PAN SECTION HYDRO STATIC HEAD & PRESSURE

 

Hydrostatic head or Hydrostatic pressure

The height of the strike

The massecuite circulation will be slow down while massecuite reaching level 1000 mm above the top tube plate.

Generally, the highest massecuite level above top tube plate is kept around 1000 mm to 1200 mm in batch pan and 300mm to 500 mm in continuous pans without using any mechanical circulator. While using the mechanical circulators, it will be increased by 200 to 300 mm height

Bottom Cone

The bottom of the pan generally takes the form of a truncated cone, but a segment of a sphere, or even a torus may be followed.

This part of the pan is situated at the bottom of the pan below the calendria making an angle of 17⁰ to 25⁰ with the extended portion of the vertical side wall of the calendria.

The  extended portion is of about 150mm to 200mm.

This is to facilitate the repairs / replacement of the end tubes of the calendria. It is advantageous for graining volume that these angles should be small but, for rapid flow of the massecuite at the moment of discharge, an angle of less than 17° will not generally be used.

Generally, cone angle is 18⁰ to 20⁰ is the value while considering all advantages and disadvantages

Downtake Diameter

The downtake diameter is generally not less than 0.4 times the pan diameter, unless a stirrer is fitted. A smaller diameter has been shown to restrict circulation.

Circulation ratio is one of the important criteria for estimating the downtake diameter.

Ideally this should be less than 2.5 to obtain a pan with good circulation, although a number of pans that have circulation ratio values up to 2.8 have given reasonable results.

Tubes

Generally the tubes are made up of Stainless steel. The diameter of the tube is 100 mm and length of tubes varies from 700-1500 mm.

Stainless steel for tubes is generally AISI 304 grade  (18% Cr + 8% Ni).

The tubes are generally, 100 mm diameter and installed on triangular pitch. The legment having 16 mm.

Tube diameters may be for low grade and smaller for high grade massecuites.

Boiling Point Rise

The boiling point rise or ‘boiling point elevation’ is defined as the temperature difference between the boiling point of the boiling massecuite at a given absolute pressure and the boiling point of water at the same absolute pressure.

i.e Difference between the temperature of a boiling sugar solution and the temperature of boiling pure water, both measured at the same pressure.

The boiling point of the massecuite depended on

1. Brix of the material (Density or Dissolved solids of material
2. Height of the massecuite above the calendria of a pan. ( Strike height)
3. Circulation of massecuite. (Mechanical and Natural circulation)

Various heights and dia of the batch vacuum pan for design aspects

( Note: These measurements will be given a approximate  values in design of vacuum pan)

Dia of Vacuum pan

D1 – Calendria shell dia  ( 1.21 P √N  –  P = Pitch of the tube  & Number of tubes)
D2-  Down take dia (0.4D1)
D3 – Vapour body dia = D1
D4 – Vapour dome dia (1.8 to 2 Dvo)
D5 – Discharge dia
Dvo – Vapour O/L dia
Dvi –  Vapour I/L dia

Height of the vacuum pan

H1 – Calendria shell height ( Depends on tube height)
H2  – Strik level height from the top tube plate  ( Depends on graining volume of pan)
H3 – Vapour space above the strik level (1200mm to 1500 mm )
H4  –  (D3-D4) x Tan ∅ /2  ( ∅ = 18⁰ to 20⁰)
H5 –  Height of the vapour dome (1.5 to 1.75 Dvo)
H6  – Bottom ring ( 150mm to 200 mm)
H7  = (D1-D5) x Tan ∝ /2  ( ∝ = 18⁰ to 20⁰)
H8  –  150mm to 200mm

Steam Entry

For pans with central downtake, it is preferable to distribute the steam through several entries placed around the calandira. The quantity of steam surging against the outer tubes immediately in front of the steam entry produces a very rapid circulation at that point at the beginning of the strike at the expenses of other zones of the Calandria. While at the end of the strike, this zone near the steam entry may produce false grain.

For this point of view it is preferable to provide each steam entry with a conical baffle to distribute the steam and avoid direct over heating of massecuite. For large capacities pans shall be provided two steam entry points.

Condensate Removal

Condensate water is removed by either inverted siphon or sealing mond. The sizing of condensate shall be calculated on the basis of velocity of condensate having 1 m/sec.

Number of condensate points will be choosen on the basis of calendriadia meter

Non Condenssible gases (NCG)  Removal

Adequate arrangements for the removal of condensate and incondensable gases must be made.

Generally non-condensable gasses are released by number of perforated pipes inside calandria and connected through main line and vented outside the pan. The total cross section are of the all NCG connection shall be required 1 cm² area for  10 m²   heating surface of vacuum pan.

Sight Glasses

The batch vacuum pans are equipped with 4 – 6 sight glasses at different height from front side and one sight glass from backside at the top level so as to facilitate easy supervision. For continuous pan equipped with sight glasses for each compartment.

Graining Volume Calculation in Batch Pan

In this article, the concepts of the Graining volume in batch pan and example for calculation of the graining volume are discussed.

The Concept of Graining Volume of the Batch Pan

The graining volume is the volume of the pan up to the level of the top tube plate, the minimum volume at which the pan can operate.

This term “Graining Volume” is used to denote the minimum volume of massecuite, which must be introduced in to the pan before opening of steam valve. It is calculated corresponding to the upper surface of top tube plate. It is also referred as ‘footing volume’ of the pan.

Graining volume generally expressed in percent of the working volume of the that pan. It is varies from 35 to 45%. Generally, all types of massecuites graining made with higher purity syrup or molasses and afterward pan is filled with low purity material. So lower graining volume is obviously of advantage for better purity control of massecuite. Also low graining volume is helps to increase the crystal size.

However some massecuites graining material preparation and developing of the massecuite with same material. In such situation, the % graining volume is not necessary to be considered for purity reduction.

Present scenario to improve the circulation of the pan use short length tubes . If tube length of pan decreases, then increase in the calendria diameter for the pan with same heating surface. The increase in calandria diameter increases the graining volume of pan.

In case of 800 mm tube length pans, the graining volume increases upto 40 % to 42%. Such pans definitely give higher circulation in pans.

The Graining volume of the batch pan calculated by adding these volumes in pan

a) Bottom cone volume

b) Bottom ring volume

c) Total tubes volumes

d) Down take Volume

e) Volume of the pan upto 50mm height from top tube plate (It may not be required to add this volume while calculating the graining volume)

Example of calculation of graining volume single discharge type:

Graining Volume of Batch Pan
Sl.no.	Description	Formula	Values	UOM
Input Data
1	Capacity of pan		60	T
2	No. of tubes	N	1306	nos.
3	Tube thickness	t	1.6	mm
4	Tube Length	H1	750	mm
5	Tube OD	OD	102	mm
6	Dia of pan	D1	5600	mm
7	Dia of the down take	D2	2400	mm
8	Height of the bottom ring	H2	50	mm
9	Angle of bottom cone	α	18	Deg
10	Discharge Dia	D3	600	mm
Graining Volume Calculation
1	ID of the tube	ID = OD – 2xt	98.8
2	Volume of massecuite in tubes	Q1 = 0.785 x ID x ID x H1 x N	7.51	M3
3	Volume of down take	Q2 = 0.785 x D2 x D2 x H1	3.39	M3
4	Volume of the bottom ring	Q3 = 0.785 x D1 x D1 x H2	1.23	M3
5	Height of the bottom cone	h = [(D1 – D3)/2 ] x TAN α	812.30	mm
6	A1=	0.785 x (D1)2	24.62	M2
7	A2=	0.785 x (D3)2	0.28	M2
8	Volume of the bottom cone	Q4 = h/3 (A1+A2+√A1A2)	7.46	M3
9	Graining Volume	Q1+Q2+Q3+Q4	19.58	M3

Example of calculation of graining volume “W” type cone

Graining Volume of Batch Pan (“W” Type cone )
Sl.no.	Description	Formula	Values	UOM
Input Data
1	Capacity of pan		60	T
2	No. of tubes	N	1306	nos.
3	Tube thickness	t	1.6	mm
4	Tube Length	H1	750	mm
5	Tube OD	OD	102	mm
6	Dia of pan	D1	5600	mm
7	Dia of the down take	D2	2400	mm
8	Dia of bottom inverted cone	D3	2200	mm
9	Height of the bottom ring	H2	50	mm
10	Angle of bottom cone	α	18	Deg
11	Angle of bottom inverted cone	Φ	35	Deg
Graining Volume Calculation
1	ID of the tube	ID = OD – 2t	98.8
2	Volume of massecuite in tubes	Q1 = 0.785 x ID x ID x H1 x N	7.51	M3
3	Volume of down take	Q2 = 0.785 x D2 x D2 x H1	3.39	M3
4	Volume of the bottom ring	Q3 = 0.785 x D1 x D1 x H2	1.23	M3
5	Height of the bottom cone	h 1 = [(D1 – D3)/2 ] x TAN α	552.36	mm
6	A1=	0.785 x (D1)2	24.62	M2
7	A2=	0.785 x (D3)2	3.80	M2
8	Volume of the bottom cone	Q4 = h/3 (A1+A2+√A1A2)	7.01	M3
9	Height of the bottom inverted cone	h2 = [( D3)/2 ] x TAN Φ	770.23	mm
10	Volume of inverted cone	Q5 = 1/3 x 0.785 x (D3)2 h2	0.98	M3
11	Graining Volume	Q1+Q2+Q3+Q4 – Q5	18.17	M3

Hydrostatic Pressure in evaporator bodies | Hydrostatic Head in Vacuum pans

This article discuss about 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 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 )

Height of Water Column in meters	Pressure in bar	Pressure in psi
1	0.098	1.42
2	0.196	2.85
3	0.294	4.27
4	0.392	5.69
5	0.491	7.11
6	0.589	8.54
7	0.687	10
8	0.785	11.4
9	0.883	12.8
10	0.981	14.2
12	1.18	17.1
14	1.37	19.9
16	1.57	22.8
18	1.77	25.6
20	1.96	28.5

Hydrostatic pressure in evaporator or Hydrosatic 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 maintained around 20% of the tube length

A significant advantage in 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 heating surface particularly in the massecuite of lower portions of the calendria.

So hydrostatic head plays key role on 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.