Hydrostatic Pressure in evaporator bodies | Hydrostatic Head in Vacuum pans

This article discusses hydrostatic pressure definition and its calculation and also hydrostatic head in Robert type evaporator bodies  and vacuum pans

Hydrostatic Head in Evaporator Bodies and Vacuum Pans

Hydrostatic pressure head definition

When a pressure ” P” is exerted on the surface of a liquid, the pressure to which the molecules of the liquid are subjected at a certain depth in the liquid is equal to “P ” increased by the pressure of liquid corresponding to the depth.

Since the boiling point increases with the pressure, if the temperature of the liquid corresponds to that necessary to produce boiling at the surface, this boiling would increase in the liquid layers situated at a certain depth.

Hydrostatic pressure calculation

The increase in hydrostatic pressure can be calculated by multiplying depth of liquid column by the density of the liquid.

Hydrostatic pressure in a liquid is calculated as follows

P = ρ g H               

Here

• • P = pressure in liquid  –  It is measured in meter of water column (mwc) or bar or atm or N/m² or PSI or lb/ft² or kg/cm²
  • ρ = density of liquid – It is measured in kg/m³ or gm/ml   or lb/ft³
  • g = acceleration of gravity – Its value is 9.81 m/sec² or  32.17405 ft/sec²
  • H = Depth in the fluid where pressure is measured or height of fluid column – It is measure in meter or feet

For Unit Conversion Factors please go through the below link

Conversion of units

Hydrostatic pressure in a water column where pressure is measured or height of fluid column –

(Density of water 1gm/ml or 1000 kg/m³ )

Hydrostatic pressure in the evaporator or Hydrostatic head in Robert evaporator bodies

In a vessel of a multiple effect evaporator, the layer of juice which is situated at the level of the bottom tube plate will be subject to a hydrostatic pressure equal to the pressure of juice which lies above it.

If, for example, the hydrostatic level of the juice in the vessel corresponds to one-third of the height of the tubes, this layer will boil at a temperature corresponding to the vapour pressure in the vessel increased by the hydrostatic pressure to which it is subjected.

As per Peter Rein

In the evaporator bodies, the average liquid boiling temperature should be evaluated at the average depth of liquid, which is 0.5 x h
where h is the boiling liquid level in meters

i.e. at a pressure P_h related to the pressure in the vapor space P_v and the liquid density by ρ_L

P_h = P_v + 0.5 ρ_L x  g x H

The optimum level is expected in Robert evaporator bodies of tube length of two meters is about 33% of the tube from the bottom.

In Semi-Kestner type evaporator bodies, the juice level is maintained around 20% of the tube length

A significant advantage of falling film evaporators is the absence of a hydrostatic head effect on evaporation.

Hydrostatic head in vacuum pan

This hydrostatic head has pronounced effect in the boiling of  massecuite as the brix of material or density goes on increasing progressively in boiling and it plays a vital role as the boiling point of massecuite increases with hydrostatic pressure.

The high hydrostatic head raises the boiling point of massecuite in contact with the heating surface, particularly in the massecuite of lower portions of the calendria.

So hydrostatic head plays a key role in messecuite level (strike height) on the above tube plate.

The boiling massecuite, at a temperature of 80 to 90 ⁰C (175 ⁰F to 195 ⁰F), has a specific gravity
of about 1.47 gm/ml, (density of approximately 92 lb/ft³ ) The pressure in the massecuite will
therefore increase by about 0.15  kg/cm² per meter (0.64 per foot) of depth.

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