Compressors

scroll compressors are currently available in sizes from about...

1-30 horsepower,

What are the application factors?

Application factors include the type of refrigerant, the size (tonnage), the pressure difference, the evaporator and condenser saturation temperatures, and the type of cooling available for the compressor.

what is the oldest compressor design and how does it work?

Open-drive compressors, are driven by turning a shaft that extends through a seal. The compressor is driven by a separate motor or engine. In direct-drive compressors, the motor shaft is coupled to the compressor shaft and driven at motor speed.

Types of Duties

air conditioning, high, medium, low, ultra low

Types of Cooling Mechanisms

air, water, and refrigerant

the two types of crankshafts on a reciprocating compressor are...

crankthrow and eccentric

what are some scroll compressor precautions ?

do not allow the suction pressure on a scroll compressor to drop too low while operating. do not run a unit with a scroll compressor with the suction service valve closed. this can cause overheated motors, bearing failure, and internal terminal arcing.

what is used to pass electrical connections through the welded steel shell of a hermetic compressor without grounding out to the metal shell

glass or ceramic insulators

All types of rotary compressors use a ____side shell

high , high-side shell, the refrigerant inside the shell is the discharge gas, not the suction gas. this means the motor in a very hot vapor as compared to low-side shell compressors. however the discharge gas is still cooler than the motor windings.

open compressors range in size from...

less than a ton capacity to several hundred tons in capacity

Types of physical Enclosure

open, hermetic, and semi-hermetic

reciprocating compressors are available in _______ designs.

open, hermetic, and semi-hermetic designs

rotary screw compressors are available in ____ rotor types.

single-, twin-, and triple-rotor types

hermetic compressor units are made in a variety of sizes, from...

tiny fractional horsepower units meant for small appliances to larger units up to about 20 tons for air-conditioning use.

why do open compressors use a shaft seal?

to prevent leaking around the crankshaft, these seals are subject to forming leaks and need to be checked during planned maintenance or service.

semi-hermetic compressors are available in sizes that range from approximately from...

0.25 tons to 125 tons cooling capacity.

Types of Displacement

positive & dynamic

centrifugal compressors are made in ____ configurations.

single-stage, multi-stage, open, or semi hermetic configurations in capacities from 100 to 1,000 tons.

What does the type of compressor depend on?

the application

what is the physical enclosure of a scroll compressor?

welded hermetically sealed design

hermetic compressor units are sometimes called...

welded hermetics, full hermetics, or sealed hermetic compressors

define surging

when there is no forward flow of gas through the compressor and a reversal flow occurs. or violent airflow oscillating in the axial direction, which means the axial component of fluid velocity varies periodically and can cause damage to the rotor/compressor.

rotary compressors are found in...

window ac units, window heat pumps, and other small units, because they have higher efficiency and are lighter than reciprocating compressors.

rolling-piston rotary compressor are commonly used in...

window air conditioners, package terminal units, mini-split systems, and appliances

Compressors are classified by 5 different categories which are...

Displacement, Physical Enclosure, Mechanical Design, Cooling Mechanism, and Duty

Types of Mechanical Design

Reciprocating, Rotary, Scroll, Screw, Centrifugal

True of False: gas volume, temperature, and pressure are related.

True, reducing volume can increase in both pressure and temperature

Open machines (unlike hermetic) can be used with ammonia refrigerant as long as...

all components are made of iron or steel (or other suitable materials).

it is possible for a scroll compressor to seal some refrigerant in it when it shuts down. always be sure to...

always be sure to vent both the high and the low sides before working on the lines, especially if brazing

the individual who operates these extremely large centrifugal compressor units, may be referred to as...

an equipment room operator or an operating engineer.

when it comes to rotary compressors, systems with a small refrigerant charge will operate as if...

as if they are undercharged for several minutes until all the refrigerant leaves the oil

what is the most common use of open compressors?

automobile air-conditioning, the compressor is driven by the pulley and belt arrangement of the main vehicle engine. open compressors are also used for transport refrigeration on trucks, tractor trailers, and shipping containers, but these applications generally use a separate engine to drive the compressor.

why is it ESPECIALLY important to keep the refrigerant clean in a system with a hermetic compressor?

because the refrigerant and oil flow directly over the motor and all internal electrical connections.

how do the valves open and close ?

it's due to pressure difference, because of this they are self-operating. they are sometimes spring loaded but the principle is the same.

TECH TIP: if a scroll compressor makes a very loud rattle sound during start up and shut down, what id probably the cause of this ?

it's probably caused by the moveable scroll rattling until the discharge pressure id high enough to seal the scroll tips and the compressor speed is high enough to hold the moveable scroll tightly against the fixed scroll. it does not indicate a problem

centrifugal compressors are widely used in...

large chillers. the chillers are used to provide cooling for large buildings or to cool industrial process machinery.

SERVICE TIP: even though semi-hermetic compressors are field serviceable, why are they often removed and sent to companies that specialize in remanufacturing compressors?

mainly cost reasons, field service work is also commonly done for large compressors that would be extremely difficult for large compressors that would be extremely difficult or expensive to remove.

most centrifugal systems are extremely large and used as part of a chiller system in large commercial buildings, most run ___ hours a day___ days a week.

most run 24 hours a day, 7 days a week.

How does a positive displacement compressor work?

moves the gas by taking it into a space, the space is then replaced by something physical like a piston. Basically shoving out the gas, thus compressing it. Discharge port blockage can damage the compressor.

what kind of hermetic compressor is usually internally spring isolated and why

reciprocating, it's to reduce the inherent vibration coursed by the reciprocating action of the pistons. scroll and rotary don't need this because they run with far less vibration, they are also spot-welded to the shell because of this.

increasing the discharge pressure and/or reducing the suction pressure...

reduces the output of the compressor

what applications are open compressors often used for?

refrigeration and industrial, where rugged, serviceable machinery is required

compact hermetic versions of rotary compressors are built in sizes from...

small fractional horsepower up to about 5 horsepower

rotary compressors are widely used in _____ sizes to power _____

small welded hermetic sizes to power small refrigerated appliances, window air conditioners, packaged terminal air conditioners, and ductless split systems.

the shaft on some connecting rods has a hole that carries oil up the piston wrist pin, other systems...

use oil in the refrigerant vapor inside the compressor to lubricate the piston wrist pin.

what type of parts can be replaced in a semi-hermetic compressor?

valve reeds, gaskets, bearing inserts, or motor stators may be replaced

pistons within the compressors may have the suction valve located in the top of the piston, this is classified as a...

valve-in-head type, or the piston may have a solid head, with the suction and discharge valves located in a valve plate or cylinder head.

liquid entering the cylinder can do great damage to the...

valves, piston, and cylinder head because liquid does not compress!

scroll compressors have ____ moving parts compared to reciprocating compressors

very few

what is a discharge manifold?

a passageway in a compressor head that connects the discharge ports of all the cylinders.

most other compressors except for rotary, use a ____side shell.

low, low-side shell. where their motors are operating in a much lower temperature environment.

the valve plate serves as...

the top of the cylinder and holds the valves.

why have scroll compressors experienced extraordinary growth in the world markets of residential and commercial air conditioning and refrigeration?

they are more efficient than a comparable reciprocating compressor

unlike most reciprocating compressors, scroll compressors are directional, this means...

they only work in one direction. scroll compressors are extremely noisy when running backwards, they also will not compress when operated backward. check for proper roasting when starting systems with three-phase scroll compressors, this can be done with gauge manifolds. if normal suction and discharge pressure are seen soon after startup, the rotation is correct. if normal suction and discharge pressures do not quickly develop, shut off the machine and revers any two-compressor power leads, then recheck.

how do rolling piston (stationary blade compressors) work

they use an offset rolling piston that seals against a blade that remains stationary and moves in and out of its slot to seal against the rolling piston. the rolling piston is a rotating off-center cam or lobe that sweeps a path inside a round cylinder. the drive shaft is centered in the cylinder. a close fitting spring-loaded sliding blade follows the piston and separates the cylinder openings for suction (inlet) gas and discharge (outlet) gas. as the rolling piston rotates, gas will flow in and fill the suction cavity due to pressure difference, much the same as in a reciprocating compressor. the piston continues to rotate, closes off, and passes the suction port and compresses the gas unit it's pressure is high enough to flow out through the discharge (reed) valve into the discharge manifold. no suction valve with its inherent pressure loss is needed, so volumetric efficiency is high. because of the rotating rather than reciprocating motion, vibration levels are reduced and are easier to dampen (externally).

some of the advantages of screw compressors include:

HIGH VOLUMETRIC EFFICIENCY• volumetric efficiency expresses the rate of internal refrigerant leaking past the internal compressor parts. very little of the refrigerant that enters the compressor at the suction port is allowed to slip back by the screws. almost all of it is forced out at the discharge port. HIGH COMPRESSION RATIOS• the compression ratio is expressed as a ratio between the absolute suction pressure and the absolute discharge pressure. a high compression ratio allows the screw to produce very cold evaporator temperatures and high condenser temperatures without damaging the compressor. POSITIVE DISPLACEMENT• positive displacement means that every time the screws turn, a fixed amount of refrigerant is moved. this means that the cooling capacity changes are directly related to compressor speed. this makes it much easier for screw compressors to have a variable capacity. HIGH-PRESSURE REFRIGERANTS• high-pressure refrigerants tend to more easily leak past internal compressor parts. however, the screw oil coating seals small gaps , thus keeping a high volumetric efficiency. LARGE TONNAGE• screw compressors are available in capacity up to 700 tons. VARIABLE CAPACITY• there are a variety of ways that screw compressors can vary their capacity to meet the demands of changing loads. REFRIGERATION OR AIR-CONDITIONING APPLICATIONS• screw compressors are capable of producing low-temperature refrigeration down to -40 degrees Fahrenheit or air-conditioning temperatures ranging up to 45 degrees Fahrenheit.

what's the difference between a rotary swing compressor and a rolling piston(stationary blade compressor?

a swing compressor has a blade that moves that uses the help of a circular bushing (or two halves of one circular bushing) and overall being more efficient when it comes to the rolling piston, which has a stationary blade , a stationary shaft, and a rolling piston. although rotary blade compressor wears in and is ultimately a better alternative to the swing compressor.

SAFETY TIP: the terminal cover should always be in place when operating a hermetic compressor

abnormally high current can create enough heat to crack the glass holding the terminals, and the electrical terminals can shoot out under high pressure. Causing an effect like a bullet followed by a flamethrower. this is known as terminal venting, a rare but potentially deadly occurrence.

TECH TIP: Discus valves in compressors make a different sound than other valve mechanisms, you might think that the compressor has a bad component or is about to fail. What should you do before jumping to conclusions?

check the nameplate, it'll be identified.

scroll compressors are classed as...

compliant or non compliant, depends on the method used to accomplish the critical task of gas sealing. compliant designs can provide a radial compliance by the use of an old ham coupling, which allowed some side to side slipping between the shaft and the moveable scroll.

The reciprocating compressor is used in a majority of _______ unit applications.

domestic, small, commercial, and industrial condensing unit applications

Hermetic compressors have their mechanical components and electric motor...

enclosed in a single welded shell, the compressor crankshaft extends into the rotor of the electric motor, eliminating the need for drive components.

the twin-screw compressor models are used for ____ applications.

larger refrigeration and air-conditioning(usually water chilling) applications. they often consist of a driven male rotor and a mating female rotor carried on bearings and rotating within a stationary housing containing gas inlet and discharge ports. either rotor can be the driven component, although it is possible to use synchronized timing gears to drive both rotors.

describe an eccentric crankshaft

looks like the cam on an engine, straight shaft with offset lobes, the connecting rods slide over these lines. the connecting rods used with eccentric crankshafts are one piece, they slide over the crankshaft when the compressor is assembled. provides smoother operation. one piece connecting rods.

describe a crankthrow crankshaft

looks like the crankshaft on an engine, they have offset sections where the rods attach. the ends of the rods bolt around the crankshaft on a crankthrow crankshaft. two-piece connecting rods

centrifugal compressors are used for moving large volumes of gas against....

low-pressure differences, centrifugal compressor machines compete with the screw machines for large-capacity chilled-water cooling plants.

what are the shells called on reciprocating hermetic compressors?

low-side shells, because the suction gas enters the shell.

How does a dynamic displacement compressor work?

moves the gas with centrifugal force, operating like a giant fan or wind turbine. The high velocity pushing against confined spaces causes an increase in refrigerant pressure and temperature. Blocking the discharge of these compressors can cause surging which can cause damage to the compressor.

how do you avoid compressor damage on a scroll compressor?

newer scrolls have an internal pressure relief valve that unloads the compressor at suction pressures below below 55 psig on r-410a compressors. the compressor will act as if it cannot pump. after system pressures equalize, the bypass will close and the compressor will pump again.

the pistons for compressors are usually manufactured smaller in diameter than the cylinder they will operate in. what is used to help block gases from "blowing by" the pistons during compression strokes?

on large compressors, they used piston rings , but on dmsll compressors they use "oil grooves" where oil collects and helps to block the gases.

typically the discharge valves are located...

on top of the valve plate and the suction valves are located on the underside of the valve plate.

screw compressors are available in ____ designs

open, semi-hermetic, or hermetic designs. semi hermetic designs are available from 35-175 horsepower and are often used in multiples for larger capacity equipment.

the benefits of hermetic compressors include:

reliable operation due to the direct connection between the motor and compressor with no belts or couplings to wear or break. no shaft seal to leak because the motor and compressor are sealed in a welded steel shell. very compact in size, permitting increased refrigerated storage space. lower sound level, ideal for domestic appliances.

three common designs for rotary compressors are...

rotary blade, stationary blade (rolling piston), and the swings compressor. they are not internally spring isolated, so external vibration isolators must be used on the mounting feet.

why is maintaining a clean system and proper lubrication so critical to rotary compressors?

rotary compressors are constructed of precisely machines metal parts with no gaskets or rings. long refrigerant lines can cause problems , because they can lose their oil in the lines and seize. small impurities can also cause them to seize.

are rotary screw compressors positive or dynamic displacement ?

rotary screw compressors are positive displacement machines made for large tonnage applications. they are relatively simple in concept but demand advanced manufacturing to very close tolerances.

a compressor will produce the greatest output of energy by...

running at the highest suction temperature and the lowest discharge temperature practical, the most work at the lowest energy rate.

are scroll compressors positive displacement or dynamic displacement?

scroll compressors are positive displacement machines, with high volumetric efficiencies.

how does a scroll compressor work?

scroll compressors compress gas between two close-fitted spiral scroll memebers. one is fixed, and the other moves( but doesnt rotate) in an orbital path. the only rotating part in a scroll compressor is the motor shaft. progressively reducing cavities compress the refrigerant gas with little or no vibration. the sides of the scroll shape are called flanks, the edge of the scroll is called the scroll tip. for a scroll machine to work well, the contact between the flanks and scroll tips are very snug.

what other types of positive displacement compressors do not have trapped reexpansion gas to deal with and thus having higher efficiencies?

scroll, rotary, and screw

Belt-driven machines offer the flexibility of...

selecting a compressor speed to match the load, but the belts require additional space and a protective guard, they require increased maintenance and increased power due to operational losses.

how does a semi-hermetic differ from a hermetic compressor ?

semi-hermetic also has the motor and mechanics in one enclosure, but the enclosure has some form of access, like bolted plates and gaskets.

semi-hermetic compressors are known by a variety of names such as:

semi-hermetic, serviceable hermetic, accessible hermetic, and bolted hermetic.

single-screw compressors use a single ____ rotor.

single helical main rotor and either one or two gage rotors. the compression volume is formed by the rotor grooves, the cylinder wall, and by the meshing gate-rotor teeth. the rotor groove fills with gas, turns, and compresses the gas as the volume is reduced by the meshing action with the gate rotor. oil flooding is used for sealing as well as lubrication and cooling.

rotor bearing may be ____ or ____ , referred to as anti friction, depending on machine design and size.

sleeve or magnetic

rotary compressors are efficient and run...

smoothly and quietly.

the moveable scroll can make a pronounced rattling sound on...

start up and shut down, some manufacturers enclose these compressors in soundproof boxes to keep sound level (db) low enough to meet standards.

rolling piston compressors are also called...

stationary blade compressors

because rotary compressors use a high-side shell, the refrigerant absorbed by the oil in a rotary compressor...

takes longer to leave than with low-side shell hermetic compressors.

besides the compressor body, the basic mechanical components of a reciprocating compressor are...

the crankshaft, connecting rods, piston, wrist pin, valves, and valve plate.

an advantage of an open-drive compressor is that...

the motor and compressor are separate units so that the motor or compressor can be repaired or replaced without having to buy a complete system. another advantage is that motors heat is not absorbed in the refrigerant so it does not have to be rejected in the condenser like most other compressor types.

why do most hermetic compressor manufacturers not like their compressors operating in a deep vacuum?

the motor varnishes used in the motor winding lose some of their insulating ability under deep vacuums, and motors can arc out very quickly, even before they have time to overheat.

the ____ is an important component in oil-flooded machines.

the oil separator. the compressor could quickly run out of oil without the oil separator returning oil to the compressor. an oil separator may be built in as an integral part of the design.

what is the connecting rod in a reciprocating compressor?

the part of a compressor that connects the piston to the crankshaft

how does a reciprocating compressor compress air?

the piston is moved up and down in the cylinder by the connecting rod and crankshaft, thus compressing the air

how are the motors in hermetic compressors cooled?

the refrigerant vapor traveling over them. operating a motor on a low charge can cause the motor to overheat because of poor motor cooling.

the clearance volume is...

the space between the top of the piston and the cylinder head. during the compression stroke, gas leaves the cylinder and passed into the discharge manifold until the pressure in the cylinder is the same as the discharge manifold pressure. at the completion of the compression stroke, this space is filled with compressed gas that is not going anywhere. this gas will have to expand on the downstroke before the pressure can drop to a point to admit fresh suction gas. this "reexpansion" represents wasted work and lost capacity. engineers strive to minimize the clearance volume while leaving adequate room for thermal expansion and for unwanted incompressible slugs of oil and liquid refrigerant.

what is a major advantage of the the triple-screw compressor, compared to the single and twin screw compressors?

the triple-screw compressor has a VARIABLE-FREQUENCY DRIVE MOTOR (VFD) that allows it to change the motor rpm, giving the triple screw a very large operating capacity range. north the single- and the twin-type screw compressors get much louder as they are unloaded, hit the triple screw gets quieter. this is a major advantage when the equipment room in a building is close to occupied space.

is it possible to do any work on components inside of the shell of a hermetic compressor?

there are no procedures for working on damaged or defective components inside the shell, the entire hermetic compressor would be replaced.

keeping condensers clean is especially important with rotary compressors because...

they are cooled by the hot discharge gas. dirty condensers lead to overheated compressors.