Working Mechanism of a Centrifugal Pump

Working Mechanism of a Centrifugal Pump

A centrifugal pump is one of the simplest pieces of equipment in any process plant. Its purpose is to convert energy of a prime mover (a electric motor or turbine) first into velocity or kinetic energy and then into pressure energy of a fluid that is being pumped. The energy changes occur by virtue of two main parts of the pump, the impeller and the volute or diffuser. The impeller is the rotating part that converts driver energy into the kinetic energy. The volute or diffuser is the stationary part that converts the kinetic energy into pressure energy.

Note: All of the forms of energy involved in a liquid flow system are expressed in terms of feet of liquid i.e. head.

Generation of Centrifugal Force

The process liquid enters the suction nozzle and then into eye (center) of a revolving device known as an impeller. When the impeller rotates, it spins the liquid sitting in the cavities between the vanes outward and provides centrifugal acceleration. As liquid leaves the eye of the impeller a low-pressure area is created causing more liquid to flow toward the inlet. Because the impeller blades are curved, the fluid is pushed in a tangential and radial direction by the centrifugal force. This force acting inside the pump is the same one that keeps water inside a bucket that is rotating at the end of a string.

Conversion of Kinetic Energy to Pressure Energy

The key idea is that the energy created by the centrifugal force is kinetic energy . The amount of energy given to the liquid is proportional to the velocity at the edge or vane tip of the impeller. The faster the impeller revolves or the bigger the impeller is, then the higher will be the velocity of the liquid at the vane tip and the greater the energy imparted to the liquid.This kinetic energy of a liquid coming out of an impeller is harnessed by creating a resistance to the flow. The first resistance is created by the pump volute (casing) that catches the liquid and slows it down. In the discharge nozzle, the liquid further decelerates and its velocity is converted to pressure according to Bernoulli’s principle.

Introduction to Centrifugal Pumps

Introduction

The operating manual of any centrifugal pump often starts with a general statement,“Your centrifugal pump will give you completely trouble free and satisfactory service only on the condition that it is installed and operated with due care and is properly maintained.” Despite all the care in operation and maintenance, engineers often face the statement “the pump has failed i.e. it can no longer be kept in service”. Inability to deliver the desired flow and head is just one of the most common conditions for taking a pump out of service. There are other many conditions in which a pump, despite suffering no loss in flow or head, is considered to have failed and has to be pulled out of service as soon as possible. These include seal related problems (leakages, loss of flushing, cooling, quenching systems, etc), pump and motor bearings related problems (loss of lubrication, cooling, contamination of oil, abnormal noise, etc), leakages from pump casing, very high noise and vibration levels, or driver (motor or turbine) related problems.

The list of pump failure conditions mentioned above is neither exhaustive nor are the conditions mutually exclusive. Often the root causes of failure are the same but the symptoms are different. A little care when first symptoms of a problem appear can save the pumps from permanent failures. Thus the most important task in such situations is to find out whether the pump has failed mechanically or if there is some process deficiency, or both. Many times when the pumps are sent to the workshop, the maintenance people do not find anything wrong on disassembling it. Thus the decision to pull a pump out of service for maintenance / repair should be made after a detailed analysis of the symptoms and root causes of the pump failure. Also, in case of any mechanical failure or physical damage of pump internals, the operating engineer should be able to relate the failure to the process unit’s operating problems.

Any operating engineer, who typically has a chemical engineering background and who desires to protect his pumps from frequent failures must develop not only a good understanding of the process but also thorough knowledge of the mechanics of the pump. Effective troubleshooting requires an ability to observe changes in performance over time, and in the event of a failure, the capacity to thoroughly investigate the cause of the failure and take measures to prevent the problem fro m re-occurring.

labyrinth piston compressor

LABYRINTH PISTON

These are vertical type reciprocating. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. In labyrinth piston compressors, an extremely large number of throttling points provide the sealing effect around pistons and piston rods. No contact seals are used. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also
having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. This increases the durability, reliability and availability of the compressor along with its economic operation.

These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant. This is unique application where lubricants are not allowed in the cylinders, which is true in case of oxygen compressor, where safety is the most important. It is also employed for applications where the process gas is heavily contaminated with the impurities.

Piston and piston rod are guided by the crosshead and the guide bearing which are located in the oil lubricated crankcase. The oil is supplied by the crankshaft driven lube oil pump
The distance piece separates the gas compressing section from the oil lubricated crankcase. Where process gas can be permitted in the distance piece, then open distance piece type compressors are used. When strict separation of cylinder from the crankcase is essential and at the same time no ambient air is also allowed in the distance piece, then Nitrogen purge is provided. Crankshaft is provided with mechanical seal to prevent the gas leaking in to the atmosphere.

ADVANTAGES:
1. Reliable operation
2. Safe for operation , environment
3. Economical
4. High availability
5. Less floor area is required.
6. As rider rings, piston rings and lubrication is not required the valve life is more.

classification of reciprocating compressor

Reciprocating Compressors can be classified as follows based on

I. Cylinder lubrication:
1. Lubricated type & Non-lubricated type
2. Dry running piston rings
3. Ringless or labyrinth type.

II. Cylinder cooling:
1. Air cooled
2. Water cooled
III Cylinder loading
1. Single acting
2. Double acting
IV Cylinder arrangement,
1. Vertical inline or V-type
2. Horizontal opposed balanced.

CYLINDER LUBRICATION:

Lubricated Compressors :
Generally big reciprocating compressor cylinders are lubricated to avoid wear and tear of liner, piston rings, rider rings and stuffing box. Lubricants are injected in drops and are lost with the process gas.

Lubrication of cylinder reduces wear of parts, enhances life of parts and also reduces gas discharge temperature. It is necessary for the lubricant to be compatible with the process gas and down stream system. Generally recip compressors above 100 KW are lubricated type.

Non-Lubricated Type :
There are many services in which oil in any form in the compressed gas, is not acceptable such as instrument air compressor and some process gas compressors. In such services oil is not injected in the cylinders, instead wear parts are made of soft material with low co-efficient of friction such as PTFE,CFT etc. The wear rate of stuffing box packing, rider rings piston rings and cylinder/liner surface may be comparatively more.

Process requirements and gas to be compressed dictate whether to use a compressor with lubricated or non-lubricated cylinder. Some chemical processes do not permit use of lubricant in the system, due to quality problems, catalyst-poising etc.

Following factors must be considered,
1. Non-lubricated compressors cost more than lubricated compressors
2. Non-lubricated compressors require more power
3. Non-lubricated compressors require more maintenance, labyrinth compressors
being exception.

TYPES OF COOLING:

Air Cooled type :
This type of compressors has fins cast as part of the cylinder to dissipate some of the heat generated by the compression of the gas. In some compressors vanes are cast as part of the flywheel or sheave to act as fan to help remove the heat from the cylinder surface. This type of cylinder cooling is used in small portable compressors.

Water-cooled type:
Water-cooled compressors are most common in industry. It is impossible to sustain cooling with air in big compressors where heat generation is very high. Water jackets are cast as part of the cylinder. Water is circulated in the cylinder jacket from external source.

In some case the cylinder jacket temperature is to be maintained few degrees higher than the process gas temperature to avoid condensation during standstill, this is possible with water-cooled cylinders by maintaining the water temperature with use of heaters.

TYPES OF CYLINDER LOADING:

Single acting
In single acting cylinder compression takes place only on one side of the piston and valves are installed only on that side.

Double acting
In double acting cylinder compression takes place on both sides of the piston. When one side is in compression, the other is in suction.

CYLINDER ARRANGEMENT:

Horizontal Type:
This type of arrangement is most common in industries. Multi cylinder reciprocating compressors with horizontal cylinders are very often designed as balanced opposed type. Balanced opposed frame is characterized by adjacent pair of crank 180 ° out of phase and separated by crank webs only. With this configuration inertia forces are balanced. The balanced-opposed design is separable frame.

Vertical Inline or V-type:
Vertical in line or V-type are used for small or moderate compression ratio and duty. Normally this type of compressors cylinder arrangement are non-lubricated type and occupy less space.

Labyrinth piston compressors
These are vertical type. In this type of compressor, rider rings and piston rings are not used as in case of horizontal type. Piston is having labyrinth type piece at the centre called skirt. Cylinder is also having serration like labyrinths on its inside surface. Piston is not in direct contact with the cylinder and close clearance is maintained in between both. These machines are very popular in the service where total dry operation is required as in case of polypropylene and polyethylene plant.

multistaging with inter cooling

It is not always desirable or possible to achieve the required rise in pressure in a single compression stage. In multistaging gas discharge from the first stage is cooled in the inter cooler up to suction temperature of the first stage before going to the second stage. This is called as perfect intercooling.

Advantages of multistaging:

1. Good volumetric efficiency as compression is done in more than one stage and hence compression ratio is controlled.
2. Lower discharge temperature and hence selection of material of construction for cylinder and its components and results in smaller size of subsequent stages.
3. Reduced work of compression, as due to intercooling, compression is closer to isothermal (gives rise to minimum work of compression). This results in to saving of power and smaller sizes of subsequent stages.
4. Limits pressure differential. This reduces excess strains in the frame.

schematic of reciprocating compressor

Suction:
When the piston moves towards BDC (backward stroke), the pressure within cylinder on the top of the piston drops below suction pressure in the header, thereby forcing suction valve to open and allows gas in the cylinder.

Compression:
When the piston moves from BDC towards TDC (forward stroke) pressure on the top side of the piston start increasing, thereby closing the suction valve.

Discharge:
As the piston approaches TDC, the discharge valve opens as the pressure inside the cylinder on top side of the piston is higher than that in the discharge pipeline. Thus gas is discharged in to the header.

Expansion:
During backward stroke the trapped gas (between piston & cylinder end cover) called clearance volume, expands.

thermodynamic Process

Compression of gas is a thermodynamic process, and can be done in many ways; some of them are listed below.

• Isobaric Process – a process wherein pressure remains constant.
• Isothermal Process – a process in which there os no change in temperature.
• Isentropic Process – a process in which there is no change in entropy.
• Adiabatic Process – a process during which there is no external exchange of heat. PVk = Constant
• Polytropic Process – a process for perfect gases, follows the law PVn = Constant n=1 for isothermal process.

basic gas laws

First Law of Thermodynamics: Energy is neither created nor destroyed, but can be converted from one form to another form.

Second Law of Thermodynamics: Energy, which exists at various levels, is available for use only if it can move from a higher to a lower level.

Ideal or Perfect Gas Laws

Gases may consist of one specific gas or may be constituent of number of gases. An ideal or perfect gas is one, which obeys below mentioned Boyle’s Law and Charle’s Law. Perfect gases do not exist in practice.

Boyle’s Law – states that at a constant temperature, the volume of an ideal gas decreases with increase in pressure.

P1V1 = P2V2 = CONSTANT

Charle’s Law – states that at constant pressure, the volume of an ideal gas will increase as the temperature increases.

V1/T1=V2/T2=CONSTANT

Amonton’s Law – states that at constant volume, the pressure of an ideal gas will increase as the temperature increases.

P1/T1=P2/T2=CONSTANT

Avagadro’s Law – states that equal volume of gas under same condition of temperature and pressure contains same number of molecules.

balance quality for rigid rotors

Based on present experience, the ISO has categorized the normally available rigid rotors into various groups and have suggested the balancing levels for them in ISO-1940 specifications. The following table gives the level to which a rotor should be balanced.

QUALITY
GRADE 'G' ROTOR TYPES - GENERAL EXAMPLES
G 4000 Crankshaft drives of required mounted slow marine diesel engines with uneven number of cylinders.
G 1600 Crankshaft drives of rigidly mounted on large two cycle engines.
G 630 Crankshaft drives of rigidly mounted large four-cycle engines. Crankshaft drives of elastically mounted marine diesel engines.
G 250 Crankshaft drives of rigidly mounted last four cylinder diesel engines.
G 100 Crankshaft drives of fast diesel engines with six and more cylinder complete engines (gasoline or diesel) for cars, trucks and locomotives.
G 40 Car wheels, wheel rims, wheel sets, drive shaft; crankshaft drives of elastically mounted fast four-cycle engines (gasoline or diesel) with six and more cylinders; crankshaft drives for engines of cars, trucks and locomotive.
G 16 Drive shafts (propeller shafts, cardan shaft) with special requirements; parts of crushing machinery; parts of agriculture machinery. Individual component of engines (gasoline or diesel for cars trucks and locomotive; crankshaft drives of engine with six and more cylinders under special requirements.
G 6.3 Parts of process plant machines. Marine main turbine gears (merchant service), centrifugal drums, fans, assembled air craft gas turbine rotors, fly wheels, pump impellers, machine tool and general machinery parts, normal electrical armatures, individual components of engines under special requirements.
G 2.5 Gas and steam turbines including marine main turbines (merchant service), rigid turbo- generator rotors, turbo-compressors, machine tools drives medium and large electrical armatures with special requirements, small electrical armatures, and turbine driven pumps.
G 1 Tape recorder and photograph (gramophone) drives, grinding machine drives, small electrical armatures with special requirements.
G 0.4 Spindles, discs, and armature of precision grinders, gyroscopes.

what if the balancing fails

As pointed out earlier it should be precisely confirm unbalance is the only problem before implementing the balancing, as there are many different problems which are having similar vibration characteristics to unbalance. Yet, after the confirmation, all attempts to balance have failed, the next recommended step is to check the rotor for repeatability of unbalance readings. We have to simply stop the rotor and remove all trial weights and balance correction weights and rerun the rotor to the condition it was in when original readings were taken. Compare readings taken in this run with the original unbalance readings. They should be the same. These exercise may be done several times to verify the repeatability of the unbalance readings. If the two sets of readings differ significantly in amplitude and/or phase, this may be due to one of the following factors.

1. The actual configuration or shape of the rotor may not have been stabilized when the first set of original unbalance readings was taken. The rotor may have had a temporary 'sag', which has disappeared now that it has had an opportunity to to run.

2. The unbalance condition of the rotor may be changing from one run to the next. For example, a rotor that is loose on the shaft may assume a slightly different position on the shaft each time it is started and stopped. In addition, fans with hollow blades or hollow shafts may have accumulated dirt or water that changes location each time the rotor is started or stopped.

3. If nothing of the above observed, the rotor should be thoroughly examined for possible cracks on the shaft or rotor. If a repeatability check reveals that the rotor is repeating the unbalance readings from run to run some other problem is usually the reason and a more thorough analysis should be carried out to determine the cause.

shop balancing

Shop balancing is always preferred in case of new machines and in case of running machines where high degree of balancing is required. Moreover, some machines such as totally enclosed motors, pumps, compressors and others not easy to balance in-situ because extensive disassembly is required to gain access to the rotor for adding or removing balance weight. In these instances, the machine is disassembled and rotor is balanced on a balancing machine.
In-situ balancing will generally produce better results in terms of vibration. However, balancing on a balancing machine will generally produce a better balance. To clarify, the balancing machine is better at measuring and correcting for unbalance, especially two plane or dynamic unbalance. It can not compensate for field installation factors. Two plane balancing on a balancing machine usually produces better results than two plane balancing in the field because the compounding field installation factors like bearing clearances, support stiffness and resonance response, additional components attached to the rotor like coupling, keys, fasteners etc. and alignment are not present. The primary advantage of balancing machine operations is that the unbalance effect is directly measured. The disturbing factors that can cause 1xrpm vibration are not present, so unbalance can be evaluated without these other complications. Balancing machine operations are not so severely troubled by cross effect because they have very flexible supports or they employ plane separation technique. Balancing can be done by either of the methods described earlier depending upon type of rotor, type of unbalance, normal operating speed of the rotor, etc. Some basics of the balancing machines are explained in the coming sections.

balancing mthods

Before actually attempting to balance a rotor, there are few preliminary factors that need consideration. The most important include:

1. Determining the type of unbalance: This is important in deciding whether a single or two plane approach will be needed. If in doubt, use a two plane approach. A single plane problem can be solved by the with a two plane approach. However, a two plane problem can not be solved with a single plane approach.

As a general guideline the length to diameter (L/D) ratio of a rotor can be useful to determine whether single plane or two plane approach should be adopted for balancing, which is described as follows:

L/D ratio Single Plane Two plane
<0.5> 1000
>0.5 rpm <> 150

2. Calculating the amount of trail weight: Great care must be taken in selecting the size of trial weights. If the weight is too small, no significant change in the unbalance readings will result and a run will have been wasted. On the other hand, if the trial weight is too large for the machine being balanced, the unbalance forces generated by the trial weight may cause extensive damage to the machine.
As a general rule, a trial weight that produces a minimum of a 30% change in amplitude and / or a 30 degree change in phase from the original unbalance readings will help insure satisfactory results. A common approach for selecting a trial weight is to use one that will produce an unbalance force equal to 5-10 % of the rotor weight supported by the bearing.

3. Correction method: Correction can be done either by weight addition or by weight removal. Weight addition is preferred than weight removal for greater accuracy. In large rotors slots for weight addition are given in pre-determined balancing planes. If weight addition is not feasible, great care should be taken when weight removal.

Balancing may be done in-situ (i.e. in installed condition) or in a balancing shop depending on certain factors like unbalance severity, type of rotor, planes of correction, rotor weight, downtime of the machine etc. The above mentioned criteria and balancing types applies to both of these. In shop balancing, measurements and calculations are done by the balancing machine software while in in-situ balancing portable vibration analyzers or polar charts may be used.

balancing types

Balancing can be classified into three types depending upon the number of correction planes used for balancing:

• Single plane balancing
• Two plane balancing
• Multi plane balancing

Single plane balancing:

As the name implies, single plane balancing is the correction of unbalance in one plane to achieve good balance. This method is always applicable for thin disks and can also be applicable to long rotors also if the unbalance is mostly in one plane. The one plane does not need be through the center of gravity. The single plane balancing corrects for static unbalance only. It can do nothing for couple unbalance. Generally "thin" overhung rotors, single impellers, motors and other long rigid rotors with unbalance vibration substantially higher at one end, are balanced with this method.

Two plane balancing:

The primary reason for doing two plane balancing is to correct for a couple unbalance or combination of static and couple unbalance, which are usually the case. This type of unbalance generates a rocking force. The correction is to apply two weights in two planes separated by some axial distance. This creates a counterrocking force; i.e. a couple unbalance is corrected by another couple. For rigid rotors, any combination of static, couple or dynamic unbalance can be corrected by two plane balancing and two planes are selected arbitrarily depending upon the accessibility and feasibility.

Multi plane balancing:

When we balance rotors externally by making corrections in planes other than where the original unbalance exists, these cause internal bending moments in the rotors. For the rotors operating above their critical speeds these internal bending moments cause deflection and change in shape of the rotors at different critical speeds as explained earlier. If unbalance is corrected in the plane where it exists, there would be no internal bending moments and the rotors would not deflect at different critical speeds. This necessitates balancing in more than two planes for rotors, which operate above their critical speeds. Rotors like high speed multistage compressors, large steam turbines, gas turbines, rolls of paper making machines etc come under this class of rotors.

balancing steps

Component balancing:

For shaft rotors, which comprise of more than one component, it is vital to balance all of the major components individually before assembly. This is done because if the rotor is fully assembled, there is no way to know exactly what contribution each component part is making to the total measured unbalance vector. In addition, if a large unbalance exists in one of the major components, within the rotor, the rotor shaft may flex at this point during high speed operation and cause significant damage to the rotating and stationary parts.

Each major rotor component must be individually balanced on a precision ground mandrel (note that expanding mandrels are not acceptable for this purpose). The balance mandrel should be ground between centers to assure concentricity of all diameters throughout its length as well as to assure a good smooth surface. After grinding, the mandrel must be precision balanced. A trial bias weight may be used to raise the observed residual unbalance readout of the balancing machine. The desired balance result is such that no matter at what angular location the bias weight is added, the balance readout is always the same. In this case the residual unbalance of the precision mandrel is as close to zero as possible. The rotor component should always be mounted to the mandrel with an interference fit, never a sliding or loose fit. If the rotor component has a key fit to its shaft, than the balancing mandrel should also have a matching keyway. After each component is shrunk on its mandrel, the axial and radial runouts should be checked to ensure that the mounted impeller or hub is not cocked on its mandrel prior to component balancing. As a general rule, runout should not exceed 0.16 mm/meter of diameter.

Progressive component stack balancing (sequential balancing):

After individual balancing of all major rotor components, the rotor must be progressively stack balanced as each major component is assembled onto the rotor shaft. Progressive or stack balancing is necessary due to the deformation of components during assembly. Components with unequal stiffness in all planes, such as those with single keyways, may deform when shrunk onto the rotor shaft. For such components, considerable deformation and resultant unbalance can occur between mandrel balancing using a light shrink fit and stack balancing on the job shaft with a heavy shrink fit.

Progressive balancing is accomplished by stacking no more than two rotor components at a time onto the rotor shaft. Component axial and radial runouts should be checked against mandrel runouts, as each component is start. In general, the start component runouts should match those runouts recorded with the components on the mandrel.

As each rotor component is start into position and the runouts checked as acceptable, the rotating assembly is to be placed in the balancing machine and trim balanced (if required) as necessary to achieve the balance tolerance. Balance weight correction is to be performed only on the most recently stacked component.

After the rotor is completely stacked, trim balancing, if required at all, should be very small to meet the tolerance of permissible residual unbalance. As a general rule of thumb, the remaining residual unbalance in the rotor should not exceed two times the residual balance tolerance prior to trim balancing.

co2 gas sensor

The CO2 Gas Sensor measures gaseous carbon dioxide levels in the range of 0 to 5000 ppm. This probe is great for measuring changes in CO2 levels during plant photosynthesis and respiration. With this sensor, one can easily monitor changes in CO2 levels occurring in respiration of organisms as small as crickets or beans! The CO2 Gas Sensor is easily calibrated using a calibration button. A chamber with probe attachment is included for. running controlled experiments with small plants and animals

types of Refrigeration systems

Carnot Vapour compression Refrigeration cycle


(a) Schematic representation (b) T-s diagram

Processes: -

1-2: Isentropic compression from state 1 (wet vapour) to state 2 (saturated vapour)

2-3: Heat rejection (QH) in the condenser

3-4: Isentropic expansion from state 3 (saturated liquid)

4-1: Heat absorption ( QL) in the evaporator

The COP of the refrigerator,

Practical Vapour compression refrigeration cycle

Application of the first law of thermodynamics to the control volume compressor, condenser, throttle and evaporator gives

(Ws)compressor=h2-h1

QH=h2-h3

h3=h4

and QL=h1-h4

The COP of the refrigerator is given by,

In the ideal refrigeration cycle, the refrigerant leaves the evaporator as wet vapour.

In some cases the refrigerant leaves the evaporator as either saturated vapour or superheated vapour.

T-s diagram for a vapour compression refrigeration cycle when the refrigerant leaves the evaporator as (a) saturated vapour (b) superheated vapour

Gas refrigeration cycle

(a) Schematic diagram (b) T-s diagram

The simplest gas refrigeration cycle is the reversed Brayton cycle

Processes: -

1-2: isentropic compression for state 1 (atmospheric air) to state 2

2-3: energy exchange with the surrounding, air is cooled

3-4: isentropic expansion to state 4

Work obtained during the expansion process can be used to run the compressor

Work done on the compressor,

Work delivered by the expander,

The net work required= CP (T2-T1-T3+T4)

The COP of this refrigeration system is given by,

Refrigeration Cycle

Mechanical refrigeration is accomplished by continuously circulating, evaporating, and condensing a fixed supply of refrigerant in a closed system. Evaporation occurs at a low temperature and low pressure while condensation occurs at a high temperature and high pressure. Thus, it is possible to transfer heat from an area of low temperature (i.e., refrigerator cabinet) to an area of high temperature (i.e., kitchen).

Referring to the illustration below, beginning the cycle at the evaporator inlet (1), the low-pressure liquid expands, absorbs heat, and evaporates, changing to a low-pressure gas at the evaporator outlet (2).

The compressor (4) pumps this gas from the evaporator through the accumulator (3), increases its pressure, and discharges the high-pressure gas to the condenser (5). The accumulator is designed to protect the compressor by preventing slugs of liquid refrigerant from passing directly into the compressor. An accumulator should be included on all systems subjected to varying load conditions or frequent compressor cycling. In the condenser, heat is removed from the gas, which then condenses and becomes a high-pressure liquid. In some systems, this high-pressure liquid drains from the condenser into a liquid storage or receiver tank (6). On other systems, both the receiver and the liquid line valve (7) are omitted.

A heat exchanger (8) between the liquid line and the suction line is also an optional item, which may or may not be included in a given system design.

Between the condenser and the evaporator an expansion device (10) is located. Immediately preceding this device is a liquid line strainer/drier (9), which prevents plugging of the valve or tube by retaining scale, dirt, and moisture. The flow of refrigerant into the evaporator is controlled by the pressure differential across the expansion device or, in the case of a thermal expansion valve, by the degree of superheat of the suction gas. Thus, the thermal expansion valve shown requires a sensor bulb located at the evaporator outlet. In any case, the flow of refrigerant into the evaporator normally increases as the evaporator load increases.

As the high-pressure liquid refrigerant enters the evaporator, it is subjected to a much lower pressure due to the suction of the compressor and the pressure drop across the expansion device. Thus, the refrigerant tends to expand and evaporate. In order to evaporate, the liquid must absorb heat from the air passing over the evaporator.

Eventually, the desired air temperature is reached and the thermostat or cold control (11) will break the electrical circuit to the compressor motor and stop the compressor.

As the temperature of the air through the evaporator rises, the thermostat or cold control remakes the electrical circuit. The compressor starts, and the cycle continues.

In addition to the accumulator, a compressor crankcase heater (12) is included on many systems. This heater prevents accumulation of refrigerant in the compressor crankcase during the non-operating periods and prevents liquid slugging or oil pumpout on startup.

Additional protection to the compressor and system is afforded by a high- and low-pressure cutout (13). This control is set to stop the compressor in the event that the system pressures rise above or fall below the design operating range.

Other controls not indicated on the basic cycle which may be part of a system include: evaporator pressure regulators, hot gas bypass regulators, electric solenoid valves, suction pressure regulators, condenser pressure regulators, low-side or high-side float refrigerant controllers, oil separators, etc.

It is extremely important to analyze completely every system and understand the intended function of each component before attempting to determine the cause of a malfunction or failure.

refrigeration-basic definations

• Temperature

Temperature scales are a way of describing how hot a substance is. A lump of matter contains energy. There are many forms of energy, one of them is Kinetic energy and measuring temperature is a way of measuring how furiously the molecules contained in a lump of substance are moving about. This molecular activity causes what we perceive as the temperature of an object. A refrigeration mechanic must be able to deal with temperatures in various scales. Traditionally the English system has been used (Fahrenheit degrees) and a whole series of familiar capacity measurements like Horse Power, BTU's, Tons, and PSI have been the norm. However the metric system which is supposed to be easier to work with is becoming popular in many locations. In both systems there are standard and absolute temperature scales. Try experimenting with the above temperature converter. Type a value in any one of the input boxes and click on the Convert Button. Here are several interesting values to try: -40 F, 0R, 40 F, 373 K, 21 C

• Fahrenheit

The Fahrenheit temperature scale was developed by no less than Fahrenheit himself back in the early 1700's. It was based on scientifically observable occurrences such as human body temperature and melting ice. Those points were assigned arbitrary values which made sense at the time. The newly created number scale was widened for easier reading and when boiling water was measured at 212 degrees, Fahrenheit changed the value of freezing water from 30 to 32 degrees to achieve the more attractive scale of 180 degrees between water's freezing and boiling points. There are 180 degrees in 1/2 of a circle and this was a temptation too great to resist.

• Celsius

In theory the Celsius scale should be much easier to work with. It is based on calling the freezing point of water zero and the boiling point of water 100. There are therefore 100 degrees between those 2 points. The Celsius temperature scale is also referred to as the "Centigrade" scale. Centigrade means "consisting of or divided into 100 degrees. I wonder what a comfortable room of 70 F would be in Celsius? If you don't happen to have a conversion calculator at your disposal you can always rely on the following 2 formulae:

• Kelvin

Scientists use the Kelvin scale, which is based on the Celsius scale, but has no negative numbers. Instead of basing it's zero point on the freezing point of water, it bases it's zero point on Absolute Zero. which is the theoretical temperature where all heat has been removed from a substance. Hence any amount of heat added creates a positive temperature. Negative numbers can mess up a scientist's mathematical calculations. You will find that in refrigeration, we too must use absolute temperature scales for some things. At Absolute Zero scientists believe that molecular motion would stop.

• Rankine

Rankine is the English version of an absolute temperature scale. Add 460 degrees to Fahrenheit temperatures to obtain the Rankine temperature. Input 0 degree in the Rankine box on the calculator above and you will see why.

• Heat

Temperature is a qualitative measurement. Heat is a quantitative measurement. The temperature "quality" of a object describes how hot it is but not the total amount of heat it actually contains. Here's a silly example which makes clear the distinction. Let's say we have two blocks of iron. One is a mere cubic inch, the other is 10 feet cubed. We heat each of them to 150 F and you verify this with some sort of thermometer. They both have the same temperature but do they both contain the same amount of heat? When you throw the little cube in your swimming pool nothing noticeable happens to the temperature of the pool water but when you toss in the huge iron chunk the pool water can be measured to rise somewhat over time. If there was a noticeable amount of heat transfer from the large chunk of iron but not from the small chunk of iron then surely the large chunk contained more heat than the small one even though they were at the same temperature. The temperature of an object is a reflection of the kinetic energy of the atoms or molecules that make it up. Fast molecules = high kinetic energy = high temperature. On the other hand heat represents the total amount of kinetic energy in an object. Heat is measured in BTU's. Recall that 1 BTU is the amount of heat required to change the temperature of 1 Lb. of water through 1 F. So it would take 2 BTU to raise the temperature of 2 Lb. of water through 1 F. And it would take 30 BTU to raise the temperature of 3 Lb. of water by 10 F. BTU's (or their metric counterparts) Larger quantities of heat in the Imperial system are described with the term Ton. 12,000 BTU = 1 Ton. A building might have a 3 Ton Air Conditioning system which would be equivalent to 36,000 BTUH.

• Specific Heat

Specific heat capacity is the amount of heat required to change temperature of a given quantity of a substance by one degree. Specific heat may be measured in Btu/lb F or kJ/kg K. Different substances have different heat holding capabilities and thermal properties. Just because 1 Lb. of water will change precisely through 1 F when 1 BTU is applied to it does not necessarily mean that the same thing will happen with 1 Lb. of copper or 1 Lb. of steel or 1 Lb. of ice cream. There is a need to be able to specify those differences and the method utilized is to compare all substances to water. Water is given a specific heat value of 1. This means that 1 BTU is required to change the temperature of 1 Lb. of water through 1 F. The specific heat of water can also be described in the metric system. The metric specific heatof water is 1 calorie per gram per degree Celsius. This value also works out to 1. In other words it would take 1 calorie of heat to raise the temperature of 1 gram of water through 1 degree Celsius. Specific heat is adimensionless quantity. It is purely a number having no unit of measurement associated with it. In Refrigeration specific heat values are used to calculate capacity requirements for refrigerating known quantities of product. For example one might need to be able to select refrigeration equipment capable of cooling 5000 Lb. of beef from 55 F to -20 F. A calculation like that must take into consideration the fact that the specific heat of a substance usually is different above and below it's freezing point.

• Latent Heat

Latent Heat is the heat given up or absorbed by a substance as it changes state. It is called latent because it is not associated with a change in temperature. Each substance has a characteristic latent heat of fusion, latent heat of vapourization, latent heat of condensation and latent heat of sublimation.

• Sensible Heat

Sensible Heat is associated with a temperature change, as opposed to latent heat. This is so-called because it can be sensed by humans. If the air in a building was to be heated from 60 F to 70 F only sensible heat would be involved. However, if the air in a building was to be cooled from 80 F to 70 F and humidity was to be removed from the air at the same time, then both sensible and latent heats would be involved.

• Insulator

Electrical wires are coated with an insulating material so electricity stays in the conductor (wire). Thermal insulation on the other hand tries to keep heat from transferring. Thermal insulation does not stop heat transfer, it only slows down the rate of transfer. The greater the amount and quality of insulation, the greater the insulating effect and the slower is the thermal transfer. There is insulation inside cooler and freezer walls and in the perimeter walls of conditioned spaces. If fiberglass batting is being used it should be noted that the glass fibers are actually a poor insulator. It is the tiny pockets of trapped air in-between the fibers that actually are responsible for the insulating effect.

• Pressure

Pressure is what occurs when a force is applied over an area. More specifically, pressure is the ratio of the force acting on a surface to the area of the surface. The equation for pressure represents this rather straightforwardly; P=F/A This equation means that Pressure equals Force divided by Area. Let's look at a couple of very simple examples. As is demonstrated in the sketches below, the same weight can exert completely different pressures depending on how much surface area it is spread out over. Note that in the Imperial System when you multiply FT by Lbs you get a unit called FT Lb. (pronounced Foot Pounds) This is a legitimate unit of pressure however. However refrigeration pressures in the Imperial System are measured in pounds per square inch not pounds per square foot. This is abbreviated to PSI. Refrigeration gauges are zeroed to 1 Atm pressure and the units are then called PSIG. (as in PSI Gauge) The calculations shown in the metric picture yield pressure units in kg/m2 (kilograms per meter squared). This is also a legitimate unit of pressure however kPa (kilopascals) are the pressure units that you will see on Metric refrigeration gauges. As with temperature, pressure has many different scales that can be used and can be described with the English system or the Metric system. We seldom deal with gravitational forces as shown in the diagram although it is an important concept to be aware of. Rather, we deal with the pressures and temperatures of gases and that is what the next section is all about.

• Energy

Energy is the capacity of a system to do work where "system" refers to any physical system, not just a refrigeration system.

• Enthalpy

Enthaply is the total amount of heat in one Lb. of a substance. It's units are therefore BTU/Lb. The metric counter part is kJ/Kg. (kilo joules/kilogram)

• Entropy

Entropy measures the energy dispersion in a system divided by temperature. This ratio represents the tendency of energy to spread out, to diffuse, to become less concentrated in one physical location or one energetic state. That spreading out is often done by molecules because molecules above absolute zero always have energy inside of them. That's why they are incessantly speeding through space and hitting each other and rotating and vibrating in a gas or liquid. Entropy is measured in BTU per Lb. per degree change for a substance.

• Mollier Charts

Mollier charts are used in designing and analyzing performance of vapour compression refrigeration systems. Each refrigerant has it's own chart which is a graph of the Enthalpy of a refrigerant during various pressures and physical states. Mollier charts are also called Pressure-Enthalpy diagrams. Pressure is shown on the verticle axis, enthalpy is on the horizontal axis. You can compare Imperial versus SI Unit Mollier Charts by clicking on the buttons below the chart.

Infrared Gas Sensors

Infrared Gas Sensors exploit the property that many gases absorb radiation in the 2-14 micron, infrared region of the spectrum. These spectral absorbance show features which may be regarded as 'fingerprints' to identify the gases and enable their concentrations to be deduced. The sensor bodies contain an infrared source and infrared detectors inside a compact and combined gas cavity/ optical cell. The detectors have infrared band pass filters placed in front, which tune them to the specific gases to be sensed. When the specific gas enters the cavity it is registered as a change in detector signal. The magnitude of this change is related to the concentration of that gas via a simple exponential formula.By utilizing different infrared filters a range of gases can be sensed and discriminated with these devices.

In cases where spectral lines overlap, then an individual sensor may show cross sensitivities to a gas range. Infrared gas sensors are very robust devices not affected by contact with a harsh chemical environment. Any changes in ambient conditions such as temperature are compensated for in software. Their dimensions and power requirements are compatible with and complementary to pellistor gas sensors. After over thirty years of successful manufacture of pellistor-based flammable gas sensors, the range of Non-Dispersive Infra-red (NDIR) gas sensors represents the first of many diversifications into other areas of gas sensor technology.

In a molecule, absorption or emission of energy can occur in transitions between different energy levels. These transitions can be associated with changes in the vibrational energy and changes in the rotational energy of the molecule. Such internal energies are quantized, so that the molecule can exist only in certain discrete vibrational and rotational energy levels. The energy related to transitions between vibrational energy levels is equivalent to radiation in the near infra-red region of the electromagnetic spectrum. Each vibrational level is associated with a set of rotational levels, which results in several closely spaced energy levels existing within a frequency band in the infra-red spectrum of the molecule. The fundamental frequencies at which the bands exist are functions of the particular bond and the mode of vibration, e.g. stretching or bending. When a molecule is exposed to infra-red radiation with an energy equivalent to a vibrational transition, the radiation is absorbed and the molecule undergoes the transition. This absorption is used as the means to determine the amount of target gas molecules present.

The NDIR technique uses a broad-spectrum source, such as a filament lamp, to expose the gas to a wide range of infra-red frequencies. An associated detector is fitted with an optical filter such that it can only monitor the intensity of a certain narrow frequency band. This frequency band is selected to match a frequency band within the absorption spectrum of the target gas and the detector output is therefore affected by the concentration of the target gas. The frequency of radiation, for our purposes, is more often expressed in terms of its wavelength, as the two terms are directly related

Thermal Conductivity Gas Sensors

Pellistors measure the flammability of a gas; they cannot be used to measure levels of gas above the Lower Explosive Limit (LEL), since the reducing level of oxygen will result in a fall-off of signal. However, a similar device can be used to monitor these high levels of gas. We have a range of thermal conductivity sensors, which are designed to complement the pellistor range in terms of electrical characteristics, so that they can be used in the same Wheatstone Bridge circuits. They are supplied with a compensator bead which is in a sealed enclosure of air. This enclosure acts as the thermal conductivity reference in exactly the same way as it acts as the reference for a pellistor.

Thermal conductivity measurements do not rely on the flammability of the gas, the technique can be used to analyze a whole range of gas mixtures, provided that there are only two gases present and that the two gases have significantly different thermal conductivities. Examples includes

• 100% Hydrogen in Air
• 100% Methane in Air
• 100% Carbon Dioxide in Air
• 100% Carbon Dioxide in Methane
• 100% Helium in Air

Thermal conductivity cannot be used for gas mixtures where the thermal conductivities of the two gases are similar. The best example of this is oxygen levels in air, as the thermal conductivities of oxygen and nitrogen are too close to give a meaningful signal.Our pellistors and thermal conductivity sensors can be obtained already packaged as complete, flameproof gas detection heads for use in fixed gas detection systems.