Exam 2 Flashcards
The Big Four
Split DX
Roof Top Unit
Air Handling Unit (AUH) / Fan Coil Unit (FCU)
Chiller / Boiler
Where is the condensing unit located in a Split DX System?
Outside
Where are the evaporator coil, expansion device, and fan located in a Split DX System?
Inside
Name the different parts of the condensing unit:
Compressor
Condenser Fan
Condenser Coil
Common size for commercial offices:
25-30 tons
Roof Curb
a device that allows the mounting of the RTU on top of the roof and provides the penetrations for duct, pipe, conduit, etc.
RTUs must have a what installed? Describe it.
Economizer.
Mechanical devices intended to reduce energy consumption, or perform useful functions such as preheating a fluid. It allows up to 100% use of outside air for cooling
What does Texas have to say about Economizers?
The energy code for most municipalities has an exception that allows the installation of RTUs without the economizer
What will happen if the condenser coils on an RTU are damaged?
They will not reject heat very well
What is Francis Hall’s floor area?
40,000 sq. ft.
CSI Section 23 00 00
Heating, Ventilating, and Air Conditioning (HVAC)
Cooling coils/heat exchangers are known as
HX
The chilled water typically exits the HX at about what temperature?
55 degrees F
In a chilled water system, the water is cooled to what temperature?
45 degrees F
Air cooled chiller systems can vary in capacity from _______ tons
10-125 tons
CSI Section 23 64 00
Packaged Water Chillers
The primary component of a typical chilled water loop cooling system
The Chiller
Compressor Types
Centrifugal for large systems (>200 ton)
Screw (150 – 600 tons)
Scroll (5 – 25 tons)
Heat Rejection
Air-cooled condenser
Cooling tower
Zachry Engineering complex cooling load:
About 1,000 tons
What type of HVAC system is recommended for Zachry Engineering complex?
4 water-cooled Chillers each of them distributing the cooling load. That way if one fails, the client would still have some cooling capacity.
Parts of a Centrifugal Chiller
Condenser water connections Condenser Compressor Motor Electrical Service Lubrication Pump Controls Evaporator Chilled water connections
Normal design operation for water-cooled centrifugal chiller
Chilled water supply = 45°F.
Chilled water return = 55°F.
Cooling tower supply = 80°F.
Cooling tower return = 90+°F.
Is the condenser barrel on a chiller typically insulated?
NO
Condensation on the surfaces is not an issue.
What is the primary difference between an air-cooled chiller and a centrifugal chiller?
Physical limitation on size. They top out at about 150-200 tons of capacity.
Chilled water cooling loop component
Chilled water pump
Building cooling load
Chiller
- Evaporator
- Expansion Valve
- Condenser
- Compressor
Cooling tower pump
Cooling tower
Any chilled water loop system MUST implement at least _______________ operation when outside ambient conditions allow such operation at the cooling towers
Partial “free cooling”
What is the primary limit on cooling tower thermal performance?
Wet bulb temperature
Where would you find parallel fan powered VAV box?
Perimeter zones in office buildings.
For very high wet bulb temperatures, the cooling tower must be sized larger or smaller?
Must be sized larger for a given capacity.
For most HVAC applications, the design engineer would specify a tower with what approach?
10 to 15° F “approach”
What does a 10 to 15° F “approach” mean?
a design point that will ensure that the cooling tower can provide cooling water back to the chiller at a temperature that is 10 – 15°F warmer than the coincident wet bulb temperature.
Two most common cooling tower configurations used in various HVAC applications
Counter flow and cross-flow
What CSI Section contains information about Air Handling Units (AHU)?
CSI Section 23 73 00 - Indoor Central-Station Air-Handling Units
What CSI Section contains information about Fan Coil Units (FCU)?
CSI Section 23 82 19 - Fan Coil Units
What CSI Section contains information about Variable Air Volume Terminal Units (VAV Boxes)?
CSI Section 23 36 16 - Variable Air Volume Units
Differences/similarities between AHU and FCU?
Differences:
Size
FCU typically has an integrated fan/motor assembly whereas an AHU has a motor separately mounted from the fan.
Similarities:
Both have fans, heat exchangers (coils) and connect via duct-work to diffusers and grilles.
How do you know if a valve is open or closed?
Parallel = open Perpendicular = close
In what type of climate would you recommend a 2-pipe chilled water unit?
Predominantly cooling (interior zones of multi-story high rise)
In what type of climate would you recommend a 4-pipe chilled water unit?
Where there is a need for both heating and cooling simultaneously
What type of system dominates the commercial HVAC market?
Variable Air Volume (VAV)
Configurations of VAV Boxes
Single duct VAV Box Single duct VAV Box with reheat (hot water or electric) Series fan-powered VAV Box Series fan-powered VAV Box with reheat Parallel fan-powered VAV Box
How does a VAV system control flow?
By maintaining a constant duct pressure
What is a common control pressure?
Around 2.0 inches of water gage (w.g.)
The variable frequency drive (VFD) is also known as the:
Variable speed drive (VSD)
The energy use with a VAV System is about _____% less than a Constant Volume (CV) based system
50%
What is the primary function of a roof curb?
Provide a secure mounting location for the RTU
Chilled water loop systems using chillers can reject heat at their condensers through:
Air or water as needed for the application.
(Either air or water depending on application because if it is a small capacity unit, we wouldn’t go to the expense of using a cooling tower. Scale (size) become important for “break point” of air vs. water.)
What “heat” is being rejected from a chiller condenser?
Heat from the refrigerant
What is the main constraint limiting the size (capacity) of air-cooled condensing for a chiller?
Physical size of the condenser
When talking to an HVAC contractor about a chiller and they mention that “insulation on the barrel needs to be protected” which barrel are they talking about?
Evaporator
Why is it so easy to identify the condenser barrel on a centrifugal chiller?
It is not insulated
“Free cooling” as described in the latest ASHRAE energy standard is referring to what?
Cooling towers are used to proved chilled water
If using a water cooled chiller on your 300,000 sq. ft. commercial tower, you would expect what kind of chiller on this project?
Centrifugal compressor, water cooled
