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Economizer is also known as what, because it does not use any mechanical cooling?
- “free cooling”
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Dry bulb economizer uses no outside? The most common placement of such a unit is where?
- A Dry Bulb Economizer uses no outside humidity reference and is most commonly placed on a roof as a roof top unit.
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Enthalpy is?
the total heat contained in a material.
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Enthalpy economizer uses temperature and humidity levels of the outside air to control its operation. The outside air temp and humidity must meet what range?
The outside Air temp must be below 65F and between 30 and 60% humidity
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Compressor compresses what? And they may be of what type?
refrigerant or other fluid. They may be centrifugal, reciprocating (positive displacement), screw (helical gear compressor), or scroll.
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Direct expansion refrigeration systems: parts include what basic components?
a compressor, condenser, expansion device and evaporator. C.C.E.E.
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What are both the condenser and evaporator?
Both the condenser and evaporator are heat exchangers, where the condensor compresses refrigerant and uses coils to remove the heat, and the evaporator does the same thing as it expands. The compressor starts the process all over again.... CCEE Compressor, condensor, expansion, and evaporator.
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condenser is a?
heat exchanger that removes heat from high pressure refrigerant vapor. Usually a coil will allow either water or air to pass by and cool the heated refrigerant.
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the evaporator medium (used to cool) is what? there are two options.
The evaporator medium are either air or water.
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The expansion device will allow the refrigerant to vaporize and absorb heat as pressure decreases. It is then piped to the….
evaporator.
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evaporator is a?
Heat exchanger that adds heat to low pressure liquid refrigerant. An evaporating medium (air or water) of higher temperature passes by the refrigerant that cools the medium. The liquid refrigerant vaporizes as it absorbs heat from the medium. It passes though the evaporator across a series of baffles and then to the inlet of the compressor.
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Cooling towers allow heat to be ejected where?
To the outside air.
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What is the duty cycles preferred for
1. scroll _______ and ________.
2. reciprocating ___________avoid %.
3. rotary screw air compressor ________>%
Scroll air compressors work well in both low duty cycles and high duty cycles. With a reciprocating air compressor, you don't normally want a 100% duty cycle. With a rotary screw air compressor, you normally don't want a duty cycle below 50%. Scrolls operate well in all duty cycles.
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Reciprocal compression units are not common over_____ tons. And may use four or more compressors and a valve can be used to allow bypass for maintenance.
100
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Chillers are between what farenheight temperature range? Include the return.
42 and 44F and the return is normally between 52 and 55F.
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the heat anticipator is wired how?
- In series with the operating contact. ¹A heat anticipator's purpose in a low-voltage thermostat is to reduce the temperature swing caused by the system, and to increase overall efficiency. It does so by creating false heat, which increases the thermostat's rate of response. Source¹: ²
- Source 2: https://www.familyhandyman.com/project/how-to-adjust-a-mechanical-thermostat/
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a cooling anticipator is wired how?
anticipator settings are not used in digital thermostats
this is wired in parallel with the operating contacts
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The purpose of the solenoid valve in cooling is to control what?
This controls the flow of a fluid as its primary purpose in cooling.
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The adding of heat to a refrigerant that has reached its vapor saturation point is referred to as?
What happens when the reading of this is too high for this? Used to indicate how much refrigerant vapor is in the evaporator.
Superheating the refrigerant is what this is called.
Superheat is a temperature value measured of vapour (steam or refrigerant) above boiling point. The reading will show the amount of refrigerant going through the evaporator and whether it is sufficient. When the reading is too high, it means that the refrigerant is not sufficient, so the system will be inefficient. ¹
Source ¹ https://andersonair.com
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Which of the following is permitted to be used pressure checking a refrigeration system?
nitrogen is the only gas allowed to check for this.
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What is a PSC motor? It is commonly used in cooling and heating applications.
PSC: Permanent split capacitor motor uses a capacitor (a device that can store and release electrical charge) in one of the windings to increase the current lag between the two windings. Both the capacitor (auxiliary) winding and the main winding remain in the circuit the entire time the motor is running, hence the name “permanent.”
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Define a positive temperature coefficient PTC device, and what it is used for?
start assist for PSC motor. A resettable fuse is a polymeric positive temperature coefficient (PPTC) device that is a passive electronic component used to protect against overcurrent faults in electronic circuits. The device is also known as a polyfuse or polyswitch.
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A thermistor may be used for what regarding a PSC motor, and what is a PSC motor?
- to provide temperature readout to assist in the starting of a PSC (permanent split capacitor) motor and to stop a motor on an overload condition? Note: Thermistors are commonly used in hot aisles in data centers to determine air temperature.
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Define a thermistor and what happens to it as it heats up?
an electrical resistor whose resistance is greatly reduced by heating, used for measurement and control.
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What mechanical piece is adjustable on a thermostat?
- the bi metal strip
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What is a primary cause of hermetic compressor motor overheating and cutting out on the internal overload device?
refrigerant under charge is a common cause of this.
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which of the following is required when the thermostat subbase designed for two 24 volt power sources is used with one power source
- A jump wire is required to be connected between RH and RC terminal. If the air conditioning thermostat has one transformer to power both the heating and cooling system, then the wire from the transformer goes directly to the RC terminal and is called the RC wire. ... This wire is also known as the RH wire.
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What is the difference between a CRAC and a CRAH regarding a chiller plant or chilled water?
A CRAC unit or Computer Room Air conditioning Unit does not use a chiller plant or chilled water but rather a condenser. A CRAH or Computer Room Air handling unit uses chilled water from a chiller plant.
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Refrigerant vapor gets hot when compressed. Why?
It has a very low boiling point and absorbs heat as it vaporizes.
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Volute is?
is the opening in a centrifugal compressor that allows you to increase pressure of the refrigerant but not the velocity.
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What does DX stand for in Heating Ventalation and Cooling systems.
Direct Expansion
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What is a VAV and what does it stand for?
It is a variable air ventilation dampener that provides control for hot or cold air by volume through a duct.
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What does the acronym VRF stand for in Heating ventilation and cooling? What type of expansion is this used with?
Variable Refrigerant Flow is VRF in HVAC. This type of cooling is used in Direct Expansion or DX
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There are two types of central air conditioning systems, what are they?
Direct Expansion (DX) type of central air condition plants and Chilled Water type of the central air conditioning plants.
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What is the difference between the set point, control point, and offset?
- 1. The set point is the desired temperature.
- 2. The control point is the actual value that a control system experiences at any given time.
- 3. The difference between a control point and a set point.
Source: Ronnie J. Auvil, HVAC Control Systems 3rd ed. 2013 American Technical Publications. PP 94
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The measurement of the results is the ___________ of the system.
feedback.
Source: Ronnie J. Auvil, HVAC Control Systems 3rd ed. 2013 American Technical Publications. PP 94
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This is what is called feedback occurring between the controller, sensor, and controlled device.
What is known as closed loop control.
Source: Ronnie J. Auvil, HVAC Control Systems 3rd ed. 2013 American Technical Publications. PP 94
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A cooling tower for a condenser water is what type of system?
This is an open system design as the piping is open to the atmosphere at some point.
Source: Ronnie J. Auvil, HVAC Control Systems 3rd ed. 2013 American Technical Publications. PP 50
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What is the GPM required for correct water flow in any branch of the system as a guideline for chillers and condenser flow through a centrifugal chiller?
- Chillers: 2.4 GPM/ Ton.
- Condenser flow through a centrifugal chiller: 3GPM/ Ton.
- Source: Ronnie J. Auvil, HVAC Control Systems 3rd ed. 2013 American Technical Publications. PP 50
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