Electrical Rules and Calculations for Air-Conditioning Systems – Part One

This is the first Article in our new Course HVAC-2: Electrical Rules and Calculations for Air-Conditioning Systems, which will list, explain, and discuss with examples all the topics covering the Electrical Rules and Calculations for Air-Conditioning Systems which will include but not limited to the following points:

• Introduction for Air-Conditioning Systems types,

• Introduction for Types of Motors/compressors used for Air-Conditioning Systems,

• Electrical Wiring for different Air-Conditioning Systems types,

• Types and locations of Disconnecting means,

• Sizing of Disconnecting means,

• Types of motor-compressor controllers,

• Sizing of motor-compressor controllers ratings,

• Sizing feeder/branch circuit overcurrent protective devices (OCPDs),

• Sizing feeder/branch circuit conductors, 

• Sizing motor-compressor overload protection.

First: Introduction for Air-Conditioning Systems Types

1- Air-Conditioning Systems Types As we explained before in our course HVAC-1: An introduction to heating, ventilation and air conditioning (HVAC) systems that the HVAC system can be divided to (2) main systems: Heating systems, Cooling systems. Here, we are interested in the cooling systems or what we will called “ Air Conditioning systems” in this course, which in turn divided to (4) sub-main systems: De-centralized systems (individual room systems), Semi-centralized systems (packaged systems), Centralized systems (central Hydronic systems), Special systems.

1.1 De-Centralized Systems (Individual Room Systems) These systems include: Window air conditioning units, Split air conditioning units, Mini-heat pumps. Window air conditioning units Split air conditioning units Mini-heat pumps For more information about Decentralized Systems (Individual Room Systems), please review Article " Air Conditioning System Configurations – Part One ".

1.2 Semi-Centralized Systems (Packaged Systems) These systems include: A- Unitary packaged systems (one pipe systems) which can be divided to: Packaged air conditioners with water cooled condensers, Packaged air conditioners with air cooled condensers. Packaged air conditioners with water cooled condensers -1 Packaged air conditioners with water cooled condensers -2 Roof Top Packaged air conditioners with air cooled condensers B- Ducted split systems (two pipes systems) Ducted split systems For more information about Semi-centralized systems (packaged systems), please review Article " Air Conditioning System Configurations – Part Two ". Also, the units used in packaged systems can be divided to: Rooftop packaged units, Indoor packaged units, Split units, A- Roof Top Packaged Units: Roof top packaged units (RPU) are an all in one air cooled A/C unit that are installed on top of the roof and can provide cooling and heating. The condensing unit and the air handler reside inside a single housing. In order to connect the unit to the inside duct work, an opening on the roof for supply and return air is necessary. The duct work distributes the air evenly through-out the space. Heating can be provided by reversing the condensing unit (heat pump) or connecting an electric heater, gas heater, or steam coil. Roof Top Packaged Units B- Indoor Packaged Units: Indoor package units (IPU) are similar to rooftop units, but because the unit is located inside, it requires a different way of cooling the condensing unit. The most common way, is air-cooled using louvered openings (horizontal angled slats that prevent outside weather to come inside, but allow air flow) to get fresh outside air. In situations where there is no access to fresh outside air, water cooled units are the best solution. Indoor units come in vertical, or space saving horizontal configurations. These units are the same as roof top units in the way they connect to duct work and have the same heating options as mention in the roof top packaged units. Indoor Packaged Units C- Split Packaged Systems: Split systems are comprised of a condensing unit (a condensing coil and a compressor) located outside, and an air handler (pushes air across the evaporator coil) located inside. Refrigerant lines for high and low pressure are run from the condensing unit to the air handler. Since the high pressure lines get cold, they are covered with insulation to prevent condensation issues and having water accumulate inside the space. The air handler comes in different configurations to best fit the desired installation. Split Systems can be divided to: Ducted (Connected to duct work), Ductless (wall mounted, ceiling suspended, ceiling cassettes, and floor standing). You may also have multiple air handlers connected to one condensing unit in computer controlled split systems.  Ducted Split Packaged Systems Ductless Split Packaged Systems

1.3 Centralized Systems (Central Hydronic Systems) These systems include: Centralized duct “All air” systems, Centralized fluid based hydronic systems “all water systems”, Combined (hybrid) water and air systems. Centralized duct “All air” systems Concept Centralized “all water systems” Concept Combined (hybrid) water and air systems Concept For more information about Centralized systems (central Hydronic systems), please review the following Articles: Air Conditioning System Configurations – Part Two. Air Conditioning System Configurations – Part Three.

1.4 Special Systems These systems include: Evaporative cooling systems, Central air washer (central evaporative air) cooling systems. Evaporative cooling systems Fixed Evaporative cooler Central air washer (central evaporative air) cooling systems For more information about Special systems, please review Article " Air Conditioning System Configurations – Part Three ".

Second: Introduction for Types of Motors/Compressors used in Air-Conditioning Systems

2- Types of Motors/Compressors used in Air-Conditioning Systems The types of Motors/compressors used for Air-Conditioning systems can be divided to: A- Positive-Displacement Compressors: They physically compress the vaporized refrigerant into a smaller volume and higher pressure, and include reciprocating, rotary, and scroll types. These deliver a constant volume of gas under a constant speed. They can be divided to: Reciprocating compressors, Screw Compressor, Scroll Compressors. B- Dynamic Compressors: They increase vaporized refrigerant pressure by the kinetic energy imparted on refrigerant by a rotating impeller. A centrifugal compressor is a dynamic compressor and is not a constant displacement type. This type includes: Centrifugal Compressors.

2.1 Reciprocating Compressors Reciprocating compressors are driven by a motor and use pistons, cylinders and valves to compress the refrigerant.  Reciprocating compressors are usually used in smaller systems up to 100 tons. These compressors are available in (2) configurations namely: Hermetic Refrigerant Motor-Compressor (welded Hermetic), Non- Hermetic Refrigerant Motor-Compressor, which can be divided to (2) sub-configurations: Semi-hermetic Refrigerant Motor-Compressor (Bolted Hermetic), Open type (Direct driven) Motor-Compressor. Reciprocating Compressors Types

2.1.A Hermetic Refrigerant Motor-Compressor (welded Hermetic) Hermetic Refrigerant Motor-Compressor Construction: A combination consisting of a compressor and the driving motor, both of which are enclosed in the same welded steel casing (housing) and the two are connected by a common shaft with no external shaft or shaft seals. This makes the whole compressor and the motor a single compact and portable unit. But since the components (compressor and motor) are not accessible for repair, the entire compressor unit must be replaced if it fails. Usage: It is widely used for the refrigeration and air conditioning applications like the household refrigerators, deep freezers, window air conditioners, split air conditioners, most of the packaged air conditioners. Window air conditioners with Hermetic Refrigerant Motor-Compressor Capacity: It is used with motor power requirements from 1/20 (for small refrigerator compressors) to to several hundred HP (for centrifugal chillers). With cooling capacity for one single unit about 10 ton but if more cooling capacity is needed, several hermetic motor-compressor can be installed in the same air conditioning unit. The smaller motors up to about 3 HP are usually of single phase design, while larger motors are invariably three phase. Advantages of the hermetic motor-compressors are: They are smaller, more compact and have less vibration than the open compressor, They have no external shaft; this eliminates mechanical problems with shafts, belts, and sheaves and concern of refrigerant leakage, In a hermetic unit the motor is located within the refrigerant atmosphere. So, the motor is continuously cooled by the refrigerant vapor flowing to the compressor section valves, Lubrication is also simplified since both the motor and the compressor operate in the same closed space with the oil. The shaft is shorter and more rigid, the bearing arrangement is simplified, the machine is quieter. The disadvantages of hermetic motor-compressors are: They are limited on capacity, They are limited on speed because the compressor has to run at the motor speed (direct drive), If the control of the related air conditioning system is poorly designed or ineptly operated, or if the compressor is allowed to start too frequently, there is a greater risk of motor burn-out than with open machines, They are not a field serviceable. Therefore, if a motor burns out in a hermitic compressor, or any other internal problem occurs, the maintenance trend is for a complete replacement of the unit. Otherwise, the entire unit must be returned to the shop or factory to be dismantled and reconditioned.

2.1.B Semi- Hermetic Refrigerant Motor-Compressor (bolted Hermetic) Semi- Hermetic Refrigerant Motor-Compressor Construction: Semi-hermetic Motor-Compressor is similar to the hermetic type but have a removable cover bolted on to the end of the casing to facilitate occasional maintenance. The motor is also part of the unit, however it is not sealed. So, Semi-hermetic compressor is almost completely accessible. Capacity and Usage: Welded, hermetic machines are used for the smaller duties, up to about 70 kW of refrigeration and bolted, semi-hermetics are used for larger loads. Packaged, water chilling units having multiple, semi-hermetic compressors are used with capacities up to about 700 kW of refrigeration. Advantage of Semi-Hermetic compressors over hermetic compressors: Semi-Hermetic compressors can be taken apart to do maintenance work or re-built a number of times if necessary giving a much longer service life. Semi- Hermetic Refrigerant compressors are direct drive and hermetic sealing.

2.1.C Open Type (Direct Driven) Motor-Compressor Open Type (Direct Driven) Motor-Compressor Construction: Open type of Motor-Compressor in which the compressor and the motor are different entities and the compressor is driven by external power source, such as an electric motor, an engine or a turbine through a direct coupling or a vee-belt. Power shaft protrudes through the compressor housing and seal is required to prevent refrigerant from leaking out of the compressor housing. Motor is cooled in a conventional manner by air that is drawn in from the surrounding. Usage and Capacity: Open Type (direct driven) Motor-Compressors use as an older technology (The motor and compressor are separated by a flexible coupling), so they are not commonly used today. The capacities for Open type of Motor-Compressors vary from a fractional ton to 400 ton in a single machine. Comparison between Open and Semi-hermetic / Hermetic Systems Open Semi-hermetic / Hermetic Open systems have motor separately connected to compressor. Not connected with refrigerant and oil charge in the system. Any problem in motor affects refrigerant charge and oil charge. Compressor being open design, can be inspected / opened by just closing the isolation valves. No botheration of removal / loss of refrigerant gas. Motor and compressor are in the same casing. Maintenance at site in case of hermetic is impossible and very difficult for semihermetic machines. Some of the internationally well known brands recommend opening of semi-hermetic also only at the factory / service centre and not at site. Since these can be attended to at site, down time is very minimal. If the compressor has to be taken to factory, long unavoidable delays will result and disrupt complete cooling. Since motors are outside the refrigerant environment, wide choice of makes and type possible. Hermetic compressors are not a field serviceable. Semi-Hermetic compressors can be taken apart to do maintenance work or re-built a number of times if necessary giving a much longer service life. Motor windings are cooled by ambient air. Motor windings are in the refrigerant vapour surroundings. Open machines are costlier compared to hermetic. Hermetic machines were basically introduced to reduce manufacturer's first cost. Motor is smaller and cheaper as it is cooled by the refrigerant. Copper and iron content are about half that of an open design. Open motors do not require any refrigeration effect and thus do not reduce the capacity of the system Hermetic motors consume cooling energy produced by the refrigerant. These vapours are to be compressed by the same compressor; Hence, effectively some cooling capacity is lost in motor cooling itself. Open system is about 10% more efficient considering loss in cooling motor and penalty in power for compressing vapour. Net effectiveness of cooling capacity and power consumption is about 10% poorer due to hermetic / semi-hermetic design. Open motors are more efficient as they are cooled / ventilated by ambient air. Hermetic motors are less efficient as they are cooled by refrigerant and are loaded beyond peak efficiency point. These motors are rotating in a much denser refrigerant atmosphere and have higher windage loss. Power fluctuations are not highly detrimental to open motors. Power fluctuations and electrical transients can produce a flash in the refrigerant atmosphere, which can break down into carbon, fluorine, chlorine etc. and will be carried into the system. When combined with moisture, hydrochloric and hydrofluoric acids are formed which can cause large – scale contamination. Insurance premia charged for open systems is much less as the damages are not catastrophic and not for long. Hermetics will take very long before they are put back into operation, hence the insurance companies charge much higher premium as the refrigerant and oil invariably need to be replaced. Overall maintenance is very fast, quick and without any problem on refrigerant side. Heavy maintenance in case of burnouts such as cleaning, flushing, vacuumising, pressure testing and recharging fresh refrigerant and oil. The refrigerant NH3 is very cheap (economical). HFC refrigerants are very costly and cannot return to soil if leaked. ODP and/or GWP problems are plenty. Oil being heavier than refrigerant, can be drained easily even during running. Oil is partially miscible and oil recovery and maintaining the oil level is a big problem.

2.2 Scroll Compressors Scroll compressors features two involutes scrolls, one stationary and one orbiting around the first. This movement draws gas into the outer pocket and the gas is forced toward the center of the scroll, creating increasingly higher gas pressures. The upper limit of the refrigeration capacity of currently manufactured scroll compressors is 60 tons.

2.3 Screw Compressors Screw compressors are based on a mechanism made up of two threaded rotors (screws) that are coupled together. The gas is compressed due to the progressive overlapping of the lobes, causing a reduction in the volume occupied by the gas. Continuous and step-less capacity control is provided by moving a sliding valve toward the discharge port, which opens a shortcut re-circulating passage to the suction port.  The refrigeration capacity of twin-screw compressors is 50 to 1500 tons but is normally used in the 200 tons to 800 tons range.

2.4 Centrifugal Compressors Centrifugal compressors are made up of a rotor located inside a special chamber. The rotor is rotated at high speed, imparting high kinetic energy to the gas, which is forced through the narrow outlet opening, thus increasing its pressure. The characteristics of a centrifugal compressor make it ideal for air conditioning applications because it is suitable for variable loads, has few moving parts, and is economical to operate.  The available refrigeration capacity for centrifugal compressors ranges from 100 to 2,000 tons.

Important Notes

1- Compressor Capacities: The size of refrigeration compressors is given in either of the following: Motor input horse power (HP), Motor input kilowatts (kW input), Refrigeration cooling capacity (kW cooling), British Thermal Units per hour (Btu/h) or Tons of refrigeration (TR): A refrigeration ton is equal to heat extraction rate of 12,000 BTU's/hr; therefore a 3 TR chiller can remove 36,000 BTU's/hr. 2- Usage Of Reciprocating Compressors hermetic Motor-Compressors is widely used for the refrigeration and air conditioning applications like the household refrigerators, deep freezers, window air conditioners, split air conditioners, most of the packaged air conditioners. Semi-hermetic Motor-Compressors are used with Packaged, water chilling units having multiple, semi-hermetic compressors with capacities up to about 700 kW of refrigeration. Open Type (direct driven) Motor-Compressors use as an older technology (The motor and compressor are separated by a flexible coupling), so they are not commonly used today.

In the next Article, I will explain Electrical Wiring for different Air-Conditioning Systems Types. So, please keep following.

 

 

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