Module 2. Food freezing

Lesson 19


19.1 Introduction

Care and Maintenance of Ice Cream Freezers and Hardening Cabinets

Primary requirement for satisfactory performance of ice cream freezers and hardening cabinets are their careful installation, proper operation and regular maintenance. The batch freezer consist of refrigerator cylinder and a rotating dasher assembly to scrap, beat and mix the frozen ice cream in the cylinder. The continuous freezer employs the same fundamental principles except that here the ice cream mix flows under pressure. The freezing cylinder is made up of a stainless steel liner pressed inside a steel or copper tube which forms the inside wall of the cooling jacket. The dasher inside the freezing chamber, carrying the scraper and beaters, is directly coupled to the driving unit. An airtight metal housing is provided on the refrigeration space around the freezing chamber which in turn is provided with insulation and protective cover to prevent moisture ingress into insulation layer. To achieve the best freezer performance the refrigeration must be good, the blades must be sharp and free from burrs , the mix must be right and the freezer must be properly operated. The following criteria are intended to furnish convenient guidelines to facilitate the regular maintenance programme for the freezer and hardening cabinets.

19.2 Dasher and Scraper Assembly

Scraper blades must function well to get the highest efficiency of the freezers. If the frozen film is not properly scraped off, the heat transfer will be low and the capacity of the freezer will suffer. The simple directions for care, maintenance and repair of scrapers involve the following:

(i) Dull blades causes slow freezing because they do not remove the frozen film from the freezing cylinder. The scraper blades should be kept free from burr and should be sharp and straight.

(ii) The burr formed on the upper side edge of the blade could be removed by drawing a fine file or stone lightly along the edge of the blade at an angle of 30º. This operation is necessary for two or three times week depending upon service performed by the freezer.

(iii) If the blade is worn down its heel become wider. The excess heel is removed by means of hand filing with a file held parallel to the main flat side of the blade. Some power operated blade-sharpener grinders are fitted with fixture to automatically hold the blade at the correct angle. The heel should be brought down to 1/32 of an inch in width for batch freezers and practically eliminated for continuous freezers.

(iv) The front edge of the blade should never be touched by the file.

(v) If the service is available it is most advantageous to send continuous freezer blades to the nearest dairy equipment service agency for professional sharpening.

(vi) If the blades are not sharp, the temperature of the refrigerant should be raised in order to allow them to scrape cleaner.

(vii) Rubber friction ring of the rotary seal for mutator shaft should be kept in good condition. Replace it whenever necessary.

(viii) Do not switch on the dasher and beater without any feed mix inside the freezer. It will result in the damage of blades.

(ix) Do not change the alignment of the dasher, it may damage the freezing cylinder.

19.3 Refrigeration System

Refrigeration system should be cheked often to avoid any undue wear and tear on the system. By replacing the parts at the proper time the system is as good as the new system which not only adds to its life but keeps operating costs and capital expenditure to a minimum. The important maintenance criteria are as follows:

(i) Pressure gauges are the main visible means to tell how the refrigeration system is operating. After being in use for some time the needle of these gauges may either stick or show too high pressure. False readings may lead to adjustment which will affect many things throughout the system. Most gauges have a screw, which will reset the needle to zero after allowing air to surround the stem. If the dial of gauges become rusty and hard to read, replace them, as the cost is very small when compared to cost of false adjustment.

(ii) Thermostatic expansions valve should be checked once a month. Under normal conditions, these valves are set for 5ºC to 6ºC super heat, but if through normal wear or because of large particles of dirt, the valve opens wider, flooding occurs which will cause a frost back to the compressor.

(iii) The float control valves are used in ammonia refrigeration system which serve to keep the liquid ammonia in the evaporators at proper level. If this valve is sticking closed, the evaporator is being starved of liquid, which cuts its capacity and increases the necessary time for compressor operation. If valve is allowed to be flooded with too much liquid, then there is a flooding of the suction back to the compressor which causes damage to compressor. Float controls should be checked very often and cartridge replaced if they do not respond to the adjustment.

(iv) Back pressure regulating valves regulate the pressure on the suction line coming from the evaporator. A pressure gauge is attached to it for adjustment of back pressure through a regulating valve. This gauge should be checked quite often for its accuracy.

(v) Defrost the evaporator coils as often as required because if frost or ice accumulates, the heat transfer is cut and efficiency of the entire system suffers.

(vi) A refrigeration system is theoretically a vacuum system, and air and non-condensable gases can infiltrate in many ways. These tend to accumulate at the top of the condenser or other high points in the systems. Whether the system has non-condensable gases or not can be detected by installing a thermometer in the high pressure vapour line near condenser. If the temperature of gas does not correspond to the pressure on condensing pressure gauge it means system contains non-condensable gases or air which needs to be purged off. Thereafter, charge the system with correct refrigerant charge.

(vii) The compressor will start to show wear after several years of operation. The simplest test is to check the compressor for over heating. Get the condition of the compressor checked as soon as the wear has been little more than normal. Do not wait until the compressor breaks down before replacing worn parts.

(viii) Loose compressor mounting bolts or vibrating unit tubing causes high level of noise and produces undue stress in the system. Tight the bolts and gently reform tubing to eliminate vibration. Check for the lubricating oil if the noise still persist.

(ix) Over charging raises the temperature and pressure above the desirable limits. Unit draws more current and consumes more power. With under charging lower than normal evaporator temperatures are maintained. The compressor motor gets overheated due to reduction in effective cooling by suction vapour. Motor winding finally burns out due to over heating. Recharge the unit with proper amount of refrigerant.

19.4 Electric Motor and Controls

The contemporary electric motor is one of the most efficient machine known today. Although they can operate with a minimum of attention, systematic motor care means not only fewer motor failures but also lower repair costs.

(i) Every effort should be made to keep water in liquid form from dripping on, splashing on or flooding on the motor unless the motor is of splash proof design. Whether the air-born moisture has produced a harmful condition in the motor can be determined by testing insulation resistance. When the resistance has dropped to a dangerous point the motor should be dried out without damage to the insulation.

(ii) Excessive bearing wear can produce misalignment and cause motor vibration. Even when the alignment is perfect vibration in the driven machine may be transmitted to the motor. Excessive vibrations can shake motor parts and electric connections loose, crystallize metal and multiply frictional wear. Vibrations should always be tracked down and it’s cause eradicated.

(iii) To protect the compressor over load protector(OLP) is provided, which starts within a few seconds and trips off, due to very high or very low voltage. Use the voltage stabilizer, if necessary. If the OLP is under sized replace it with correct size OLP.

(iv) If the unit hums and shuts off it may be due to the low voltage or the inoperative relay. At low voltage current drawn by the motor is not sufficient to keep plunger of relays on and it drops earlier. At normal voltage if relay is of oversize, the start winding is not kept in circuit by relay for sufficient time. Use voltage stabilizer and the correct sized relay.

(v) Inoperative thermostat or incorrect thermostat setting may sometimes results in too cold cabinet temperature. Correct the setting or use the lower setting of thermostat. The thermostat may be re-calibrated by varying cut in and cut out temperatures.

In order to achieve the desired performance objectives, it is necessary to determine periodically the performance of each item of the ice-cream freezers and hardening cabinets and auxiliary equipment under actual operating conditions and correct any digression from the established norms. Immediately after new equipment is placed in service it begins to deteriorate. Even under idealized conditions the efficiency and performance of equipments progressively decreases with normal usage. Through periodic checks the actual performance can be determined. The laxity of operating and supervising personnel also needs to be detected and corrected. Set up a convenient file containing all manufacturers specifications, instructions, drawings and performance data for the units. No mal-performance should go unheeded or uncorrected. This would help the plant to remain in optimum performance condition at all times. Electric motor should be provided with soft starters and other means to maintain power factor above 0.95.

Last modified: Friday, 12 October 2012, 7:16 AM