How Cooling System Operates?

The operational cycle of a standard cooling system is shown on the diagram below. There are 3 main elements in a cooling system. The compressor, evaporator and condenser. There are other equipment also in the cooling system to control the refrigerant flow like the expansion valve, capillary pipe, thermostat, drier-filter, liquid separator, sight glass, pressure gauge, thermometers, etc. Finally, mention must be made for the refrigerant and lubrication oil.

The function of the cooling compressors in the cooling system is to displace the heat laden refrigerant in the evaporator and coolers to give way to following cooled refrigerant to ensure uninterrupted flow and to increase the pressure of the refrigerant in vapor state the level that corresponds to the condensing temperature at the condenser.

Following control characteristics are requested for an ideal compressor:
a.    Continuous capacity control and adopting wide range of load fluctuating-operation regime,

b.    Lowest startup moment of rotation as much as possible,

c.    Consistent efficiency even under partial loading,

d.    Safety and reliability under various operation conditions,

e.    Consistency in the vibration and noise levels under partial or full loading at various operational conditions,

f.    Trouble free operation in long service life,

g.    Stability in performance in terms of unit cooling rate at specific power, and

h.    Low acquisition and operation cost as much as possible.

It is likely that there are hardly any compressors available in the market meeting all these requirements. The selection is usually made for the compressor that meets foregoing characteristics best under given operational conditions.

Depending on their general constructions, the cooling compressors are classified as follows:

a.    Piston compressor
b.    Rotary wane compressors
c.    Rotary helical – screw compressors

1B) Centrifugal Compressors

1A/a) Piston Compressors

The rotational of the drive motor is converted into linear motion via the crankshaft - connecting rod mechanism in these compressors where the compression is performed with a piston that moves up and down motion in the cylinder. It is possible to see obtain the same function as in the case of previous double-action compressors that were driven by the horizontal piston steam engines. Contemporary piston compressors for cooling purposes are in general designed and manufactured as single-action, high speed and multi-cylinder machine with open type (belt – and pulley or direct coupling) drives or as hermetic compressors (except ammonium.

The area of application of the piston compressor are suitable for the refrigerants that require cylinder volume per unit refrigerant cooling capacity is low, but suction / discharge pressures differences are extremely high.

Conventional cylinder arrangement of the open type piston cooling compressors are in general, single-acting, vertical in I, V and W, arrangements, with cylinder numbers up to 16. Horizontal and double-action compressors are almost abandoned completely. For sealed hermetic type motor-compressors, vertical axis crankshaft and motor and horizontal axis cylinder arrangements are frequently.

1A/b) Rotary Wane Compressors

The rotary compressors perform the compression motion while rotating, instead of reciprocation motion of the piston compressors. The rotary motion is made use of in numerous ways (single or double gear, single wane, multi vane, etc.). Helical gear type rotary compressors operating with a double gear principle will be described below. Here, two most common types of the wane rotary compressors are described as follows.

1A/b1) Single/Stationary Rotary Wane Compressors

Usually incorporated in the design of the small-size and full capacity hermetic compressors, such wane rotary compressors have the wanes installed on the exterior enclosure and do not take part in rotary motion, but moves in linear direction in line with the rotary motion of the rotor. The most important feature of such rotary motion compressors is that they don’t usually suffer from the surface roughness and change in the operation gaps of mating parts without a leak and good lubrication that ill ensure the friction / abrasion at a minimum level. The leakage in these compressors is kept under control by hydrodynamic effect. Here, what is meant or obtained by the hydrodynamic effect includes such factors like operating allowances, relative motion speed, viscosity of the lubrication oil and surface machining smoothness. The clearance of the wane rotary compressors can be made very small and therefore, the volumetric efficiency is very high. Furthermore, their vibration and noise levels can be made very low with good manufacturing practices, when compared to the piston compressors. But the noise and vibration level increases proportional to the size of compressor. Large-size wane rotary compressors are usually complete with the noise suppressor (silencer) at the discharge port. The main components and features of these compressors are outlined as follows.

Compressor Capacity Control Mechanism

Various types capacity control mechanisms are developed for the purpose energy saving and stable operation of the system under high efficiency just like operation under heavy loading when the compressor is operated under small cooling loads. The methods available to this end include (i) keeping the suction pressure under control by choking the suction port, (ii) keeping the discharge pressure under control, (iii) Partial or total bypassing of the compressed refrigerant to the suction side, (iv) including expansion valve on the refrigerant circuit, (v) adjusting the piston stroke, (vi) closing the compressor discharge and revert the refrigerant to the suction side, (vii) adjusting the compressor speed, (viii) complete closing the cylinder opening.

a. External body/Cylinder

The housing consisted of rotating parts, bearing, refrigerant entry and exit ways, discharge side valve, electric motor (hermetic compressors), drive shaft and other component, in general high quality leak-free cast iron. The internal wall that comes in contact eccentric rotor and mutual side walls are machined to the close tolerances, grinded and honed. For hermetic compressors, the vertical arrangement is in general adopted for motor / compressor.
b. Rotor

The rotor is installed on the drive shaft in an eccentric manner and sweeps the internal wall of the housing while rotating. The rotor is made from high quality steel machined and glazed at tight tolerances.

c. Wane

The compressor efficiency greatly depends on the tightness of the wanes and therefore they are machined and finished at tight tolerances (0.08). They are made from either high quality gray cast iron or steel, aluminum, carbon (ASTM Type “A” Graphite). The wane thickness is selected to resist to the loads (up to 0.005% of the wane length). The compressive force of the spring pressing the wane onto the rotor is determined as to be 25% higher then the refrigerant condensing pressure.

d. Drive shaft

The most important feature demanded for the drive shafts of these compressors is their rigidity without to much flexure, stability in their operation range and keeping oil film. Therefore, the shaft designed to have suitable size should be made from hard material (Min. 52 Rockwell), and very fine surface finishing (grinded and honed down to 0.125) should be provided.

e. Bearings

The bearings should maintain specified tolerances with the rotating parts in order to maintain proper oil film thickness. If the tolerance is too tight, the abrasion and wear increase, resulting heat build up. If it is too large, the tightness cannot be maintained. The bearings should be reliable, resistant to ambient condition, featuring long service life and maintenance free. The bearing made from steel casing on the cast iron core has been extremely successful. Likewise, good results are obtained with the bearings made form high quality ferrite-free gray cast iron with 180-220 Brinnel hardness (pore-free-tight perlite cast).

f. Valve

There is no need to use the suction valve for the wane rotary compressors, since flow is continuous. At the discharge port, high quality, generally leaf type valve (flap) is provided. The flap thickness ranges between 0.1 mm and 0.3 mm, depending on the compressor size (less than 5 HP). The valve surface and edges should be smooth for the tightness concern.

1A/b2) Rotary multi-wane compressors

This design type is general used for large-size compressors (there are also small-size hermetic compressors with two wanes). The wanes take part in rotating motion with the rotor. They are in general used with the refrigerants like R-12, 22 and Ammonium, designed as single stage at regular evaporation temperature levels. They provide ideal solution for cascade system deep cooling applications, where high sweeping flowrates are necessary for lower stages. Likewise, they are successfully used as the booster compressor for multi-stage deep cooling applications between -20 and – 90°C). The power rating of contemporary designs ranges between 10 HP and 600 HP. The properties of these compressors are as follows: They are successfully used at extremely low evaporation temperatures thanks to the smaller sizes and lighter weights when compared to other compressors at the same capacity, let alone the robust and durable design suitable for industrial applications. The main application areas include cold storage, freezing process of the goods and industrial and chemical processes that involve cooling operations. The number of wanes of these compressors ranges between 4 and 16. There are numerous advantages in increasing the number of wane with respect to the compressor capacity. The compression ratio should be designed to ensure the least power consumption per unit cooling capacity. The compression ratio in contemporary multi-wane compressors is kept under 1/7. On the other hand, suction and discharge pressure difference should not be kept high, otherwise, the wanes may undergo excessive stress (flexing), let alone the increased loads on the bearing and flexure of the rotor. The lubrication for these compressors is extremely important to ensure minimizing wane-housing / wane-rotor friction, keeping the cooling at maximum level and back-leakage of the refrigerant.

These compressors are normally equipped with the capacity control mechanism. The capacity control limits are performed within 20% to 100%. The control is provided for continuous-steeples manner. The most suitable control mechanism is to make adjustment by the valves installed on the side surfaces of the cylinder. These valves also provide protection against the excessive pressure like a pressure relief valves, while capacity control is performed on the basis of the signal from the pressurized oil from the lubrication equipment. When the oil pressure is low, the valve is fully open and therefore, the compressor is started at the minimum load. The valves ensure decreasing the discharge pressure to control the capacity by providing the bypass of the compressed refrigerant to the section side through the gaps between the wanes.

1A/c) Helical rotary compressors

These compressors are considered in the general group of positive compressed compressors. There is different type of these compressors with diverse constructions. Common helical rotary compressors frequently encountered in cooling applications can be classified in two groups with apparent differences; i.e., (i) Single-screw helical type, and (ii) Screw/helical rotary compressors. There are, however many common features of these compressors in terms of principle of operation and construction. For example, both compressors are characterized with the spray lubrication for the purpose of lubrication as well as tightness during the compression stroke. This is also performed to remove the heat built up in the housing. Likewise, the similarities are evident for both compressors in compression ratios, capacity control mechanisms and heat economizer arrangements.

1A/c1) Single Gear/ Helical Rotary Compressors

The operational principle of contemporary single helical rotary compressors was first introduced in 1960s, which were used mainly for the air compressors for 10 years afterward. These compressors were also used in cooling applications and eventually they found application in meeting higher cooling capacities.


1A/c2) Helical/Screw Rotary Compressors

Similar in the operational principle of screw rotary compressors, these compressors are comprised of a pain of male and female helical gears. The helical pair gears are installed in housing and supported by the bearings at both ends. There are suction and discharge ports on the housing. The flow of the refrigerant in the helical gear gaps is both in the radial and axial directions. One of the helical gears (generally male gear) is driven by the electric motor, while other follows the rotation of the driven gear. On the other hand, there are other design types, where both gears are driven with a synchronous rotational power source.

It is possible to differentiate the operation of the suction and discharge sides of the helical/screw rotary compressors in 4 parts. They are, (A) Suction, (B) Advance, (C) Compression and (D) Discharge. This operation is repeated by each one of the teeth of two helical gears. Previously, the tooth profile of the helical gear was made circular, however, in recent years it is changed to a special asymmetric profile for more efficient operation and better tightness.

The screw compressors can be designed to operate dry/oil-free or else, they usually include spray lubrication. Dry type screw compressors are characterized by limited compressive pressure (difference in suction/discharge pressure difference) and high rotational speed (above 3600 rpm). These limitations are eliminated in spray lubrication types (at 45 to 50°C temperature) to a great extent.

The spray lubrication also provides the cylinder cooling, reduced noise and wear. The compressor may cope with the moisture content in the refrigerant better, which is the significant property in the cooling applications. The screw type spray lubrication compressors can be used with the refrigerants like R-12, 22, 502 and ammonium (featuring high condensing pressures) very easily. Now they are available at the power ratings between 20 HP and 1500 HP.

The performance characteristics of the helical/screw rotary compressors are as follows:

a.    Smooth (uninterrupted) refrigerant flow,

b.    Smooth torque level,

c.    Positive compression feature,

d.    Minimum level of vibration in wide range of load variation,

e.    High volumetric efficiency and adiabatic efficiency,

f.    Proven design without section and discharge valves and thus reduced the source of failure and pressure loss, and

g.    Small-size and lighter construction when compared with other compressors.

In spray lubrication type, the compressors should be complete with an oil separator to collect the oil from the refrigerant. The type of the oil separator depends on the system properties and the refrigerant used.

Following properties are requested from the oil system:

(1)    Lubricating the cylinders housing the helical gears, reducing in back-leakage of the refrigerant, contributing to the refrigerant cooling, etc.

(2)    Lubricating the main bearings,

(3)    Hydraulic balancing of the axial forces on the rotor, and

(4)    Contributing to the operation of capacity control mechanisms.

The heat extracted from the refrigerant is collected from the oil cooler and disposed.

The centrifugal compressors providing cooling by compressing the gas refrigerant are characterized by making use of the centrifugal forces as opposed to piston, rotary or wane compressor performing positive compression. It is possible to move the refrigerant with high specific volume (larger volumes of the gas refrigerant) in the centrifugal compressors and therefore they are suitable in large-size deep cooling (up to -100oC) applications. The centrifugal force is directly proportional to the square of the speeds and therefore, the difference between the suction and discharge pressure can be increased by increasing the speed or diameter of he rotor or number of stages.


COMPRESSOR PERFORMANCE

The performance of equipment is the degree of the fulfillment of the predefined task assigned for that equipment. The compressor performance is in particular determined by the design that combines the physical conditions of the compressor and motor to conduct the following tasks:

1.    1.     Trouble free and long operation

2.    2.     Maximum cooling effect obtained under minimum power input

3.    3.    Minimum cost

4.    4.    Wide range of operation conditions

5.    5.    Acceptable vibration and noise generation

There are two beneficial criteria for the compressor performance; one is capacity in terms of the compressor displacement, other is the performance factor.

The system capacity is the cooling effect the compressor provides. This is the difference between the enthalpies of the coolant at such pressures and temperatures when it is entering and leaving the compressor. It is expressed in term of kJ/kg.
The performance factor of a hermetic compressor is an indication for the common efficiencies of the motor and compressor.

The Performance factor (hermetic compressors)

Recently, the performance factor has been crucial for the industry due to the concerns on the energy saving. To this end, the term PER (Power to Efficiency Ratio) is used. The real performance of cooling and air conditioning devices are approved and enlisted under ARI regulations. By this way, the contractors, experts and energy companies are able to compare the relative efficiencies of the equipment.

With respect to the compressor performance, there is one another classification that provides three definition and criteria used in particular by the design engineers, if not practical for the cooling technicians at all.

The compressor efficiency is related to what happens in the compressor. This is something related to the deviation of the real compression from ideal and defined by the work performed in the cylinder.

The volumetric efficiency is defined as the ratio between the volumes of the coolant after it is compressed by the pistons.

The real capacity is a function of ideal capacity and overall volumetric efficiency.

The brake horse power (bhp) is a function of the mechanical and volumetric efficiencies of the power input to the compressor with respect to ideal conditions.

This lesson is on the real capacity and power input necessary for the compressor or condenser depending on a given operational condition range, rather than defining the tasks of a cooling technician.

The compressor manufacturer performs detailed testing (Figure S11-4) on their compressors in order to conform to ASHRAE and/or ARI requirements. There are two types of compressor testing. First is for the determination of the capacity, efficiency, noise level, motor temperature, etc. Second testing is also important, since it is performed to determine the possible service life. Testing for the service life should be performed under the conditions simulated for operation during anticipated life. In this operation, rules for the operational safety should also be observed.

Testing operations may provide the beneficial information on the operation and performance of the compressor, which might be used for further improvements.

The capacity values are published as the tables and curves that include following data:

1.    Definition of compressor, i.e., number of cylinders, bore, stroke, etc.

2.    Excessive cooling circuits or expression on the correction made on the data for zero degree excessive cooling

3.    Compressor’s rpm
4.    Cooling fins

5.    Suction temperature of superheated coolant

6.    Compressor ambient conditions

7.    Outdoor cooling conditions, if necessary

8.    Maximum power and maximum operation conditions and minimum operation conditions with no load.

Figure 4.1 shows typical capacity and power curve of a hermetic piston compressor. Observe the parameters already provided: The refrigerant (R-22), super cooled (6°C) refrigerant (11), superheated gas (1°C) and compressor speed, 1750 rpm. The capacity is given in kW on the vertical axis at the left. The power is given in kW on the vertical axis at the right. Bottom horizontal axis shows the evaporation temperature range. The condensation temperatures are on the diagonal curves. (Note: This is the condensation temperature of the refrigerant; it should not be confused with air or water cooled compressor terminology. The compressor is unaware the type of the condenser it incorporates; it is interested in the temperature and pressure of condensation.) Let’s assume that the evaporator temperature is – 3, 9°C and condensing temperature is 46.1°C to define the cooling capacity. Locate – 3.9°C at horizontal axis and go up to find 46.1°C on A curve. Then go left to read the final figure that is 30.76 kW. In order to determine power input, go up from - 3, 9°C to locate 46.1°C on the intersection point, B an then go right horizontally. The figure to be read is about 11.5 kW.

We may observe rapid decrease in the capacity of a simple displacement machine as a result of decreases in the evaporator temperature due to the coolant with low densities pumped at the fixed pressure. On the contrary, it is interesting to see the power curves that do not respond to the increase in the resulting pressure required for the same low density coolant gas corresponding to the operation at those levels. It is therefore, the relative conditions of commercial cooling and air conditioning systems are quite variable.

It is clear that it is not practical to use this type of compressor under any condition at all. It is possible to make use of equilibrium graphics similar to the one used in the cooling system. The evaporator capacity values provided by the coil manufacturer is based on entry air temperatures with variable evaporation temperatures. Similar curves might be drawn for the equilibrium conditions of the compressor-condenser equilibrium points under various ambient temperatures. Let’s assume that the evaporator load at 4.4°C suction air temperature is 16.11 kW. This calls for an evaporation temperature of – 3.5°C or a temperature difference of 8°C (point A). Again, let’s assume that the compressor / condenser operate at 37.8°C ambient temperature. Its capacity under evaporation temperature of at – 3.3°C will be about 17.28 kW (point B). That is, there is a difference of 1.17 kW. If ambient temperature is maintained at 37.8°C, the evaporating temperature will drop if its load does not changed and therefore, evaporator’s temperature difference and capacity will be increased. Meanwhile, the compressor capacity will drop until the system is stabilized at 16.7 kW and –4.4°C evaporator temperature. (Cooling Technique)