Term
159. Cooling liquid ammonia feeding low temperature evaporators serves to:
Reduce air unit capacity
Increase the amount of oil accumulation.
Reduce the amount of flash gas handled by the booster compressor
Reduce condenser load |
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Definition
| Reduce the amount of flash gas handled by the booster compressor |
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Term
160. Operating a compressor at high compression ratios
Results in higher operating costs
Reduces oil migration
Reduces heat of compression
Improves overall system efficiency |
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Definition
| results in higher operating costs 9-2 |
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Term
161. A cascade system may be used:
To reduce initial equipment costs
To improve evaporator capacity
To obtain greater efficiency than an intercooler
To obtain -100f or lower temp |
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Definition
| to obtain -100F or lower temp 9-25 |
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Term
162. What is the advantage of two stage compression in systems operating low temp
Only a two stage system can achieve the overall high compression ratios necessary for low temp
The two stage system uses less equipment
The two stage system has lower initial cost
The two stage system is more efficient (lower kWh per ton) when operating at low temps |
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Definition
| the two stage system is more efficient (lower kWh per ton) when operating at low temperatures 9-13 |
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Term
163. High speed ammonia, V and W style, compressors are limited to what practical compression ratio?
4:1
8:1
75:1
14:1 |
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Definition
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Term
164 Higher compression ratios
Consume more power
Increase refrigeration effect
Stabilize the operation of the system
Improve oil flow in the compressor |
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Definition
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Term
165 in a single stage system, what happens to theoretical compressor discharge temperature as evap temp is lowered by lowering the suction pressure?
Compressor discharge temp and evap temp are not related
As suction pressure is lowered, compressor discharge temp decreases
As suction pressure is lowered, compressor discharge temp increases
Theoretical compressor discharge temp becomes unstable as suction pressure is lowered |
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Definition
| As suction pressure is lowered, compressor discharge temp increases 9-13 |
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Term
166 as compressor discharge temp increases…
Compressor oil’s lubricating quality is reduced
Compressor volumetric efficiency increases
Compressor oil heaters are turned on
Condenser rated capacity is increased. |
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Definition
| compressor oil's lubricating quality is reduced. 9-7 |
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Term
167 in a two stage system the booster compressor discharge gas is normally fed to
The condenser
The discharge port of the high stage compressor
The suction side of the high temperature evap
A desuperheating device, then the high stage compressor. |
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Definition
| a desuperheating device, then the high stage compressor 9-4 |
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Term
168 what is intermediate pressure?
Any pressure reading between the high stage compressor and the condenser
The discharge pressure of the high stage machine on a two stage system
The pressure reading between the low stage discharge and the high stage suction
The suction pressure of the low stage machine on a two stage system |
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Definition
| the pressure reading between the low stage discharge and the high stage suction 9-15 |
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Term
169 what is the effect to system equipment selection of subcooling liquid refrigerant before going to the low temp evap?
There is less flash gas in the intercooler
The booster compressor runs at a lower compression ratio
The booster compressor can be smaller
Subcooling liquid refrigerant before going to the evap is an advantage to the condenser not the booster compressor |
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Definition
| the booster compressor can be smaller 9-16 |
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Term
170 what changes occur to the high pressure liquid entering a flash type intercooler?
The pressure drops rapidly to intermediate pressure which causes some of the liquid to flash into vapor
The pressure drops gradually as the liquid is cooled by contact with the saturated liquid in the intercooler
The liquid is contained in an evaporator bathed in low temp liquid. This creates high pressure subcooled liquid.
Some of the liquid flashes as the pressure drops, creating a subcooled liquid in the intercooler |
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Definition
| The pressure drops rapidly to intermediate pressure which causes some of the liquid to flash into vapor. 9-17 |
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Term
171 why may it be difficult to detect leaks in the low side of a two stage ammonia system operating in a vacuum?
Air is leaking in instead of refrigerant leaking out
There is little pressure so a lot of gas escapes
The purger does not cycle as often
Leaks are detected by a decrease in compressor discharge pressure. |
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Definition
| Air is leaking in instead of refrigerant leaking out. 9-19 |
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Term
172. a cascade system is
A refrigeration system in which the high temp evap is the condenser for the lower temp section
A refrigeration system in which a single refrigerant is used and the low stage compressor is discharged to a high stage compressor.
A system with two or more compressors mounted on a single oil separator vessel.
Any system where the heat absorbed in the evaporator travels through multiple components, cascading from one to the other. |
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Definition
| A refrigeration system in which the high temp evap is the condenser for the lower temp section. 9-20 |
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Term
173 what pressure characteristic does a low temp refrigerant for a cascade system have?
It operates above atmospheric pressure in the evap
As a liquid, it can be stored at low pressures
It cannot be stored as a gas because the pressures are so high.
It must be kept cool so that it can be stored at operating pressure |
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Definition
| It operates above atmos pressure in the evap 9-20 |
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Term
174 why would two different refrigerants be used in a cascade system?
To assure different lubricants are used in the high and low pressure portions
To avoid operating at tremendously high compression ratios.
So that centrifugal compressors can be used for both pressure ranges.
A cascade system only uses one refrigerant. |
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Definition
| To avoid operating at tremendously high compression ratios. 9-21,22 |
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Term
175 what advantage would a high pressure refrigerant bring to the system in the low temp portion of a cascade system?
It eliminates the need to operate in in a vacuum
It is less expensive
It is easily stored
It does not need a heat exchanger. |
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Definition
| It eliminates the need to operate in a vacuum 9-25 |
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Term
176 does the cascade system require balancing oil charges between systems and why?
Yes, because the oil migrates to the lowest temp in the system
No, because the system used two refrigerants which do not mix.
Yes, because the oil is entrained in the refrigerant and accumulates in the thermosiphon oil coolers, starving the low temp system for oil.
No, because the system is normally equipped with AMOT valves to balance the oil |
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Definition
| No, because the system uses two refrigerants which do not mix. 9-26 |
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Term
177. the primary refrigerant in a vacuum cooling process is
Ammonia
Carbon dioxide
Water
Decided on by the designer |
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Definition
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Term
178 two stage systems without additional evap loads at the intermediate pressure
Have booster compressors that are smaller displacement and lower horsepower than the high stage compressors.
Have booster compressors that are greater displacement and higher horsepower than the high stage compressors.
Have equal booster and high stage compressors.
Have booster compressors that are greater displacement and lower horsepower than the high stage compressors. |
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Definition
| have booster compressors that are greater displacement and lower horsepower than the high stage compressors 9-14 |
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