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FOOD FREEZING

Food products should be stored at approx -15 Deg or below in order to preserve them in their original fresh state for longer periods.Such storages are known as frozen storages. Products which are commonly preserved in frozen storage includes fruits,vegetables,fruit juices,meat,poultry,bread,pastries,and wide variety of pre-cooked foods.
Vegetables and fruits to be frozen should be harvested at the peak of maturity and frozen as quickly as possible after considerable processing(cleaning,washing,blanching in hot water etc )

Methods Of Food Freezing

1. slow/sharp freezing : Food products are placed in low temperature rooms and allowed to freeze by the aid of natural convection. Temperature at a range of -17 deg to -40 deg is maintained.Time required for this type of freezing is considerably large.It may be from three hours to three days depending up on the bulk of products & conditions in the freezer.

2. Quick Freezing : In this method forced circulation of cold air is necessary.Quick freezing is generally done in

(a) Immersion Freezing : Food products are immersed into low temperature liquids,liquid medium used should be non-toxic and not produce any bad effect on the immersed foods.Liquids used for this purpose are sodium chloride,brine,propylene glycol etc.This freezing method produces a thin coating of ice on surface of product resulting in prevention of dehydration.

(b) Indirect Contact Freezing : Food product is freezed by the direct contact with metal surface cooled by any of refrigerants such as ammonia ,R-12, R-22 etc. For this purpose various types of plate freezers consisting of metal plates with storage space and refrigerant circulating through them are used.

(C) Air Blast Freezing : Food products are freezed by contact with cold air.Air blast freezing is widely used because it provides excellent quality of good among all other types.In this method,a very low temperature air is circulated with a very high velocity,around all parts of product kept in insulated storages.Velocity of air varies from 30m/min to 120m/min according to the type of food to be freezed.Air blast freezing is used for freezing fishery products like shrimp,fish fillets,streaks,scallops or pre-cooled products .

Marine Air Conditioning

General principles of Air conditioning which apply to land installations are also applicable to marine air conditioning, provide all the factors affecting construction and operation of a ship are given proper consideration.The system should be flexible to work under different climates ranging from extreme hot to extreme cold as the ship may pass through regions of different climates.

        In case of cargo ship the system should be capable of providing any temperature between -23.5 deg C and 12.5 deg C.The reciprocating compressors with R-12 as refrigerant is universally employed.The compressors are of shell & Tube type which uses sea water for cooling.Corrosion resistant cup nickel materials used for tubes and end covers.

Factors to be considered for designing air conditioning system in ships are as follows:

1. Objectionable noise and vibration of ducts and piping should be avoided by using proper dampers.

2. Equipment should be designed to occupy minimum space because of limited space on ship on board.

3. The low ceiling rooms in the ship need special attention for layout of supply and return air ducts,attention should be given to room dimension,volume of air handled and temperature difference between supply and room air.

5. Certain quantity of spare parts of essential items and extra refrigerant charge should be carried as the period of voyage may be long.

Leakage Detection of refrigerants

 Leakage of refrigerant from a refrigeration system affects the performance of a sealed system  adversely.  Both suction as well as discharge pressures reduce due to loss of refrigerant. There will be less liquid and more flash gas, which has negative effect on several components of the system. For example, it is possible that in stead of liquid, a mixture of liquid and vapour may enter into the expansion valve leading to its malfunctioning. Due to lower suction pressures, the lubricating oil return becomes difficult. If the suction pressure due to refrigerant leakage falls below atmospheric pressure, then there is a possibility of air leaking into the system. This will bring moisture into the system and the presence of moisture affects the system adversely.

Two widely used leak detection methods are:

i.  HALIDE DETECTION

ii. ELECTRONIC DETECTION

The halide leak detector works on the principle of change of colour of a flame in the presence of the refrigerants. When a fluorocarbon based refrigerant such as R12 or R22 is sucked through a sampling tube and passed over a surface whose surface temperature is high (around 500^oC), then the refrigerant vapour breaks down and forms a foul smelling gas known as phosgene (COCl₂). When this gas is passed over a glowing copper (heated by the flame of the torch itself) it forms copper chloride, which changes the colour of the flame from pale blue to bright green. The halide torch usually burns methyl alcohol, butane gas or acetylene and is similar in construction to that of a blow lamp with a provision to draw the air for combustion through a sampling tube. If the air consists of refrigerant (due to leakage) then it is detected by the change in the colour of the flame. Using halide leak detectors leaks as small as 1.5 to 2 oz. Per year could be detected. However, use of halide leak detectors requires certain precautions to be taken while using. It cannot be used with hydrocarbon refrigerants.

Electronic leak detector is based on the principle that when halogen vapour is heated, positive ion concentration is increased and this increase is suitably magnified to an audible or visual signal. Thus the leak detector consists of a sampling tube through which air around the refrigeration system is drawn by means of a small fan. The air sample is passed over a heated platinum element. Under normal (clean air) conditions positive ion current flows under a voltage of 240 V between the ion emitter and collector (anode and cathode), However, when a sample containing halogens is drawn into the probe, an immediate increase in positive ions results and this is magnified by the electrical circuit so that an audible or visual signal is produced. Exact location of the leak can be determined as the strength of the signal increases as the probe is moved towards the leak and decreases when it is moved away from the leak. In some leak detectors a reference leak is built for comparison and adjustment of sensitivity of the detector. It is also possible to make compensation for background contamination. Electronic leak detector is highly sensitive and it can detect leaks as small as 0.25 to 0.50 oz. Per year. The operation is clean, quick and free from any fire hazard. However, it is important to maintain the input voltage for good sensitivity of the detector. They are designed to detect refrigerant leaks but should not be immersed are kept in refrigerant streams, for example from the refrigerant cylinder.

                       

Physiological Hazards Resulting From Heat

The Body can dissipate heat only through evaporation of sweat.When heat loss due to due to evaporation is unable to cop up with the heat gain,there will be storage of heat in the body and the temperature of the body rises.Several physiological hazards exist,the severity of which depends upon the extend of time duration of body temperature rise.Following are some of the physiological hazards.

1.Heat Exhaustion :   It is due to the failure of normal blood circulation.The symptoms include fatigue,headache,dizziness,vomiting and irritability.It does not cause permanent injury to the body and recovery is usually rapid when the person is moved to a cool place.

2.Heat Cramp :  It results from loss of salt due to excessive rate of body perspiration. It causes severe pain in the calf  and thigh muscles.The heat cramps can be avoided by using salt tablets.

3.Heat stroke :  It is the most serious hazard.When a man is exposed to excessive heat the and work ,the body temperature may rise rapidly to 40.5 deg (105 F )or higher . At such elevated temperatures sweating ceases and the man may enter a comma, with death imminent. A person experiencing a heat stroke may be avoided by taking sufficient water at frequent intervals.It has been found that a man doing hard work in the sun requires about one litre of water per hour.

Factors To Be Considered In Load Estimation Sheet For "Comfort" Application.

1. Load estimate sheets are designed for limited application with respect to building type,construction,                          internal  heat  gains and exposure.

2.Application of load forms require estimates of time peak loads or range of time of peak loading.

3. Commercial load forms are based on significant internal heat gains and relatively lower external heat gain.Do not use residential forms for commercial applications and vice-versa.

4. FW sunlit windows,doors and walls :

        a. Determine the exposure at peak load time.

        b. In calculating the solar load.orientation of wall is Critical.

        c. Walls--use wall area exposed to sun during the day.

 5. Transmission gain for walls,windows,doors and partition :

        a. Area to include all windows exposed to outside air.

       b. Partitions may be considered as shaded walls if no factor is listed.Partitions exposed to unusual conditions such as kitchen,boiler room etc,may be handled by multiplying partition factors by 2,if no special allowance factor is listed.

6. Internal heat gains

       a. Evaluate occupancy load carefully

      b. Adequate exhaust and hoods over appliances may reduce the loads from these appliances by as much as 50%

      c. Careful consideration should be given for lighting load , frequently this is the major internal load               particularly in office space or brilliantly lighted stores.

Air Filter Replacement

In our busy schedule we  forget to check the filter of our Air conditioner,but its something that you need to do regularly,otherwise it can cost us in many ways.

* Air cannot flow easily through a clogged filter ,thus the system have to do extra work which results in wastage of energy and a hype in our power bills.

* If the device is forced to work harder due to this dirty filter, breakdowns will be more common. Longevity of device can be ensured by regular filter replacement.

* Indoor air quality will come down if we fail to replace the filter, Dust and other particulate circulating in the room can cause respiratory ailments.

* Remove the filter once in a month and hold it in sunlight,if it is still transparent you can use it again ,if you can't see light through it ,then its time to replace the filter.
Check the owner's manual to check the best suited filter

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