Gravity feed system for evaporator

[sandybapat]

I have two queries regarding gravity feed and pump feed evaporators.

With pump feed the circulation ratio from 3 to 6 is used. With gravity feed what is the circulation ratio?

If an evaporator coil is designed for pump circulation ratio 4 and the same is fed with gravity feed what effect it will have on the coil performance? Gravity feed is ofcourse with proper liquid head to take care of evaporator and vapour return line losses.

[Magoo]

The surge vessel on a gravity feed coil needs to be at a specfic height above coil depending on what refrigerant is applied.

[sandybapat]

Yes Magoo

What is the circulation rate expected in simple gravity feed system?

[Magoo]

There is a major difference between liquid overfeed coils and gravity feed evaps., mainly that gravity fed coils generally have a greater effective surface area because you replacing evaporated refrigerant from the coil so effectively the control level in surge vessel is equal to the evaporation rate, as in 1 : 1 ratio. Obviously the fead rate should be higher potential as boiling effect in coil saturates and becomes closer to design. With pump fed coils you have the advantage of effectively supplying super cooled refrigerant to coil surface area and the nett effect is basically the same as an indirect refrigerant, so a smaller effective coil surface area

[sterl]

A simple finned coil for air cooling at evap temps near water-freezing will tend to operate between 6 and 16 overcirculation. Actual depends on heights, coil circuiting, heat flux and similar.

There is a considerable treatment on this and similar in Prof Stoecker's book on Industrial Refrigeration.

[basilyousuf]

In pumped ammonia evaporators you have central vessel seperatingliquid and gas from the suction vapour. the pump send liquid ammonia to the evaporators. the evaporator is made of two manifolds for in and out and there are tubes in between.
in the gavity evaporators you got the liquid ammonia from the main reciever with high pressure equal to the condensing pressure, intering at first a vessel with float expansion valve to act exactly like the seperator vessle in the pumped system but with one defferance thet is this vessel is used for one evaporator in time in the pumped system the vessle serves all evaporators.