1-Compressors-Tep09 Flashcards
Why is the compressor such an important part of the heat pump cycle?
Uses most energy. COP is determined by energy input and output
What are the three main groups of piston compressors?
Hermetic,semi hermetic and open
What is the difference between hermetic and semi hermetic compressors?
Hermetic are wielded together and impossible to open without breaking the compressor. Semi hermetic are also closed but screwed together which makes them possible to open.
Describe open compressor
The motor is a separate part than the piston and the rest
Put the types of compressors in order from lowest displacement volume to highest.
First is piston compressor, second is screw compressor and third is turbo compressor
Describe equation for power consumption for a compressor
w=-k/(k-1)RT_1*[(p2/p1)^k/(k-1)-1]
What are the pros of piston compressors?
Low purchase and service cost and simple control
What are the cons of piston compressors?
Low efficiency, pulsation of fluid during operation and high noise level.
What effect has clearance volume on the efficiency of piston compressor?
Increasing clearance volume decreases efficiency
What other leakage losses are there in a piston compressor?
Losses through piston and valves, pressure loss trhough valves or ports, heat exchanger losses, absorption of gas in oil and non-tightness losses of the construction
What is isentropic efficiency?
n_is=W_is/W_shaft
What is indicated effieciency?
n_ind=W_is/W_ind
What is mechanical efficiency?
n_m=W_ind/W_shaft
What can influence isentropic efficiency?
Stroke and cylinder diameter and especially the relationship between them, rotational speed and valve construction
What is the recommended piston compressor load regulation techniques?
Reduce rotational speed, close one or more cylinders and turn off one of the parallell compressors
What are most common of single screw and double screw compressor?
Double screw compressor
What are the most common screw compressor load regulation techniques?
Capacity slide control, variable volume ratio slide and speed control
Why is there such a high oil need in screw compressors?
None of the screws are physically in contact, everything is sealed by the oil.
What happens if condensing pressure is changed?
The efficiency of the cycle is reduced. Increased condensing pressure means under compression that gives reduced energy output for the condenser. A reduction of condensing pressure leads to over compression with extra compressor work
What are the advantages of screw compressors?
Simple construction (except for oil cooling), low vibration, low discharge temperatures, non-pulsating , large capacities and and high compression ratio
What is the general comparison of piston and screw compressors?
Piston compressor has higher isentropic efficiency. Screw compressors has higher volumetric efficiency
