Dairy Engineering

Module 5. Pasteurizers

Lesson 19
CARE AND MAINTENANCE OF PASTEURIZER

19.1 Introduction

The performance of the pasteurizer is an important parameter for the dairy plant as a whole, as this is the heart of the dairy plant. Its efficiency in pasteurizing the milk with minimum of services requirement reduces the cost of operation of the plant. One of the important parameter to judge the pasteurizer efficiency is the Regeneration Efficiency.

19.2 Regeneration in HTST Pasteurizer

Thermal regeneration is a technique devised to reduce service loads and thus improve energy usage. Percentage regeneration may be defined in several ways and due to these different methods; heat exchangers having identical thermal performance may have, seemingly, different regeneration efficiencies

One definition of Regeneration efficiency is “The temperature range through which the product is heated by regeneration, expressed as a percentage of the total heating range”.

A simple heat exchanger is considered where a product is to be heated from 5°C to 85°C by 80% regeneration and then finally in a heating section. Having heated the product to 85°C it is held in holding tubes and returned to the regeneration section at 85°C.

The total temperature heating range is 85 -5 = 80°C

The temperature range through which the product is heating by regeneration must be 80% of 80°C = 64°C.

Thus the incoming product will be raised in temperature from 5°C to 69°C by regeneration and will be heated from 69°C to 85°C in the heating section. The heated product is then returned to the regeneration section at 85°C where it is cooled regeneratively by the cold incoming product at 5°C. This can be depicted as follows.

From such a temperature profile diagram.

Considering now the heating loads involved and assuming a flow rate of Q then.

(i) Steam load in heating section = Q (85 – 69) = 16Q
(ii) Total heating load = Q (85 – 5) = 80Q

Thus the steam load expressed as a percentage of the total heating load is.

The heat load in the heating section is 20% of the total heating load. Therefore the regeneration percentage (or efficiency) of 80% truly represents 80% of the total heating load.

19.3 Preventive Maintenance of HTST Pasteurizer

The plant should be inspected externally for leakage of all kinds. It should be monitored continuously throughout the process and any abnormalities noted be investigated and rectified. The flow rate of milk and media should be checked by measuring the time to process a known quantity of milk.

19.3.1 Plates

1. The Pasteurizer should be CIP cleaned regularly after day’s operations.

2. The plates have to be cleaned manually periodically. It should be done with nylon or fiber brush or coir. It should never be cleaned with metal wire brushes.

3. The proper tightening and dismantling procedures have to be followed. Care should be taken not to tighten beyond the mark, provided by the manufacturer. The tightening has to be done such a way that plates move parallel to each other and the measurement of length at the top of the plate pack should be same as at the bottom.

19.3.2 Gaskets

Gaskets must be of nitrile rubber which can with stand up to 120ºC. Use of aggressive acid solutions during CIP cleaning might damage the gaskets. They should be replaced as a set instead of one at a time, to prevent misalignment of plates. New gaskets should be tightened to the minimum. As gaskets get older, tightening should be increased just to avoid leakage. While replacing the old gaskets with new gaskets, the remnants of old gaskets should be carefully removed from the gasket groove. Old adhesive, grease, dirt etc. should be removed by acetone and the plate groove thoroughly cleaned and dried. The new gaskets for replacement should be slightly sand papered at the back and should be cleaned and dried. A thin coat of adhesive should be applied to the gasket groove and back of the gasket with a small brush for even distribution. While the adhesive is still sticky, the gasket should be fitted in to the gasket groove. A slight pressure on plate for some minutes will help in the fixing of the gaskets. Excess of adhesive should be removed if any still present.

19.3.3 Frame

The frame having threads and spindle should be lightly coated with grease, to help easy movement of the plates.

19.3.4 Filter

Filter cloth should be checked from time to time for any tear or worn out patches. It should be washed after dismantling. Cloth should be replaced if any damage is observed. So is the case with wire gauge filter screen protecting the filter cloth.

19.3.5 Instruments

All thermometers should be checked periodically for accuracy. All air operated instruments should be supplied with clear dry air. The air filters should be inspected periodically, cleaned or changed when required. The components of controlling instruments should be replaced when ever damaged or functioning inaccurately. Leakages in the control instruments of steam supply should be checked, as they are susceptible for corrosion because of higher temperatures.

19.3.6 Pumps and motors

The pumps and motors should be thoroughly overhauled once in a year. The shaft seals if leaking should be replaced immediately. Motors if noisy should be checked for damaged bearings. Misalignment is one of the most common sources of pump troubles. Scales in hot water pump should be removed.