chosing the right evaporator

[odza]

Can someone please tell me the difference betwwen flooded and non - flooded shell and tube type evaporators; based on capacity, condition of the vapur leaving the evaporator, heat transfer effectiveness, construction and control.

Thank you in advance

[A_Ahed]

In flooded type u have a shell semi-full of refrigerant like a pool and the compressor sucks from this shell (no expansion valve)but in tube type as the name implies liquid evaporates inside the tubes.
To best of my knowledge in large capacities flooded type in prefered

[US Iceman]

Shell & tube heat exchangers:

Flooded - this means the refrigerant is on the outside of the tubes and inside of the shell (usually called the shell-side of the heat exchanger). The refrigerant exit condition is a saturated vapor.

Heat transfer is by pool boiling or nucleation. The liquid boils around the tubes and the vapor generated travels upwards and back to the compressor(s).

Direct Expansion - this means the refrigerant is expanding inside of the tube (usually called the tube-side of the heat exchanger). The refrigerant exit condition is a superheated vapor if the TXV is set and controlling properly.

Heat transfer is by forced convection boiling. As the liquid evaporates less liquid is available. Therefore the vapor condition is continually increasing at a saturated condition, until all of the liquid is evaporated. Superheating of the vapor then occurs.

[TXiceman]

As a point to note, the flooded chillers have a a thermal expansion valve. You have to have some point of expansion and on a flooded chilller it is ether a control valve controlled by the level on the chiller (low side control) or a high side level control located at the condenser/receiver. With a high side float, you can have wither a critical charge system or a non-critical charge system.

DX control uses a thermal control vlave for expansion. This is valve is normally either a filled capilliary type witha filled charge in the bulb, or an electronic stepper type valve. You can also either feed the liquid through capilliary tubes and distributor or just an orifice.

As US Iceman explainted the flooded has thetubes on the shell. You can also use a thermo siphon design which is a flooed desigh, but the tubes are on the refrigerant side.

Now if you are talking about unit coolers, you can have a flooded design that has the coil tubes fed by a thermisiphom drum. Some people also refer to a pumped recirculation or over-feeddesign as flooded.

Isn't referation a wonder thing. When we have so many ways to describe the saame thing.

Ken

[odza]

whats the best refrigerant for perishable application

[yuhonghai]

I think the flooded evaporator is better for big capacity personally,but there is another problem of refrigerant charge,too much refrigerant charge sometimes influence the evaporating temperature,for small scale capacity unit,dry evaporator is better.

[TXiceman]

Best refrigerant depends on size of the unit as well as the temperature. If the system is small enough, I would use a DX and probably R507. I don't like to use the 400 series as they are all blends where 507 is an azetrope. If it is a cooler application, look at R-134A.

For larger plants, look at pumped recirculator systems on ammonia.

My 2 cents worth.

Ken