Why Amonia DX evap need surplus capacity ?

[Yudha99]

Dear All,

Why do we need to add capacity if design amonia evaporator dx ? is there somebody can explain technically (from thermodynamic side) ?

For example I need evap dx with refr NH3, Evap temp -2C, Air in temp 5C. It means DT 7K and capacity 23kW. And he suggest to use unit capacity 23kW with surface reserve 46%. If I set surface reserve 0%, actually the evaporator has capacity 31.2 kW.

BR,
Yudha

[Sandro Baptista]

Dear All,

Why do we need to add capacity if design amonia evaporator dx ? is there somebody can explain technically (from thermodynamic side) ?

For example I need evap dx with refr NH3, Evap temp -2C, Air in temp 5C. It means DT 7K and capacity 23kW. And he suggest to use unit capacity 23kW with surface reserve 46%. If I set surface reserve 0%, actually the evaporator has capacity 31.2 kW.

BR,
Yudha

I don't think that it's related with the fact be R717 dx but rather availability of models that closes to the capacity you want. 23 kW with 46% surface reserve means that you have more surface than that you need to comply with the capacity you want. In fact that evaporator gives 31,2 kW on that conditions.

If you multiply 23x1,46 it will give you a little more than 31,2 kW (33,6 kW) » because you would need to also increase the airflow on the same proportion of the evaporator to maintain the same LDTM (considering that the global heat transfer coefficient would be constant).

I hope this would help you.

[Sandro Baptista]

"...because you would need to also increase the airflow on the same proportion of the evaporator..."

I meant "because you would need to also increase the airflow on the same proportion of the surface..."

Sorry

[Yudha99]

Hi Sandro,

Thanks for your advice, but I think the purpose is not only to maintain LMTD. This is straight regulation that must be fulfilled. The information I get, there will be problem if surface reserve smaller and even zero percent. I'm not clear about the hapened phenomenon. As I know bigger difference temperature between evap temp and air inlet the bigger surface reserve you need. In this case 7K, if you have design 8K you will need surface reserve bigger than 46%

[mad fridgie]

It could be one of two things or a combination of the two things
One being required superheat being a DX coil, and the other being thermal length (bypass)
vapour it not a good transfer medium, and thermal length, the air does "not" have "enough actual time" to cool, (even though straight energy transfer says it does) so you increase the surface area to effectively give it more time

[Sandro Baptista]

Hi Sandro,
The information I get, there will be problem if surface reserve smaller and even zero percent.

I can't see what problem can happen if you only need 23 kW why would you need have more surface reserve? Your compressor guarantee an evaporating temperature such as you can get the 23 kW of capacity, right?

If you want have more margin or more surface heat exchange to accumulate more ice before you make a defrost that's another discussion.

[mbc]

how about you have ice in your Evaporator.
More surface give you more time between two defrost time and avoid to liquid returning to compressor in (DX) system,
in flood system give you more capacity.
if you compressor was small your suction pressure goes up( D.T) K comes down until balancing capacity.and...

[Sandro Baptista]

how about you have ice in your Evaporator.
More surface give you more time between two defrost time and avoid to liquid returning to compressor in (DX) system,
in flood system give you more capacity.
if you compressor was small your suction pressure goes up( D.T) K comes down until balancing capacity.and...

"More surface give you more time between two defrost time and avoid to liquid returning to compressor in (DX) system," » It's a conception of design and all depend of the working conditions

"if you compressor was small your suction pressure goes up( D.T) K comes down until balancing capacity.and..." If is that so the evaporator should be selected with the corrected TD (lower than 7K on this case) and see what is the real capacity on that conditions


"in flood system give you more capacity." The selecting of the evaporator already have in count that the feeding system is dry expansion (DX)