suction pressure regulation

[chillyblue]

Hi

Does anybody have any ideas on maintaining the suction pressure above 0deg. C on a small system as detailed below.

duty 1kw
evaporating no lower than +0.5 to =1deg C (don't want the coil to freeze and restrict airflow as i am trying to de-humidify the air)
TEV expansion.

I have used evaporator regulators in the past to maintain a pre set suction pressure but not on a system this small, i have also used discharge bypass regulators.

I don't believe i can rely on a pressure switch as it will be on and off like a yoyo, causing compressor damage.

If there are valves small enough to control this who is the manufacturer and distributors.

Cheers CB:cool:

[nike123]

Look this valve:
http://tinyurl.com/63k379

[chillyblue]

Thanks Nike, but i'm looking for something smaller if poss, that has a rated capacity of 3.6kw on R404a (forgot to mention the gas used).
Will the capacity matter?? as long as it is too big rather than being too small??
My pipe sizes are 3/8

CB

[nike123]

Thanks Nike, but i'm looking for something smaller if poss, that has a rated capacity of 3.6kw on R404a (forgot to mention the gas used).
Will the capacity matter?? as long as it is too big rather than being too small??
My pipe sizes are 3/8

CB
Capacity doesn't matter for correct functioning untill reaching that indicated. Biger valve also could be mounted and it will make job as smaller, but it is uneconomical and sometimes size matter. Like fitting water valve of 3" on pipe of 1". But if you fit valve of 1" to pipe of 3" it does matter.

[wambat]

Why not use an automatic expansion valve which maintains a constant pressure in the evaporator. In operation, the valve feeds enough liquid refrigerant to the evaporator to maintain a constant pressure in the coils. This type of valve is generally used in a system where constant loads are expected. :o

[SteinarN]

An oversized suction pressure regulator tends to regulate the pressure more narrowly or closer than a "normal" sized valve. Suction pressure ragulators like Danfoss KVP has a noticeable P-band. When using an oversized valve, you only use a small part of the P-band and you get a narrower or closer regulation of the pressure you wish to maintain.

[nike123]

Why not use an automatic expansion valve which maintains a constant pressure in the evaporator. In operation, the valve feeds enough liquid refrigerant to the evaporator to maintain a constant pressure in the coils. This type of valve is generally used in a system where constant loads are expected. :o

KVP maintain pressure constant regardless of capacity change (until reaching his indicated capacity)

[chillyblue]

Why not use an automatic expansion valve which maintains a constant pressure in the evaporator. In operation, the valve feeds enough liquid refrigerant to the evaporator to maintain a constant pressure in the coils. This type of valve is generally used in a system where constant loads are expected. :o

That sounds good, only one valve (keeps costs down)What manufacturer and model would you and use (danfoss preferebaly)???
Do they still operate ok if the load is not constant?
CB

[chillyblue]

Why not use an automatic expansion valve which maintains a constant pressure in the evaporator. In operation, the valve feeds enough liquid refrigerant to the evaporator to maintain a constant pressure in the coils. This type of valve is generally used in a system where constant loads are expected. :o


Also what are the down sides to the valve.
for every solution there is always a bad point.

CB

[Billy Ray]

chillyblue

i have never been a great fan of evaporator pressure regulators (EPR) on systems with one compressor & one coil where you may operate at low load conditions.

During operation at low load conditions, the EPR closes to maintain the set pressure within the coil. This causes the effect of the compressor suction pressure operating at a lower balance point & possibly 'pumping down' & cutting out on LP.

No sooner as the compressor has cut out on LP, the EPR opens & the compressor re-starts.

This efect is generally seen on very low load conditions only.

Capacity control is really needed on the compressor, or some form of hot gas injectioin into the coil to maintain suction pressure (Danfoss CPCE & LG mixer).

If it is applied right & works - OK.

If the set-up is not good then you be forever revisiting site.

The key to this i think is knowing the minimun conditions the plant will operate at!

Billy Ray

[wambat]

With an automatic expansion valve (AXV) the condition you seek with freeze-up protection; is provided by keeping the evaporator above the freezing point at low loads. In addition, a constant evaporator pressure is maintained under all load conditions, and there is no remote bulb to attach to the suction line. As I said, if you have dramatic load (as opposed to gradual) changes then you might consider an EEV but they are expensive. At low-load conditions, the AXV maintains a higher suction pressure, thus preventing coil freeze-up. Additionally, at higher load conditions, the AXV maintains a lower suction pressure, thus reducing power consumption and protecting the compressor against overload

[BESC5240]

If you're looking for a small, precise and cheap HGB valve, you can go for a TUH9:
http://www.ra.danfoss.com/TechnicalInfo/Literature/Manuals/01/PDHK0A202.pdf
This is not an TEV, it only looks like that.
Only disadvantage : can only be bought by the box (45 pces)

[750 Valve]

do not go with epr on a small single system for the reasons billy ray has suggested.

have you thought about a variable speed compressor?

[Billy Ray]

Hi wambat,

can you set/adjust the system suction pressure by an automatic expansion valve (AXV)?

or does the AKV suction setpoint come factory set?

Billy Ray

[wambat]

It's adjustible and can be set at whatever pressure you like
Go here http://www.free-ed.net/sweethaven/MechTech/Refrigeration/coursemain.asp?lesNum=3&modNum=4

[chillyblue]

If you're looking for a small, precise and cheap HGB valve, you can go for a TUH9:
http://www.ra.danfoss.com/TechnicalInfo/Literature/Manuals/01/PDHK0A202.pdf
This is not an TEV, it only looks like that.
Only disadvantage : can only be bought by the box (45 pces)

looks like thats the way to go, are they availible in the UK though??

Do you need to connect it via a proper mixing valve??

cheers CB