HVACR Practice Test Kit

The symbol or letter for impedance is Z and it is represented by writing its magnitude and phase.

The NEC and NFPA has designated green as a ground.

The opposition to the passage of current flow is resistance.

Power laws P = I x E. 480 x 5 = 2400 watts.

Ohmmeters measure resistance and cannot be used in a live circuit, therefore must have its own power source.

Fuses are rated in voltage and amps.

The 60 watt bulb will act as a conductor and only the lower watt bulb would light. There is a small amount of voltage drop across the 60 watt bulb but usually not enough to make it light.

When capacitors are wired in parallel, the formula used for combining the capacitors is C₁ + C₃ = C_t : 25 + 5 = 30

The input side of a transformer is known as the primary side.

480 watts divided by 240 volts will equal 2 amps, therefore Ohm's law will apply voltage divided by amperage equals resistance:

240 ÷ 2 = 120 Ohms

Other formulas which can be used are as follows:

W ÷ I² = R or E² ÷ W = R

Parallel circuit laws state that total resistance in a parallel circuit is always less than the smallest resistor in the circuit.

The smallest resistor is 24 ohms, therefore the total must be less than 24. One of the parallel circuit laws state that the total resistance is always less than the smallest resistor in a parallel circuit. There are two formulas for finding total resistance in a parallel circuit:

ECM motors are designed to maintain the proper air flow based on the static pressure to a predetermined limit set by the manufacturer.

Connecting an ohmmeter to live voltage can damage the meter and possibly harm the person testing with the meter.

An inductive load is a load consisting of a coil of wire which will cause current to lag voltage; such as a motor or transformer.

The ratio of turns is 10 to 1, therefore 240 divided by 10 equals 24 volts secondary.

When current flows through a conductor, a magnetic field is created and extends out from the conductor.

Inductive reactance is a phenomena that takes place in an alternating current circuit involving a coil of wire and the reversing magnetic fields which will create a voltage in the opposite direction. This counter EMF will act like resistance to the applied voltage.

Current type motor starting relays are used for assisting in the starting function of single phase AC motors of resistance start, induction run and capacitor start, induction run types. The primary function is to remove the start capacitor or start winding from the circuit.

Alternating current motor speed is based on the number of poles in the motor.
Examples:
2 pole motor would be 3600 RPMs
4 pole motor 1800 RPMs

By reversing any two leads of a three-phase power supply, the rotation of a three phase motor will reverse

The motor starter incorporates a contactor and overload relays to open the circuit in the event of an overload to prevent the motor from overheating.

Contactors are rated by the amperage draw through the contacts and cause minimal arching.

A pictorial wiring diagram is a simplified conventional representation of an electrical circuit. It shows the components of the circuit as simplified shapes, and the power connections between the devices. This wiring diagram usually gives more information about the relative position and arrangement of devices and terminals on the devices.

Any time there is a change of temperature indicated by the thermometer the heat level or intensity can easily be measured. In the writings of the early scientists who provided the foundations of thermodynamics, sensible heat had a clear meaning in calorimetric.

Heat, which causes a change of state without a change in temperature, is known as latent heat. An example of latent heat is water changing to ice or vice a versa. Latent heat is the heat released or absorbed by a body or a thermodynamic system during a process that occurs without a change in temperature.

Superheat is best described as heat above the saturation (change of state) temperature of a vapor; it is any heat added causing a change in temperature after all the liquid has boiled away.

Heat transfer is a discipline of thermal engineering that concerns the conversion, and exchange of thermal energy and heat between physical principles. Heat transfer is classified into various methods, such as thermal conduction, thermal convection, and thermal radiation.

Radiation is the transfer of energy through space by way of electromagnetic waves The same laws that govern the transfer of light govern the radiant transfer of heat.

When refrigerant gas is compressed, the pressure and temperature rapidly increases above saturation temperature. The gas enters the condenser which will cause the gas to de-superheat, change state (Vapor to liquid) and finally become subcooled. During this whole process the heat is being released by sensible and latent processes.

When gas travels from a direct expansion evaporator to the compressor, it is in a superheated state. This will assure that no liquid will enter the compressor that can cause damage. Being superheated means that no liquid is present.

Pounds per square inch absolute (psia) is used to make it obvious that the pressure is relative to a vacuum rather than atmospheric pressure. The reverse is pounds per square inch gauge (psig), indicating that the pressure is relative to atmospheric pressure. Since atmospheric pressure at sea level is 14.7 psia, 14.7 must be subtracted from any psia reading above atmospheric to achieve a psig reading.

The compound gauge is mounted on the left side of the gauge manifold set and connects to the low side of an air conditioning system. The compound gauge allows the HVAC technician to measure pressure above atmospheric pressure and vacuum pressure (below atmospheric pressure). Some systems may operate in a vacuum to obtain a low evaporator temperature.

The main function of any air condition condenser is to remove both sensible and latent heat from the refrigerant by causing the gas from the compressor to go through three stages; de-superheat, change of state and sub-cool.

The pressure should not change entering or leaving the condenser. The condenser's primary design is to expel the heat from the refrigerant that was absorbed in the low side of the system. The compressor will maintain the pressure and only the temperature will change.

When the refrigeration cycle is super imposed onto a pressure enthalpy diagram, de-superheating is located between points 2 and 3.

Crankcase pressure regulating valves are designed to prevent overloading of the compressor motor during and after a defrost cycle or a normal pull-down period by limiting the crankcase pressure to a predetermined maximum value.

Water medium is much more efficient than air cooled condensers, therefore a higher than 10° F split will cause high head pressure and the system would not operate efficiently.

When a system has a restricted filter drier, it causes a pressure drop across the drier, thus restricting the flow of refrigerant, and the difference in pressure results in a temperature drop across the drier at the point of the restriction.

When oil coats the walls of the evaporator coil it will act as an insulator thus reducing heat transfer. This condition could cause liquid flooding back to the compressor. It will also rob the compressor of oil needed for proper operation. The causes of oil logged evaporators are too much oil, improper piping practices and liquid migration to the compressor during off cycle.

Moving the probe any faster than the recommended speed of one inch per second may result in leaks not being detected.

Refrigerant must be recovered with approved equipment according to EPA Section 608. Failure to recover could result in loss of certification, fines and having to appear in federal court.

The water level in the trap must be low enough that a pressure higher than the static pressure of the air-handler would be required to make it flow back into the drain pan. The higher the air-handle static pressure, the more vertical drop is required.

The Btu output along with the temperature difference across the furnace can easily be calculated and applied to the sensible heat formula. CFM = Btu divided by (1.08 times the temperature difference).

Inverter driven compressors operate at varying speed by changing the AC voltage and frequency.

An inverter changing the voltage from AC to DC then pulses the on and off time of the applied DC voltage to the motor from 0 to approximately 120 cycles.

Some digital electronic thermostats can use a shielded 2 conductor cable with a 1 positive and 1 negative connection between the indoor and out sections of a split system.

Two wire data communicating controls are called direct digital controls.

A sling psychrometer is a device that consist of two thermometers fitted in a hinged bracket used for spinning the two thermometers. One of the bulbs is kept moist with distilled water on a sock or wick placed over the thermometer's tip. As the thermometers are rotated, evaporation of water from the wick lowers the temperature indicating a lower temperature than that of the dry-bulb thermometer. This is the Wet bulb temperature which will be lower than the dry bulb and can never be above dry bulb measurement. The lower the relative humidity, the lower the wet bulb temperature. The difference in the two bulbs readings can be applied to a psychrometric chart to calculate relative humidity and other properties of air.

Compression ratio is found by dividing the absolute discharge pressure by the absolute suction pressure. PSIG is converted to absolute by adding 14.7 to the pressure.

In this problem, discharge pressure 235.3 plus 14.7 equals 250 PSIA and the suction pressure 35.3 plus 14.7 equals 50 PSIA.

250/50 = 5

The ratio is 5 to 1

The inlet of the hot gas bypass valve is piped from a tee in the discharge line at the outlet of the compressor. The outlet of the bypass valve is piped to a tee between the expansion valve and the distributor of the evaporator coil.

When the load or demand drops in the evaporator, hot gas is piped into the evaporator and mixed with saturated liquid. This keeps the pressure up to prevent short cycling due to low capacity.

Bulb pressure is the opening force of a thermal expansion valve. When the bulb charge is lost, the spring and evaporator pressures valve will cause the valve to close or slow down the feed to the evaporator dropping the pressure.

By increasing water flow, this will cause the discharge pressure to drop. More water at a cooler temperature will absorb more heat and lower the pressure. Water cooled condensers are designed to maintain a 10 degree differential between the refrigerant and water temperature.

A dirty air filter will decrease air flow resulting in lower evaporator pressure and temperature; once the temperature is below 32°F the evaporator will frost over, acting as an insulator then the suction line will develop ice to the compressor.

Efficiency of reciprocating compressors are reduced due to the gas that re-expands in the clearance space. The greater the clearance between the top of the piston and bottom of the valve plate, the more the gas will re-expand on the down stroke of the piston. The suction valve will not open until the pressure is lower than pressure on the other side of the valve.

Used to maintain the proper head pressure in the condenser, the water regulating valve has a diaphragm that opens the valve as the head pressure rises increasing the flow of water. It works off the principle of refrigerant pressure/temperature relationship.

Five gallons of water is equivalent to 41.65 lbs, therefore the BTUs absorbed by water is equal to the BTUs given up by the refrigerant. Since the water will absorb 10 BTUs of heat, the temperature will rise 1 degree per lb. Water enters at 80°F and would leave at 90°F.

The primary function of a vibration eliminator is to absorb compressor vibration and stop the transition of vibration into the piping and components of the air-conditioning system. By isolating the compressors vibration, the risk of damage to piping and the connecting components are reduced.

Vibration eliminators should be installed perpendicular to the direction of vibration (parallel with the compressor crankshaft).

Enthalpy is the total amount of heat contained in one pound of a substance and is calculated from an accepted temperature base. The temperature base for refrigerants is minus 40°F.

Non-condensables in an air conditioning system are gases that will not condense at the same temperature and pressure in which the refrigerant will condense. Non-condensables take up valuable space in the condenser which is used for removing the heat from the refrigerant allowing it to condense into a liquid. If the refrigerant cannot condense into a liquid taking up less space, the gas will cause the temperature and pressure to rise causing high amperage draw.

Un-loaders are devices used on reciprocating compressors to control capacity. Un-loaders will disengage one or more cylinders hydraulically or electrically lifting the suction valves off its seat thus controlling capacity. When suction valve is lifted off its seat the gas enters the cylinder on the down stroke and when the compressor on the up stroke will force the gas back into the suction port. This will reduce capacity and energy consumption of a compressor.

Systems with multiple evaporators must have a receiver to store refrigerant, due to the different demand and feed of refrigerant to each evaporator.

The EPR valve is used to permit two or more evaporators, to work at different load temperatures and to function with one compressor. They maintain the evaporator pressure it is connected to according to a set point and are not dependent on the loads. The evaporator pressure is used to control the operation of the valve.

Flooded evaporators are normally used for 100 tons or above cooling capacity. This metering device maintains the entire evaporator at a particular level of liquid. As some of the liquid refrigerant boils off, the float opens a valve allowing more liquid to enter the evaporator. Many flooded evaporators are also used in industrial process refrigeration equipment.

In a direct expansion evaporator the refrigerant enters as a liquid and as it absorbs heat the liquid boils changing into a vapor. After the change to a vapor it absorbs more heat in the last rows of the evaporator and becomes a superheated gas.

The pressure must be set above the working pressure for an R-410A system. The design condensing temperature is from 65°F to 120°F. The pressure is 417 psi for 120 degree condensing temperature. The cutout must be set approximately 45% - 50% above the operating pressure.

Due to the negative pressure, without a P-trap air will be sucked through the drain and will block the flow of condensate.

Depending on the size of the evaporator and liquid line, the condenser may not be large enough to hold all the refrigerant during pump down.

A receiver is used to store liquid refrigerant for varying loads, multi evaporators and pump-down.

Oil pump is located inside the compressor, therefore it must compensate for surrounding refrigerant pressure. The difference between the measured oil pressure and the crankcase pressure is the net oil pressure.

By piping the hot gas back into the suction line the compressor pumping efficiency is reduced.

The regulating valve must sense the pressure that is trying to control.

The (G) terminal is used to control the indoor blower relay on most thermostats.

When multiple compressors are used in conjunction with each other it is important to keep the oil level the same in all the compressors.

Due to pressure temperature relationship, thermostatic expansion valves must sense the correct pressure at the location of the thermal bulb to maintain the correct superheat. Pressure drop is usually caused by size or capacity of the evaporator taking into account the number of U-bends.

Direct type evaporators are designed to operate most efficiently with approximately 10 degrees superheat. In order to have superheat, all liquid must be boiled away before reaching the end of the evaporator.

A valve that is in the maximum open position will have liquid refrigerant flowing at a rate faster than the evaporator could boil it away; thus creating a flooded coil and liquid refrigerant going back to the compressor.

The condenser causes the refrigerant to go through three processes: first de-superheat, second a change of state and finally the refrigerant is cooled below saturation temperature. The refrigerant is sub-cooled at this point. The amount of sub-cooling is critical to the system efficiency.

All of the above condensers are classified as water cooled type condensers. Tube within a tube condensers are used in systems such as: residential water source heat pumps and shell type condensers are used in commercial applications.

Efficiency of reciprocating compressors are reduced due to the re-expanding gas that is in the cylinder clearance space . The greater the clearance between the top of the piston and bottom of the valve plate, the more the gas will re-expand on the down stroke of the piston reducing the pumping efficiency .

The most likely place for an open type compressor to leak is around the shaft that extends through the shell of the compressor and the seal. Friction will eventually wear out the seal.

High compression ratios conditions are caused when the spread between suction and discharge pressure increases. This will cause the efficiency to decrease. Each type of compressor will have its own ratio guide based on manufacturer's design.

The automatic expansion valve is designed to maintain a constant pressure in the evaporator. This valve can only be used when the evaporator load does not change very much. When the valve responds to higher load demands, it functions opposite to TEV valve, it will temporally cause the valve to close as demand increases.

The high velocity fans are delayed from coming on to prevent any water left on the evaporator from been blown into cooler space causing an ice safety hazard.

An off cycle defrost method is used when the coil is slightly frosted over from operating below or at 32°F refrigerant temperature. Defrost is dependent upon warm air in the cooler above (32°F) to melt the ice before the next on-cycle.

Short cycling is caused by discharge pressure leaking back to the low side through the compressor suction valve.

The automatic expansion valve may be used to prevent the refrigerant pressure, thus the evaporator temperature from dropping below the freezing point.

The discharge pressure is controlled by cycling the condenser fan(s). When the ambient temperature drops, a decrease in head pressure will result. Reduced refrigerant pressure reduces the flow causing decreased capacity.

When ambient temperature drops the compressor can become cold and refrigerant may migrate to the oil or condense in the compressor. As the suction pressure drops on start-up, the refrigerant will boil causing the oil to foam and be pumped from the crankcase. To prevent this, a crankcase heater heats the oil to prevent the refrigerant from mixing with the oil.

All compressors that are hermetic are of the welded type, and cannot be easily taken apart. Semi-hermetic compressors are designed to be taken apart and usually have the bolted type housing.

As water is evaporated from the cooling tower, minerals that are left behind will form scales. After a period of time the scaling will cause poor heat transfer and inefficiency. De-scaling is preformed with chemicals at least once a year.

Cascade systems are an arrangement of two or more refrigeration systems that are used in series to achieve ultra-low temperatures. The evaporator cools the condenser of the other system.

High temperature refrigeration applications normally are 47 to 60 degrees box temperature. The storage of such things as flowers, candy and some fruit and vegetables must be maintained at these temperatures.

When the fan is not running, some of the refrigerant will not boil away in the evaporator and liquid will flow out of the evaporator to the compressor. A suction line accumulator is used to prevent liquid refrigerant from entering the compressor.

A service valve is a manually controlled valve with three ports: line port, compressor port and a service port. This valve has three positions: back-seated position for normal operation, front-seated position for isolating a component and cracked position when servicing the system. When the service valve is in the cracked position all ports are open.

A low pressure control can be used like a thermostat to control temperature. The switch works off the principle of pressure temperature relationship. By controlling evaporator pressure, temperature can also be controlled.

Solenoid valves are shutoff valves that are opened or closed by an electro magnetic coil which controls an armature inside the valve. It is normally used for controlling refrigerant flow in the liquid line.

An oil separator is a device installed in the discharge line between the compressor and condenser to help remove oil from the gaseous refrigerant. The oil is returned to the compressor by the use of a float or some other means. If during the off cycle the oil separator is allowed to cool, some gaseous refrigerant may condense and displace the oil and flow back to the compressor causing damage on start-up.