Dairy Process Engineering

Module 1. Evaporation

Lesson 8
CARE AND MAINTENANCE OF EVAPORATORS

8.1 Introduction

The dairy plants mostly use tubular falling film evaporators for concentrating milk to the desired level of total solids. Most of the equipment, fittings, gauges, valves and density measuring devices are selected by individual plant manufacturer, but special care is needed in their selection and operation to get satisfactory results. The flow rates and flow pattern and the temperature gradients are of vital importance to efficient operation. Thus, loose jets, spindles and seats in steam valves can produce a fluctuating vacuum.

8.2 Operation of the Plant

After going through general discussion on various components of the plant, we can now look into the details of operating parameters which affect the performance of the complete plant.

The performance of the evaporating plant is generally based on economy in the use of steam and the output capacity of the given plant. The economy is improved by either increasing the number of effects or by utilizing the outgoing vapour separated from milk. The increase in number of effect involves additional cost of calandria, pumps, piping system and also the operating cost. The other limitation is due to smaller temperature difference between heating medium and product as the numbers of effect are increased. Hence three to five effect plants are common in our country. On the other side, the thermo-compressor system is less costly and can give economy of steam equivalent to one additional effect, if steam pressure and other operating conditions remain steady as per the design of thermo-compressor operation. The thermo-compressor is most suited where high pressure steam is available and the evaporator can be operated with low pressure steam. Space limitation would favour a thermo-compressor installation.

The following table gives steam and cooling water consumption of evaporator.

Table 8.1

Sr. No.	No. of effect	Kg of steam to Evaporate one Kg of water	Kg of water to condense one Kg of vapour
1	Single effect	1.17	20.00
2	Single effect using recompression of vapour	0.57	9.00
3	Double effect	0.57	9.00
4	Double effect using recompression of vapour	0.37	7.00

The term capacity of evaporating plant gives the output in terms of water evaporated per hour. It depends on the surface area of heat transfer, temperature difference and the overall heat transfer coefficient. The surface area may be used on the external or inner diameter of the tube. The number of tubes and the length of tube will decide the area of heat transfer with fixed diameter of tubes. Thus the capacity of plant increases in direct proportion to the number of tubes of same diameter and length. With larger number of effects, the total surface area may be distributed among different effects keeping in mind the product flow rate. Thus the first effect has always larger number of tubes while the subsequent effects will have reduced number of effect.

The other important parameter is the temperature difference which is the difference in temperature of steam condensing in the first effect jacket and the temperature of vapour condensing after the last effect. With first effect temperature of 70 °C, the steam temperature may be assumed to be around 80 °C and the last effect vapour may be assumed to condensing at 45 °C. Thus, the available temperature difference (80-45=35) is about 35 °C. With the increase in number of effects the available temperature difference will decrease and hence the capacity will remain constant or may even decrease because of increased loss of heat from surface of calandria.

The last and one of the important parameter is the overall heat transfer coefficient expressed in W/m2.K. The U-value or the overall heat transfer coefficient is affected by number of factors, such as flow velocity, thermal conductivity of metal and the scale and the turbulence of the liquid product flowing down the tube. The change in specific gravity with change in concentration also affects the U-value. Thus the U-value is much higher for the first effect but goes on decreasing with increased concentration of liquid flowing in subsequent effects. The surface tension and viscosity indirectly affects the flowing velocity and thickness of film causing reduced heat transfer rate. The fouling of milk contact surface with hard scale is the main cause of reduction in U-value with time. The scale formation is basically due to inverse solubility of calcium and phosphorus salts present in milk at the tube wall temperature and protein denaturation at temperature above 70 °C occurring at heat transfer wall. In addition to this low flow rate or inadequate wetting of tube wall causes burning of the thin film at the tube wall even at slightly low temperature. Therefore, care is required in the operation of the plant to avoid conditions for hard and tough scale formation in the evaporator calandria. Over hard scale deposit, softer deposits are also formed causing reduction in heat transfer rate.

The other factors which affect the plant operation are live steam pressure, flow rate of product, even distribution of feed to all tubes, maintaining constant vacuum, preventing leakage of air and operation of condenser for rapid condensing and removal of condensate. If thermo-compressor is used, the working of plant is affected by variation of steam pressure at inlet to thermo compressor. The provision of separate steam main from boiler with regulating valve will solve the problem. The flow rate can be easily controlled by feed pump of adequate size and type and needle valves for flow regulation. Automatic flow controller and flow measuring device may be used.

The uniform distribution of product to all the tubes is another major problem in the design and operation of falling film evaporator. The term wetting rate indicates the relationship between product feed rate and heating surface. Every tube requires a certain minimum quantity of product to cover the entire surface. Therefore in a single pass evaporator, the number of tubes per calandria will decrease from effect to effect, because the milk volume is constantly reduced as the concentration is increased. The first effect is always the largest, and in large evaporators, the calandria will be divided in two sections in order to overcome the problem of distribution of the product over a very large number of tubes, and to obtain the correct wetting rate. When the first effect is in two sections, the milk flow will be in series, but the steam flow will be parallel.

8.3 Care and Maintenance

1. Follow the preventive maintenance norms from the manufacturer guidelines.
2. Check the air leaks which may develop around valves, joints, cover and observation posts, as it fluctuates the operating vacuum and temperature.
3. All gaskets must be changed periodically.
4. Avoid the use of high pressure steam.
5. Economy of cooling water to the water cooled condenser should be checked.
6. Keep about 3 °C temperature difference between condenser discharge water and cooling water for better economy.
7. Ensure heating surfaces are clean and free from deposits.
8. Scaling of heat transfer surfaces constitutes a problem and it should be cleaned after an optimum operation time. Descale the plant at least once a year with a suitable acid solution.
9. Check water vacuum system (working of vacuum pumps).
10. Periodical detection of steam coil leakage by hydraulic pressure test.
11. Condensate must be removed properly from the heating surface.
12. Release vacuum immediately in case of sudden power failure. Also steam valves and cooling water supply must be shut off at once.