DX v/s Gravity

[Hemant Anand]

Hi,

This is a proposed solution to an old problem. We have a PHE ( Thermowave 250) that is installed as a DX chiller using R22 as refrigerant. It has not been giving the rated heat transfer.

Acc to the manuf. catalogue it should be installed as a Gravity chiller to deliver the desired capacity.

Our contractor has proposed a 'semi flooded system' that allows 75% of the PHE volume to be occupied by liquid R22 and balance 25% as expansion space. He is proposing a 65 lt accumulator mounted along side the PHE to achieve this.

Can someone explain the difference between a gavity system and a semi flooded system?

What are the possible operational difficulties with the above systems as compared to a DX system?

[US Iceman]

Our contractor has proposed a 'semi flooded system' that allows 75% of the PHE volume to be occupied by liquid R22 and balance 25% as expansion space

To my knowledge there is nothing called a "semi-flooded system". In a normal gravity flooded system the heat exchanger will be full of liquid at low load or 0% capacity. Since there is no heat load there is no boiling. So, the exchanger is full of liquid.

When heat is applied to the exchanger the boiling begins to occur. As the boiling increases, the volume of vapor generated also increases. At full capacity the volume of refrigerant vapor in the exchanger will be approximately 25%. The remaining volume will be liquid (75%).

He is proposing a 65 lt accumulator mounted along side the PHE to achieve this.

Three points to consider...

The diameter of the accumulator (surge drum) should be large enough to provide gravity separation of the liquid so that only vapor leaves the accumulator. This is dependent on the total capacity of the PHE.

The accumulator should be located slightly above the PHE, not below the top of the PHE.

The refrigerant piping to and from the PHE must be over sized to allow gravity recirculation. These pipes will be larger than a normal suction and liquid lines for a DX PHE.

What are the possible operational difficulties with the above systems as compared to a DX system?

If the flooded system is allowed to operate with fast suction pressure reductions the compressor can be slugged with liquid if the accumulator is not large enough.

You will also need some type of oil return on the accumulator, or the oil will concentrate in the accumulator.

[US Iceman]

Here is a quick sketch to show the relationship of the accumulator/surge drum to the PHE.

If the accumulator is mounted too high over the PHE, the static head of the liquid can increase the evaporating temperature in the PHE.

You should also ask your contractor for proof the accumulator is sized properly. After all of the problems you have had on this project, you should ask them to prove the capacities instead of waiting to see what happens afterwards.

[Hemant Anand]

Thanks a lot for this informtion gents. I had never heard of a semi flooded system before but I am no expert in refrigeration.

The contractor has not been paid in full yet I have withheld all installation & commisioning charges amount to nearly 35% of his fees. - and he will not be paid till he proves the capacity. Your suggestion about waiting to see what happens is very welcome.

Regards,

[Chopper]

If the accumulator is mounted too high over the PHE, the static head of the liquid can increase the evaporating temperature in the PHE.

US Iceman, please excuse my ignorance, but could you explain this one a little further.
Would i be right in saying that with too great a liquid head that the desired saturated temperature would only be achieved higher up the "leg" thus requiring a lower evaporating temperature to achieve the required heat transfer?
If my assumptions are right, would this not only be the case with "blends" or maybe just more noticable?

How do you size the static head?

Kind Regards

Chopper

[US Iceman]

Hi Chopper,

Static head is the increase in pressure (at the bottom of the liquid column) due to the height of liquid above.

The refrigerant pressure is still the same. However, when liquid is stacked up in a pipe, the column of liquid generates an additional pressure known as static head.

The refrigerant pressure + the static head is the total pressure. If you take the total pressure and the temperature of the refrigerant and look at a pressure/temperature chart you will see the total liquid pressure at that temperature does not correspond.

The saturation temperature is higher for the higher pressure. The difference between the higher saturation temperature and the one measured is another form of subcooling.

If the PHE is to achieve the rated evaporating temperature, the suction pressure at the compressor must be lowered to compensate for the higher saturated temperature due to the static head.

This allows the evaporating temperature to be lowered to the desired temperature. Of course this also reduces the compressor capacity since the compressor is operating at a lower suction pressure.

This happens with any fluid (liquid or vapor). However, it is more noticeable with liquid due to the higher fluid density.

This is the same reason scuba divers experience higher pressures at lower depths. The deeper you go, the more the pressure increases.

To find the static head in IP units: 144 sq. in./sq.ft. divided by fluid density (lb_mass/cubic foot). For water; 144/62.4 = 2.308 feet. This means for every 2.308 feet of height you will generate a pressure of 1 psi. If the column of water is 23.08 feet tall you would see a pressure of 10 psi at the bottom of the water column.

Note: for those trying to cancel units in the IP system there are some other constants required here. I did not show them to keep this simple.

Sorry for the IP units, I'll have to look at this from an SI perspective a little later.

[frank]

For Water, with a density of 1000kg/m3, a rise of 10m vertical will give a static head of 1 bar. (SI Units)

[binman54]

Hi,
Our contractor has proposed a 'semi flooded system' that allows 75% of the PHE volume to be occupied by liquid R22 and balance 25% as expansion space. He is proposing a 65 lt accumulator mounted along side the PHE to achieve this.

Bitzer uses the term semi-flooded to describe NH3 evaporators using injectors. I have never worked on one so I don't know enough to comment on how they work.
CIMCO, a refrigeration contractor in Canada had, I believe, a semi-flooded ammonia ice rink evaporator.
Could the contractor be putting the accumulator too low? It would certainly be a semi-flooded evaporator then.