Common Problems and Remedies in a Commercial Refrigeration Plant

In the previous topics, we have discussed about what care and maintenance should be carried out for each component of a refrigeration plant. All that care/maintenance is given the name as Preventive Maintenance. Because it prevents any problem or fault to appear during the operation of plant and halt its working. But in spite of keeping full preventive maintenance as per the set maintenance schedule, some problem, minor or major, may come at any time during the operation of plant. The minor problems generally have minor causes and remedies, which the operator may himself know and rectify. Hence the operator of refrigeration plant should know about the cause and remedies of some routine problems in plant. However, if the problem is major, such as any tube of evaporator or condenser or refrigeration pipe has punctured or any electric motor or component has burnt or the compressor has gone under heavy wear and tear or any other such miscellaneous problem by which the whole refrigeration plant breakdown, it should be attended by expert maintenance staff.

Here we will discuss some common problems, which appear in day to day working of refrigeration plant, and their possible cause and remedies in brief.

i. Compressor is not starting
1. Fault in electric supply to driving motor

•  Power may not be available. Check with voltmeter or test lamp.
•  Voltage may be very low
•  Any wire or electric connection may be burnt
•  Any defect in motor starter
•  Any one phase in 3-phase supply may be dead

2. Faulty overload cut-out/relay. Check it.

3. Faulty thermostatic switch or temperature controller which may keep the compressor in off position even at higher temperature of cold room due to faulty adjustment or wrong position of its probe. Check and readjust.

4. Faulty H.P./L.P. cutout switch due to faulty adjustment. Readjustment is required.

5. In case the motor is rotating without rotating the compressor, drive belt may be loose or worn out. Replace it.

6. There may be any fault in motor or compressor i.e. either may stuck. Remove the connecting belt and try to rotate both by hand. If any one is stuck, it need repair.

ii. Compressor Runs for too Long or Cooling is Slow

1. There may be excessive cooling load. Check if this is the case.

2. Too much frost may have accumulated on evaporator coil, which is cutting the heat transfer and hence cooling rate. Defrost the evaporator.

3. Thermostatic expansion valve may not work properly and may keep less flow of refrigerant even at full load. It may be either due to wrong placement of feeler bulb or less gas in the bulb. Flow of refrigerant may be judged by looking at sight glass or on the scale of rotameter, if fitted in refrigerant line.

4. The temperature controller or thermostatic switch may be faulty.

• It may be set for too low temperature. Recheck.
• The temperature sensing probe is placed at wrong position i.e. in the warmer region. Check it.
•  There may be some inside fault in its circuit. Take suitable corrective measures.

5. Poor condensation due to high discharge pressure in the condenser. There may be many reasons for high discharge pressure. Purge the air and non-condensable gas, if any. See if the condenser needs cleaning or de-scaling.

iii. Frequent Starting and Stopping of Compressor

1. Fault in setting of cut-in and cutout temperature in temperature controller. The difference in cut-in and cut-out temperature may be very less. Reset the temperature controller.

2. The probe or bulb of temperature controller/thermostatic switch is placed directly in cold air stream in front of blower. Due to that the temperature at probe decreases at a fast rate thus switching off the compressor before the whole space in cold room comes at designed low temperature. Check and re-position the probe.

3. Due to less suction pressure or less amount of refrigerant vapours at the suction of compressor LP cutout may switch off the compressor. There may be many reasons for low suction pressure, such as

•  Less quantity of refrigerant in the system.
• -Inefficient heat transfer in evaporator either due to heavy frost or evaporator surface dirty or excessive amount of oil in evaporator or blower is not working, etc.
•  Faulty expansion valve, which does not allow the sufficient flow of refrigerant.

4. Faulty overload relay. Check and repair/replace.

5. High discharge pressure due to ineffective condensation of refrigerant vapours in condenser, which switch off the compressor frequently through H.P. cutout.

There may be many reasons for high discharge pressure as follows:

•  Air in the condenser tubes.
•  Too much dirt, scale or sludge has accumulated in the condenser tubes.
•  Inefficient supply of cooling air or cooling water

6. Faulty H.P. and L.P. cut-out. Check and re-adjust.

iv. In-sufficient Cooling/Cold Room is Less Cold

1. Excessive cooling load than that is designed.

2. Too much crowding of food containers which may restrict the diffusion of cold air. Arrange suitably the food containers.

3. Faulty setting of temperature in Temperature Controller. Reset it.

4. Frequent starting and stopping of compressor or less running time of compressor due to various reasons explained in the previous section. Take corrective measures.

5. In-efficient refrigerant flow rate due to less charge in the system. Check the correct pressure of refrigerant in the system.

6. In-efficient compressor, which is not compressing the gas efficiently because of heavy wear and tear or loose driving belt. Take necessary action.

7. Condenser becomes ineffective and need cleaning and servicing.

8. Evaporator coil is less effective due to heavy frost or excessive oil in tubes. So,defrost it and drain the oil, if any.

9. Faulty thermostatic control valve, which does not increase the flow rate of refrigerant liquid to evaporator on increase in superheating in evaporator or increase in cooling load. Check the feeler bulb and capillary. Also check for sticky valve.

10. In case of float valve, the valve may stick in the valve seat thus starving the evaporator from liquid refrigerant. Float valve needs servicing.

v. Heavy Cooling/Cold Room is too Cold

1. Fault in temperature controller setting.

•  Setting of temperature is disturbed and it is set for a lower temperature. Reset it.
•  Temperature controller is not working i.e. is not switching off the compressor

when designed temperature is attained. Check and adjust it.

•  Temperature sensing probe is placed at some dead zone, which is warmer than the cold room. Place it correctly.
• Faulty probe which is not giving correct temperature. Replace the probe.

vi. Excessive Power Consumption of Refrigeration Plant

1. Due to increase in cooling load by

•  Increase in temperature of products at the time of storing.
•  Surrounding temperature is very high due to hot season.
•  Doors of cold room are kept opened for longer times than required.
•  Door sealing/ gasket has worn out
•  Insulation may become less effective by absorbing moisture from any broken layer

2. In-efficient compressor which need overhauling or replacement.

3. Capacity of condenser is either small or it has become less effective due to other reasons.

4. Capacity of evaporator is small than the designed load or it has become less effective due to other reasons.

5. Quantity of refrigerant is either more or less than that required i.e. suction and discharge pressures are not in designed range.

All the above causes are required to be rectified for reducing the power consumption or running cost of plant.

vii. Noisy Operation

If any part of refrigeration system is producing abnormal noise during running of plant, mostly it itself indicates that what is wrong with it. And so one can judge easily by common sense that what remedial action is required.As the compressor has many moving parts, it can produce different types of abnormal sounds due to any type of wear and tear, which should be checked by some expert maintenance staff.

Preventive Maintenance of Controls of Refrigeration System

We know that there are different types of indicating and safety devices and also the expansion/control valves in a refrigeration plant. These devices sense the various properties like pressure, temperature, flow, liquid level etc. of refrigerant flowing indicates the value of these properties so that the operator and maintenance staff may see these values and have an idea of properly functioning of the plant. The safety devices sense the value of these properties and control accordingly the starting and stopping of the plant to keep these properties in a set range so that to ensure the safe and efficient working of the plant. The expansion/control valves also automatically control the flow and expansion of refrigerant through the system depending on the cooling load and evaporator temperature. In this way we see that the safe and efficient working of the whole plant depend on proper functioning of these control and safety devices just like the functioning of our body depend on functioning of our mind. Thus the ‘timely checking, setting or replacement’ of all the indicating/control devices is very much important for the safe and efficient operation of the plant. Many times a minor fault in the setting/adjustment of a control device greatly increase the wear and tear and hence the operation and maintenance cost. Controls should be checked at least four times a year, or often, if there is a fine adjustment to make. Making adjustments before hot weather operation is particularly important. The care and maintenance required in each type of these devices is discussed one by one as follows:

i. Thermostatic Expansion Valve

It is the main control valve in a refrigeration plant, which controls the flow of refrigerant depending on amount of superheat in the evaporator. It should be checked frequently at least once a month. Under normal operation, these valves are set for10 o C superheat. But through normal wear in the needle valve and valve seat or also sometimes because of a large particle of dirt or any other foreign material, the valve may open wider even at less superheating. It results the excessive flooding of refrigerant in the evaporator, which will cause liquid traces of refrigerant to pass on to the suction of compressor causing damage to the compressor and loss in efficiency. On the other hand if sometimes the gas leaks out of the feeler bulb, the valve will be closed due to spring action and will not allow liquid refrigerant in the evaporator, thus stopping the cooling. The thermostatic valve should be serviced or replaced as per the manufacturer’s recommendations.

ii. Float Control Valve
It keeps the liquid ammonia at proper level in the evaporator. If this valve is sticking closed, the evaporator is starved of liquid, which cuts its cooling capacity. It increases the running time for compressor. On the other hand if due to normal wear and tear or due to any dirt at the valve seat, the valve remains open, even at proper liquid level, it allow too much liquid in the evaporator. It may cause liquid traces of refrigerant to pass on to the suction of compressor. It not only causes damage to the compressor, but also circulates more ammonia than is necessary, which means a loss in efficiency. Thus float control valves should be checked very often, and float and valve kit should be replaced at least once every 6 months or more frequently if they do not respond to adjustment.

iii. Solenoid Valve

It is an electrically operated valve in which the force of a magnetic coil operates valve-stem. Normally, it remains in closed position, but when electric current is supplied to the magnetic coil, magnetic force generates, which open the valve.When current is stopped, it again comes to the closed position. These are used tostop the flow of either liquid or gas at different points in the system to isolate that particular cooling unit or room from the main suction and liquid lines. These valves are usually trouble-free, except for the burning of the magnetic coil, which lifts up the valve stem. On burning of magnetic coil it needs replacement.

iv. Back Pressure-Regulating Valve

In some refrigeration systems ‘back pressure regulating valve’ is fitted in the suction line of compressor. The function of this valve is to maintain a constant suction pressure and also so the temperature in the evaporator. Any faulty working of this valve may cause the temperature in evaporator too low allowing frost or ice to accumulate between the fins of evaporator and clog the entire unit. Hence regular check-up and setting for accuracy is needed for the back pressure-regulating valve.

v. Relief Valve

At some top point above the condenser, relief valve is given for purging of air and to release excessive pressure in the system. After the excessive pressure has been brought down to normal, it should reseat automatically. However, a particle of dust can hold the valve slightly open and allow ammonia to leak out. If it remains unchecked, a large amount of ammonia can escape through it. So, the valve must be checked and reseated at regular intervals.

vi. Water Flow-regulating Valve

In the shell-and-tube type condenser, a water flow-regulating valve is fitted, which is usually actuated by the condenser pressure. This valve automatically regulates the flow of cooling water so as to keep a constant pressure in the condenser regardless of the variation in flow rate and temperature of refrigerant vapours. If this valve is not checked occasionally, many thousands of liters of water can go waste.

vii. Filter/Strainer

We know that the working of a control/expansion valve is very much sensitive to the presence of any dirt, scale or rust particle in the flowing refrigerant liquid, which may come through fresh charging or rusting in pipes. Any such particle can be filtered from the refrigerant liquid before flowing to the control valve. Thus to install a filter/ strainer before the control valve in the refrigerant line is a usual practice.On running of the plant, this filter/ strainer may get clogged after some period and then needs to be cleaned/ washed or replaced. It should be checked at least once a year.

viii. Pressure Gauge
These are the main indicating devices, which give us the idea that how the refrigeration system is operating. After being in use for some time, the pointing needles of these gauges may either stick or show too high pressure. Any false readings may lead to wrong adjustments, which will affect proper functioning of the refrigeration system. There is an adjusting screw by which the pointing needles can be set to zero. If gauges become rusty and hard to read, replace them, as the cost is very small when compared with the loss due to false readings.

ix. Thermometers

These are installed in the various suction lines, compressor discharge lines, cooling water inlet and outlets, storage rooms, and at other convenient points in a refrigeration plant to indicate the temperature of flowing fluids. These should be frequently checked to see that they are not broken, easily visible and installed properly at the site.

x. Rotameter and Watermeter

These are some other types of indicating devices, which indicate the flow rate of refrigerant and cooling water through the condenser. These should be in working condition and should be replaced if not repairable.

xi. Temperature Controller

It must be kept properly adjusted and set to the required value, as wrong adjustments either will keep higher temperatures than desired in the refrigerated space or lower temperature which will tend to keep the compressors operating longer. The higher temperature than designed may effect the preservation of food products in a cold storage. And lower temperature than that designed will increase the power consumption of the plant. So any wrong adjustment or improper functioning of temperature controller effects the working of refrigeration plant to a large extent and its timely checking and setting must be taken care of.

xii. L.P and H.P. Cut-outs

These should be checked occasionally for their correct operation. Any wrong adjustment will either switch off the compressor unnecessarily or will not work even at dangerous limit of pressure in the suction and discharge line and leave the system unsafe. Thus from safety point of view correct setting of these pressure switches must be ensured.

Preventive Maintenance of Condenser and Evaporators

The main function of condensers and evaporators is to exchange heat between the refrigerant and outside environment. Thus these are maintained so as to exchange the maximum possible heat or to keep the heat transfer resistance at a minimum level. Various types of routine problems that come in operation of condenser and evaporator and their remedies are discussed as below:

i. Cleaning and De-scaling of Condenser
A condenser is either air-cooled or water-cooled or both i.e. evaporative type.Whatever be the type of condenser, its function is to transfer the latent heat of refrigerant vapours flowing inside the bundle of tubes to the surrounding air or water. This heat transfer can be badly affected if the outside surface of tubes is dirty or any scale or sludge is deposited on it. This problem is more serious in case of water-cooled condenser where the naturally available water is used for cooling purpose in condenser. This water may contain many types of salts and minerals,which deposit in the form of scale or sludge on the tube surface and cut the heat transfer. Deposit of dirt also cannot be avoided because the condenser is situated in open. So what we can do is only to follow the proper cleaning and de-scaling schedule to maintain the effectiveness of condenser. A compound named as potassium permanganate is generally added to the water to kill the fungi and keep away the slime deposit from accumulating upon the surfaces.

ii. Purging of Non-condensable Gases and Air
Theoretically, a refrigeration system is closed system. That means only the pure refrigerant flows through the system from one component to the other. But practically it is not completely closed. During the suction stroke in compressor, the pressure of refrigerant vapours above the piston becomes lower than that of atmospheric pressure. Due to low pressure in the compressor, outside air can leak through stuffing box and piston rings into the low-pressure refrigerant vapours. Air can also leak through the repair of compressor and other leaks in the suction line. Some air may also be initially present in ammonia while charging in the system.

Now, you must be thinking that what is the bad effect of presence of this air in ammonia vapours. Actually what happens, when the system runs, the air and other non-condensable gases do not condense in the condenser because their condensing temperature is considerably higher than the condensing temperature of ammonia.And due to their lightweight they collect at the top of the condenser or other high points in the system. The presence of these causes an excess condensing pressure, which increase power consumption. These also put an extra heat transfer resistance and make the condenser ineffective. Thus the air and any other non-condensable gas must be extracted or thrown out of the system for its efficient working. For this, the system is first shut down before allowing the purging of these gases through relief valves.

iii. Oil Traps Draining

In the compressor of a refrigeration plant, the lubricating oil layer between cylinder and moving piston come in contact with refrigerant vapours. Now if the refrigerant is miscible with lubricating oil, it will dissolve in the lubricating oil but the traces of lubricating oil will not mix in refrigerant vapours. It is the case with freon refrigerants and the system is safe against the problem of oil traces going with vapours.

However, the choice of lubricant oils in freon compressor need very much care because if lubricant oil is not of right type, its lubricating property can be damaged by dissolved freon refrigerant vapours. On the other hand if refrigerant is immiscible in lubricating oil, it will not dissolve in lubricant oil but the oil traces will mix in refrigerant vapours. It is the case with ammonia, which is most frequently used in large size refrigeration plant of a dairy due to its other favourable properties. These oil traces will go to condenser and evaporator along with the ammonia vapour and form a layer on the inside surface of evaporator tubes and decrease the heat transfer. If some method is not used to prevent this oil to accumulate in evaporator,it will make the evaporator ineffective. For that oil traps are used after compressor which separates out the oil traces from ammonia vapours and prevent these to pass on to the condenser and evaporator tubes. In this way this trapped oil accumulates in the oil trap and must be drained at regular intervals. And whatever amount ofoil is added to a crank-case of compressor the same must be drained out from the
oil trap.

iv. Purging of Oil out of Evaporators

In spite of that oil trap is installed before condenser in an ammonia refrigeration plant. Some oil traces may pass through it and enter the condenser and evaporator coil along with refrigerant. As the temperature is low in the evaporator, the oil traces accumulate at the inside surface of evaporator coil and cut the heat transfer.In this way lubricating oil layer is more harmful in the evaporator and makes it ineffective. This oil must be purged out at regular intervals by warming up the evaporator coil and then draining the oil after disconnecting the evaporator from rest of the system.

v. Defrosting of Evaporator Coil

This is the another problem with evaporator coil, which cuts the heat transfer, and makes the evaporator ineffective. As the temperature at the surface of evaporator coil is very less, the moisture present in the surrounding air of cold room condenses on the coil surface and then freeze there in the form of frost. This frost puts an extra resistance to exchange of heat from air to refrigerant inside the tubes. This problem is common in case of natural convection evaporator. But in most of the refrigeration plants, a blower type (forced convection) cooling coil is used in which a blower pass the air over the cooling coil with high speed. The speedily passing air does not allow the frost to accumulate on the coil. But sometimes due to very low temperature or failure of blower, frost may come and need to be removed.

There are many types of defrosting methods used in the plant:

•  By blowing hot air over the evaporating coil.
•  By using electric heater for heating the evaporator and melting the frost.
•  By blowing hot discharge gas in the evaporator coil, etc.

Preventive Maintenance of Compressor and Checking its General Efficiency

The compressor is the most important component of a refrigeration system. As it has many moving parts, wear and tear is more. So it requires more care and maintenance. The routine maintenance of compressor has been divided into three different phases as mentioned below:

i. Daily Check-up

Some daily check-ups as given below are made on start of compressor:-

1. Lubricating oil level in the crankcase should be slightly above the middle of sight glass.

2. There should be no oil leakage.

3. The oil pressure and discharge pressure should be in the given range.

4. Driving belt should not be too tight or too loose.

5. No electric wire or connection should be loose or in poor condition.

6. There should not be any abnormal sound or vibration.

7. There should not be any overheating.

ii. Servicing of Compressor

To minimize the wear and tear, the servicing of compressor should be done as per the schedule recommended by manufacturer. Oil should be changed at regular intervals. The oil strainer should be opened and the screen should be removed and cleaned at the time of each oil change. Use only the grade of oil, which is suitable for the type of compressor and also recommended by the manufacturer.The main shaft of compressor is rested on the stuffing box on the crankcase side of compressor. On rotation of shaft, the bearing surface of stuffing box and shaft undergoes heavy wear and tear and the lubricating oil inside the crankcase start leaking through it. So it needs timely care. Do not simply tighten the bolts and replace the seal to prevent the leakage, but check to see if the crankshaft is scored.

If so, replace the shaft rather than putting in a new seal. No kind of seal will stop leakage if the shaft is badly scored and arrangements should be done to repair or replace it.This is the routine care of compressor against normal wear and tear of moving parts. But this care is not complete. Under the preventive maintenance schedule, check the condition of compressor by opening it up at least once a year. Pay particular attention to cylinder liner, piston rings, valves, valve seats and all bearings.If there is any undue wear and tear of any part, repair or replace it accordingly.

On scoring of piston rings, cylinder liner, suction and discharge valves and their seats, the efficiency of compressor to increase the pressure of refrigerant gas will fall down. Also the leakage of refrigerant vapours through the piston rings will increase, which will be a total loss. Mixing of oil traces in the refrigerant gas will also increase. However before dismantling of compressor, a general check on its efficiency can be made. If the efficiency has actually fallen down then the compressor can be repaired accordingly.

iii. General Check on Efficiency of Compressor

A general checking of the efficiency of compressor can be done at any time by adopting the following steps:

1. Close the suction and discharge service valves of compressor.

2. Attach a pressure gauge to the discharge valve gauge port and a compound pressure gauge to the suction valve gauge port. (Compound gauge shows pressure and vacuum of refrigerant simultaneously).

3. Start the compressor and throttle the discharge valve until a head pressure of about 125-psi is maintained.4. Close the suction valve slowly and when it is completely closed, note the vacuum on compound gauge fitted on suction gauge port.

5. If the compressor produces and also maintain vacuum of 20 inches or more, it may be considered as operating satisfactorily. Otherwise it may be found that wear and tear has occurred in the compressor.

Another simple test in case of a multi cylinder compressor is to place the hand on the discharge line of each cylinder. If there is difference in the temperature it indicates that one of the cylinders is not efficient. If a cylinder is overheating, it may be due to some dirt under compressor valve or undue wear in the valve and valve seat.

After checking the condition of a compressor, make the necessary repairs if the wear seems to be more than normal. Do not wait until the compressor breaks down before replacing worn parts, because the breakdown may come at a time when the compressor is desperately needed. In this way, by properly following the preventive maintenance the compressor remains in a good condition, which not only increases its life but also keeps operating costs and capital expenditure to a minimum.

General Check Up of a Refrigeration Plant

Some general check-up must be followed before and after running of the refrigeration plant. Some of the important steps are given below:

1. Check all the electrical wires and connections and make sure that all these are in order and well tight.

2. Any wiring diagram and instruction manual should be carefully read and followed.

3. Check the HP, LP cutouts, water flow control valves, pressure regulating valves and all other safety controls and adjust these, if required.

4. Check the temperature controller for its normal functioning.

5. Observe oil level in the compressor crankcase. It should be slightly above the center of the sight glass. Use only proper grade of oil as recommended by the manufacturer for the particular type of compressor.

6. Switch on the condenser fan motor or water pumps in case of water-cooled condenser.

7. After ensuring the proper working of condenser, compressor is switched on.Also check the drive belt tension before starting the compressor. In case the compressor does not start, either there may be some electric problem or some problem in setting or working of LP and HP cutouts or temperature controller.Check all these satisfactorily. If the compressor starts then do not leave system unattended till it has reached its normal operational conditions and again check the following points.

8. The line voltage and load drawn by each phase should be recorded by using proper instruments. The load drawn by each phase should be equal.

9. Check the suction and discharge pressures in the pressure gauges. If these are not within designed limits, determine, why it is so and take the corrective action.

10. Check the flow rate of refrigerant in sight glass or in ‘rotameter’ and if there is an indication of less quantity of refrigerant, check for any leakage. Then switch off the plant and thoroughly repair the leaky points before adding refrigerant.

11. Again observe oil level in the compressor crankcase through the sight glass and add oil if necessary.

12. If there is a thermostatic expansion valve, feeler bulb must be in positive contact with the suction line and it should be filled with the right amount of gas. It should also be ensured that there is no liquid refrigerant feedback to the compressor crankcase.

13. Recheck all the control valves and safety devices for proper operation and adjust these if necessary.

Maintenance and Repair of Commercial Refrigeration System

We learnt about the principles of refrigeration, components of refrigeration systems and various types of refrigeration equipment used in the dairy industry. Until now we know only that how these refrigeration equipment are constructed, what is the function of each component, what are different refrigeration processes, etc. But we don’t know what steps are to be taken and what are the steps involved in initial inspection, efficient operation and control in running of a commercial dairy plant. What type of periodic check up and periodic maintenance is required for efficient running of the plant? What trouble may come during running of the plant and what can be its solution? Or in the nutshell what should be done,what should not be done during operation, control and maintenance of refrigeration equipment of a dairy plant. All this will be discussed in this chapter. One should go through this unit, very carefully as it is of most practical utility. The knowledge gained in previous chapters will serve as a base to understand all the basic principles of operation and maintenance.

To ensure safe and efficient running of plant, a routine checkup and maintenance schedule is followed. By following this schedule, various parts/components are either readjusted or repaired or replaced. In spite of routine check up and keeping maintenance of the refrigeration plant, some problem may further come during its operation. To each problem there are some causes and remedies which a plant operator should know about. If the causes and remedies are minor sometime he can himself rectify these. However, if these are major, must be attended by expert maintenance staff.

The routine check up and periodic maintenance including servicing, cleaning, setting etc. is given the name as ‘preventive maintenance’. Preventive maintenance is done even if the refrigeration plant is working properly. It minimizes the sudden breakdown of plant at the peak load, so prevents failure and saves maintenance cost. On the other hand, any repair, which is done at breakdown of plant, is given the name breakdown maintenance. Breakdown maintenance need to be immediately attended and is generally very costly. So it is usually minimized by properly following the preventive maintenance schedule.

Insulation Application & Management

After discussing various types of insulating materials and their properties, it is now the turn of knowing about their application at the site and proper care and maintenance thereafter. One major threat to the insulating material is moisture. So, it should be applied and maintained in the way that it should always remain waterproof and air-proof. Other major consideration is for mechanical strength required at the place of application. It is more important in case of floor insulation when it has to bear load also. Now we will discuss one by one the insulation of different parts of a cold storage.

i. Insulation of Refrigeration Pipes

In a cold storage, the compressor sucks ‘low-pressure and low-temperature vapours’ from evaporator coil through the suction line. This suction line generally passes through normal temperature area outside the cold room. If it is not insulated, heat from surrounding area will pass naturally to low-temperature vapours and increase their temperature. The bad effect of this increase in suction temperature is the increase in compressor load. Hence, the suction line of compressor including pipe fitting must be insulated to retard the increase in suction temperature.

Before applying the insulating material pipes, & fittings must be cleaned and then painted with a heavy coat of waterproof asphalt paint. Then the pipes are covered with ready made cork or thermocole lengths which come in half hollow sections,matching with diameter of pipe as shown in Fig.7.7. The insulation covering should be securely wired with copper wire. The seams and any hollow space between the insulation packing should be filled with cork-dust. Then again the whole surface is coated with waterproof asphalt paint.

ii. Insulation of Cold Room

This is the main part of application of insulation. You already know the importance of effective insulation of a cold room. If properly applied and maintained, the insulation reduces the compressor running time to a great extent and saves the running cost of plant. No area of a cold room should be left without insulation. We will discuss here the insulation of different areas of a cold room.

a) Insulation of walls

Walls of a cold room are generally made of brick and cement. As shown in Fig.7.8 the inside surface of wall is first finished by applying water proof cement plaster or mastic. Then the corkboard or sheets of any other insulating material are fitted on the whole surface with nails. Care should be given on making of good tight joints. Thereafter a second layer of water proof cement plaster is applied by first fitting wire mash on the surface of corkboard. The wire mesh helps in applying and staying in place the layer of cement plaster.

b) Insulation of ceiling

It is based on the same general principle as that of wall insulation. If possible, the insulation is applied on the top of ceiling slab. In case if it is applied at the bottom of slab from inside the cold room, it is necessary to use more fastening nails or it may be hanged by a strong iron bar structure.

c) Insulation of floors

It is also based on same principle as in case of wall and ceiling but the difference is that the floors should be mechanically strong to bear the weight of storage material. Generally a thick concrete floor serves our purpose. However, when insulating material is applied, it should be mechanically strong and also it should be applied as air tight and water tight.

d) Insulation of doors

The double wall galvanized-sheet insulated doors are available as ready made and fitted on the site with strong hinges and other fittings. All the door fittings also must be galvanized i.e. of corrosion resistant material. The gasket or door seal is fitted on the circumference of door and it fills the annular space between door and doorframe. The door-seal forms the airtight joint between door and door frames and hence serves as an effective insulation against heat and moisture.

Types of Insulating Materials

There are several types of insulating materials either available in natural form or manufactured in the convenient size and shape from natural materials. So these are broadly classified into two groups as follows:

(i) Natural insulating materials

(ii) Insulating materials manufactured from natural materials.

i. Natural Insulating Materials

Some materials which are already available in nature and posses good insulating property are cotton, animal hair, wool, asbestos, silica, rock or slag, saw dust,wooden shavings, cork etc. Many times these can be used directly by filling in the space between double walls of refrigerated space. But, sometimes a binder is mixed in some of the natural materials to apply them in the shape of plaster coverings.

ii. Manufactured Insulating Materials

These are manufactured from naturally available insulating materials. Some of these materials are as discussed below:

a) Cork Board: These are manufactured by drying the cork and then pressing to shape by baking at moderate temperatures. On baking, the natural gum binds the mass together leaving the air cells in it. These may be available in varying thickness. These are commonly used in cold storage of dairy plant.

b) Thermocole: It is an outstanding insulating material used in cooling application.Its closed cell structure gives it excellent resistance to moisture. It also has good mechanical strength.

c) Kopak: It is a silky fiber taken from Caba tree. It is manufactured in the form of slabs which are light in weight. It can also be used in its loose form.

d) Celotex: It is produced from sugarcane.

e ) Insulating Wool: - Some insulating materials are melted and blown by air into fibrous form so as to give insulating material in the form of wool. This wool contains too much air space, which increases its insulating property.

Properties of Insulating Materials

In the previous topic, we came to know that how insulation is important in cold storage and how the rating of insulation is done. Depending on that, we can decide the type of insulating material and its thickness in a particular application. But the heat resisting property of insulating material is not the sole requirement in refrigeration. There are some other properties also which play an important role in the selection of an insulating material. One of these properties is mechanical strength.

It comes in picture when some insulating materials are also to be used as load bearing floors and to form self-supporting partitions. Another important property required in an insulating property is that it should be moisture resistant. It should not deteriorate or loose its heat resistive property in the event of moisture collection on it. Thus some other properties, which are considered while applying an insulating material, are as mentioned below:

a) Mechanical strength

b) Moisture resistance

c) Low odour level

d) Safety to health

e) Inflammability

f) Repellence to insects

Rating of Insulation

In general all materials offer some resistance to the flow of heat. But those materials, which offer heavy resistance to the flow of heat through them, come in the category of insulating materials. And the heat-conducting behavior of a material is indicated by the property named as thermal conductivity. The letter ‘k’ denotes it. Thermal conductivity of a material is equal to the amount of heat in kcal transmitted per hour per square meter area per degree Celsius difference in temperature for one-meter thickness. So, if we know the thermal conductivity of an insulating material, k and all other conditions i.e. area of heat flow, A, thickness of insulation material t and temperature inside and outside the cold room in T i and T o , we can calculate the amount of heat passed in kcal/hour through the insulating material as per the given formula:

Heat passed in kcal/hour =((T o - T i ). k. A)/t

We can judge from this formula that heat flow will decrease if the value of k is less, heat flow area is less and thickness of insulating material is more. The factor t/ k is called heat resistance of a material and it should be higher for an insulating material. However, in actual the wall of a cold storage or any refrigerated cabin is not made of insulating material only. It is made up of more than one material.

For example in the wall of a domestic refrigerator outside layer is of sheet metal and inside layer is of plastic and in between the insulating material such as glass wool is fixed as shown in figure.

The calculation for overall thermal conductivity of this type of wall is made like:-

Where, k 1, k 2, k 3 are the conductivity of different material used and t 1 , t 2 , t 3 are the thickness of layers of those materials.In this way, rating of insulation is done and the calculation for heat entering to cold space through all the walls, roof, floor, etc. is made.

Design Considerations

Until now we have discussed that cold storage is required to store the food/dairy products at low temperature so as to preserve these for a set period of time. Thecooling coil of refrigeration system installed inside the cold storage extract the heat of the product and lower down their temperature. After lowering down the temperature to a set range, this temperature should be maintained so as to preserve the products. To maintain that temperature it is necessary to extract all that heat energy which enters the cold storage by some means. Thus to know the capacity of refrigeration plant required, we must know the amount of heat to be extracted in a given period of time i.e. the cooling load on refrigeration plant. There are various types of cooling loads that are taken into consideration in a cold storage.

All these are explained as follows:

i. Heat of Products

This is the main part of heat, which is required to be extracted and is known as major cooling load. When the products are stored in a cold storage, these are initially at higher temperature. To lower down their temperature, their heat is required to be extracted. It is divided in three groups:

(a) Chilling load above freezing

Thus in case of chilling only, first type of load is considered. In case of freezing,first and second type of load are added and taken into consideration. In case of deep freezing, all three types of loads are taken into consideration.

ii. Heat Conducted in through Walls, Roof and Floor of Cold Storage
A cold storage is generally situated at such a place in a dairy, which is not exposed to direct sun. All the walls, roof, floor are insulated i.e. insulating material is fixed around which prevent the heat flow from surroundings. But, in spite of that there may be some cooling loss to the surroundings, however, it may be negligible,depending on the difference in temperature inside and outside of cold storage and type of material of wall and insulation. This continuous heat flow through walls,roofs etc. is added in the cooling load of refrigeration plant.

iii. Heat Gain from Product Handling Labourers

The products are stored and arranged in cold storage by labourers. They also loose some heat in cold store due to its low temperature. This heat also puts some cooling load on refrigeration system.

iv. Heat Gain from Lighting and Power equipment

There may be a variety of appliances like fan or blower motor, any safety device,lights etc., which continuously loose heat in cold storage and put some cooling load on the refrigeration plant.

v. Heat Gain due to Infiltered Air
While loading or unloading the product in cold storage, the door is opened. During this period, the cold air of cold storage escapes out and hot air from surrounding come in, putting extra cooling load on the system, which depends on the time for which door is kept opened.

vi. Heat Gain through Ducts

After evaporation in the evaporating coil, the refrigerant gas, which is at lower temperature, flows to compressor through pipes. These pipes are exposed to higher temperature of surrounding and some heat gain to refrigerant gas takes place.However, it can be generally reduced by properly insulating the suction line of compressors outside the cold store.

All the above-mentioned heat gains are required to be removed by refrigeration plant of cold storage. Thus all these contribute to the cooling load which is considered before designing or finding out the capacity of refrigeration plant required.