Unit 14 Review Questions
What would the expected condensing temperature be for an air-cooled condenser on a 100°F day?
100°F + 25°F to 30°F = 125°F to 130°F
How much additional cooling capacity would 20°F of subcooling have on a refrigerant system?
20°F x 0.5%/°F = 10% increased capacity
What does a cooling tower do?
A water-cooling tower cools the majority of the water flowing through it by evaporating a small portion of the water. This cool water is then sent back to the condenser.
List the three types of condensers used with HVACR systems.
Air cooled, water cooled, evaporative
What can happen to a cooling tower if the water is not treated?
Biological organisms can grow, fouling the tower and condenser. Scale can form when the pH gets too high.
List the methods of reclaiming condenser heat?
Desuperheating hot water heater or a heat reclaim condenser to use heat from refrigeration machines to heat the store.
List some of the factors that can cause the condensing temperature to be too high on an air-cooled system?
Dirty condensers, mechanical damage to the condenser fins.
List the different types of water-cooled condensers.
Double pipe (tube in tube), shell and coil, shell and tube.
Explain how an evaporative condenser works.
Evaporative condensers are similar to closed loop cooling towers, except the coil is a refrigeration coil instead of a water coil. They work by evaporating water to cool the refrigeration coil.
What are some ways that an air-cooled condenser can be made more efficient?
Increase the coil size to increase the heat transfer area Increase the airflow rate to remove more heat Decrease the condenser tube size so that more tubing can be added which increases the heat transfer rate
Why would a building need air conditioning if the outdoor temperature is 35°F?
Internal heat loads from people, lights, and machines.
Why is subcooling important?
It ensures that a solid stream of liquid will enter the metering device. It adds to the cooling capacity of the system at a rate of about 0.5% of the total cooling capacity per degree of subcooling.
What can happen to a water-cooled condenser if the refrigerant is recovered improperly?
Low refrigerant pressure and temperature can drop below freezing, freeze the water in the condenser, and damage the condenser.
List some of the factors that can cause the condensing temperature to be too high on a water-cooled system.
Low water flow, scaled condenser surfaces, or fouled condenser surfaces.
What type of water-cooled condenser is easiest to clean?
Shell and tube.
How much heat must a condenser be able to dissipate?
The amount of heat absorbed by the evaporator + the heat added by the compressor
What advantage are there to using a remote air-cooled condenser?
The compressor can be placed inside the building out of the weather and separate from the condenser. Also the heat will be exhausted outside rather than indoors which in the case of a kitchen, may already be a very warm place.
What problems can be created if the system high side pressure is too low?
The metering device may not feed correctly, leading to low evaporator pressures and reduced capacity.
List the different types of low ambient controls for air-cooled condensers.
The most common are fan cycling, fan speed control, damper control, and condenser flooding.
What is the purpose of a condenser?
The purpose of a condenser coil is to reject unwanted heat from a refrigeration or air conditioning system. It does this by condensing the high pressure, high temperature gas to a liquid.
Describe the changes in the refrigerant as it passes through the condenser.
The superheat must be removed first before the refrigerant can condense. This occurs in the first portion of the condenser known as the desuperheating section. Approximately 15% of the heat is removed here. Then the vapor is condensed throughout most of the condenser. Approximately 80% of heat is rejected through condensation. After all the vapor is condensed to liquid, the liquid is subcooled in the last portion of the condenser. Approximately 5% of the heat is rejected during subcooling.
What is the difference between open and closed cooling towers?
The water circulating in an open loop system is also the water circulating through the condenser. Closed loop systems use a heat exchanger to separate the water in the condenser loop from the water in the tower.
What is the most important operating characteristic to check whenever working on a water-cooled system?
The water flow.
What is the purpose of the fins on finned tube coils?
To increase the surface area exposed to the air and increase heat transfer efficiency.
What can be placed in the water line on a wastewater system to keep the head pressure constant?
Water pressure regulating valve
Compare the discharge pressure and operational cost of air-cooled condensers versus water-cooled condensers.
Water-cooled condensers operate at a lower discharge temperature and pressure than air-cooled condensers, approximately 15°F lower. This increases their efficiency.
Which requires more maintenance: air cooled condensers or water-cooled condensers?
Water-cooled condensers.
What do environmental concerns have to do with water-cooled condensers?
With water resources becoming a precious commodity in many parts of the country, wastewater systems appear to be an extravagant use of a precious resource.
What water conditions must be monitored to keep a cooling tower system working properly?
pH, mineral content, and biological growth