1-Compressors-Tep09

Question 1

Why is the compressor such an important part of the heat pump cycle?

Answer 1

Uses most energy. COP is determined by energy input and output

Question 2

What are the three main groups of piston compressors?

Answer 2

Hermetic,semi hermetic and open

Question 3

What is the difference between hermetic and semi hermetic compressors?

Answer 3

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.

Question 4

Describe open compressor

Answer 4

The motor is a separate part than the piston and the rest

Question 5

Put the types of compressors in order from lowest displacement volume to highest.

Answer 5

First is piston compressor, second is screw compressor and third is turbo compressor

Question 7

Describe equation for power consumption for a compressor

Answer 7

w=-k/(k-1)RT_1*[(p2/p1)^k/(k-1)-1]

Question 8

What are the pros of piston compressors?

Answer 8

Low purchase and service cost and simple control

Question 9

What are the cons of piston compressors?

Answer 9

Low efficiency, pulsation of fluid during operation and high noise level.

Question 10

What effect has clearance volume on the efficiency of piston compressor?

Answer 10

Increasing clearance volume decreases efficiency

Question 11

What other leakage losses are there in a piston compressor?

Answer 11

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

Question 12

What is isentropic efficiency?

Answer 12

n_is=W_is/W_shaft

Question 13

What is indicated effieciency?

Answer 13

n_ind=W_is/W_ind

Question 14

What is mechanical efficiency?

Answer 14

n_m=W_ind/W_shaft

Question 15

What can influence isentropic efficiency?

Answer 15

Stroke and cylinder diameter and especially the relationship between them, rotational speed and valve construction

Question 16

What is the recommended piston compressor load regulation techniques?

Answer 16

Reduce rotational speed, close one or more cylinders and turn off one of the parallell compressors

Question 17

What are most common of single screw and double screw compressor?

Answer 17

Double screw compressor

Question 18

What are the most common screw compressor load regulation techniques?

Answer 18

Capacity slide control, variable volume ratio slide and speed control

Question 19

Why is there such a high oil need in screw compressors?

Answer 19

None of the screws are physically in contact, everything is sealed by the oil.

Question 20

What happens if condensing pressure is changed?

Answer 20

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

Question 21

What are the advantages of screw compressors?

Answer 21

Simple construction (except for oil cooling), low vibration, low discharge temperatures, non-pulsating , large capacities and and high compression ratio

Question 22

What is the general comparison of piston and screw compressors?

Answer 22

Piston compressor has higher isentropic efficiency. Screw compressors has higher volumetric efficiency