would probably select a pump in the case of a cold store with V/Stations and blower coils at 10m high at around the 150Kpa mark. Keep adding pumps to get to the flow rate with on extra pump for redundancy.

Case in point - I have a pump (WITT: GP51A - note the "A" in the model #) running on a 6 pole motor. It runs with a differential pressure of 0.3 bar (30Kpa) and services 1 x user (a remote Nh3 / Glycol PHE) which is +/- 15m from and +/- 5m above the pump. The pump delivers a duty of 750KW at a recirculation rate of 1.5:1 = +/- 5.9m3/h.

So it is about flow rate - not pressure.

I hope this helps.

[Superfridge]

Superfridge,
Have seen plants with 150 up to 450kpa pump pressure.
Agree 300 kpa would be most common.
Had new plant with 450 kpa where penthouses over 4 stories high.
As long as you have pressure to furtherest point so gets liquid.
If high pump pressure Reg valves set to suit.
Danfoss has best set up on Coolselector to accurately set their valves.
Just need evapoator capacity, suction pressure, valve size.
You actually find different pump pressures do not change settings to much.
You more than likely have Hansen in your part of the world.
Pump has to stay in it’s pump curve to avoid cavitation (design engineer territory).

Do you have an issue?

No, no issue.
Just trying to learn more about those things covered in ice under the suction separator that get removed every few years for service, that otherwise give very little trouble and when discussed among colleagues there are a lot of blank looks and then pass on what I have learnt.

[Superfridge]

Hi Superfridge,

The evaporator data sheet will advise the flow rate of refrigerant required (based on a 3:1 or 4:1 Recirculation Rate for "blower coils"). You require enough pressure to deliver the refrigerant to the hand expansion / flow regulating device only. The pressure should then fall away and the coil can do its job - boil the refrigerant. Too much flow rate then the coil performance will decrease (brining occurs).

There is no firm and fixed rule on what to select the pump flow rate duty pressure at. The factors are as follows: temperature of refrigerant (for density), the geodetic head that the pump must deliver to (highest blower coil from Nh3 pump) and the pipe friction loss.

NH3 (ammonia) at -10 deg. C has a density of +/- 652kg/m3 (0.65 of water) and at -40 deg. C it has a density of +/- 690kg/m3 (0.69 of water). With water 10m is 1 bar. With NH3 = 10m at -10 deg. C is 0,65 bar (65 Kpa).

I would probably select a pump in the case of a cold store with V/Stations and blower coils at 10m high at around the 150Kpa mark. Keep adding pumps to get to the flow rate with on extra pump for redundancy.

Case in point - I have a pump (WITT: GP51A - note the "A" in the model #) running on a 6 pole motor. It runs with a differential pressure of 0.3 bar (30Kpa) and services 1 x user (a remote Nh3 / Glycol PHE) which is +/- 15m from and +/- 5m above the pump. The pump delivers a duty of 750KW at a recirculation rate of 1.5:1 = +/- 5.9m3/h.

So it is about flow rate - not pressure.

I hope this helps.

Yes, GrantD most defiantly helps.

Some good words there to digest. As a tech on the ground flow is not generally measured whereas pressure is hence the question but I think your reply will get me thinking flow more.

Cheers

[RANGER1]

Superfridge,
Some more reading, but gives some insight to piping design.

http://www.ishraepune.org/documents/Liquid%20overfeed%20systems%20by%20R.%20P.%20Paranjpey.pdf

https://web.iiar.org/membersonly/PDF/TC/T268.pdf