sc 110 gas compression - fricktraining.blob.core.windows.net

SC 110 Gas Compression

1

The Roots Blower This invention was the Grandfather of the screw compressor. It was a very successful commercial

product – still used today in automobiles. The Roots blower has the advantage of being rotary – not

stopping and starting, and could handle large volumes. Its limitation is compression ratio – try to get

too high a pressure on discharge and the undercompression in the blower will cause temperatures on

the rotors and casing to get high.

The Compressors Role The compressor controls the low side of the system – loading and unloading of the compressor

maintains the low

side pressures.

The compressor

adds energy to

the gas in the

form of pressure

and temperature

rise.

The high side

pressure that the

compressor

pushes the gas to

is determined by

the condensers.


2

Compressor Terminology Positive Displacement – compressor that works by expanding and shrinking the compression chamber

Swept Volume – the capacity of a compressor in volume of gas taken in.

Volume Ratio – Suction volume divided by discharge volume

Compression Ratio – Discharge pressure divided by suction pressure

K factors and saturated pressures are available through the downloads in the class materials. You will

need to be able to look up pressures and K factors to determine volume ratio.

The Frick Family The FRICK Model number of the compressor is the compressors swept volume – in cubic meters per

hour divided by 10. The model number is the swept volume. (2) RXF-50 compressors are just about

equal to (1) RXF-101 unit in capacity.


3

Compressor Performance What happens to a fully loaded compressor when you change either the suction or the discharge

pressure?

Suction Pressure First realize that suction pressures don’t change as much as discharge pressures can change. Suction

pressures vary small

amounts typically –

unless the system is

pulling down. So there is

not much

“thermodynamic”

change to the cycle

when suction pressure

changes. However, the

density of the gas does

change - and that

increases the mass flow

through the

compressor.

Raising suction pressure causes mass flow and brake horsepower to increase. However, the brake

horsepower only goes up a little – and the mass flow can go up a lot.


4

Changing Discharge Pressure Higher discharge pressures result in higher

compression rations, increasing the brake

horsepower used by the compressor. Since

discharge pressure change a lot – they can have

a thermodynamic effect as well.

The effect discharge pressure has on

compression ratio is obvious. What is not

so obvious are the thermodynamic cycle

changes that occur.

Lowering discharge pressure moves the

condensed liquid to a lower energy state

(more left on the diagram) which is a good

thing for the refrigeration cycle. It

increases the refrigerating effect – the

amount of heat removal per pound of

refrigerant evaporated.

Lowering discharge pressure is an easy way

to increase system efficiency.


5

Screw Compressors In a 4/6 rotor

combination, the

male rotor has 4

“points of

contact” with the

6 lobe female

rotor.

A screw

compressor

operating at fixed

speed will take in

a fixed volume of

gas.

The male lobes

sliding in the female flutes cause both the suction on the back side and compression of the gas in front

of the lobe. Suction to the screw compressor is continuous, without pulsation. Discharge from the

machine pulses 240 times per second at 60 Hz.

Unloading a Screw Compressor Changing rotational speed allows the flutes to fill up and exhaust more slowly – lowering the swept

volume of the compressor.


6

The slide valve allows trapped gases to get back to suction without the compressor performing any work

on them.


7

Volume Control

R717 Part Load - SV - 193mm vs VyperTM

0

20

40

60

80

100

120

0 20 40 60 80 100

% of Full Load Capacity

% o

f F

ull

Lo

ad

BH

P

2.5CR

3CR

6CR

10CR

19CR

Vyper


8


9


10

Economization The economizer increases system efficiency by removing flash gas at a higher than main suction level.


11

A caution with economized compressors using slide valves for unloading. The slide valve will quickly

(80%+-) uncover the economizer port – exposing it to main suction pressures. This defeats the energy

advantage of the economizer port. Solenoids are often put on economizer lines to shut them off when

operating at low slide valve conditions.


12

Table 1. k Values (Average Specific Heats) k values for commonly used refrigerants and gases are as follows:

Refrigerant k-value 1/k value Refrigerant k-value 1/k

value

R-11 1.110 .9009 R-12 1.120 .8929

R-13 1.130 .8850 R-13b1 1.130 .8850

R-14 1.150 .8696 R-23 1.180 .8475

R-50 1.290 .7752 R-113 1.070 .9346

R-114 1.070 .9346 R-134a 1.100 .9090

R-142b 1.110 .9009 R-170 1.170 .8547

R-218 1.060 .9434 R-401a 1.130 .8850

R-402a 1.110 .9009 R-404a 1.100 .9090

R-410a 1.150 .8696 R-500 1.120 .8929

R-502 1.135 .8811 R-503 1.150 .8696

R-507 1.100 .9090 R-508 1.120 .8929

R-508b 1.120 .8929 R-600 1.090 .9174

R-600a 1.090 .9174 R-717 1.300 .7692

R-718 1.330 .7519 R-728 1.400 .7143

R-729 1.400 .7143 R-771 1.132 .8834

R-1150 1.220 .8197 R-1270 1.145 .8734

R-22 1.170 .8547 R-290 1.120 .8929


13

Using the XY Button on a Scientific Calculator Scientific calculators have XY buttons to allow for functions that involve powers and exponents. The XY

button allows the user to multiply a number by a selected power.

Using the XY Button

First input a number on the calculator, then push the XY button, then input the desired power. The

calculator will give the result of the number and the power.

Powers, Exponents and Bases

The number you input first, before clicking the XY key, is the base number. It is multiplied by the

exponent, or the number selected after clicking the XY key. The process of calculating base numbers by

exponents is known as "raising to the power." In these calculations, the exponent is called the "power."

Calculating Powers

In math, the exponent is shorthand for repeated multiplication of the base by itself. In XY calculations

the base represents the number and the exponent the amount of times the number will be multiplied by

itself. For example: If 4 is the base number and the exponent is 3, the occasion would be 4 x 4 x 4, or 64,

the equation can also be understood as 4 to the power of 3, also 64.

Why X and Y?

The symbols XY represent the common form of writing questions to do with calculating the power. For

example, "2²," means the base number or X is two and Y or the exponent is also 2. In mathematics,

unknown variables are represented by letters starting with x, y and z. The calculator is asking for the

user to input the X and Y in an equation about powers.

Example:

Calculate the Vi for an ammonia compressor operating at a CR of 3:1.

𝑉𝑖 = 𝐶𝑅1/𝑘

k for ammonia (R717) is 1.300

1/k = 1/1.3 = 0.7692

CR = 3

Solution

𝑉𝑖 = 30.7692 = 2.328


14

Exercises 1. What is the compression ratio for an ammonia compressor with suction 20°F and 95°F

discharge (both saturated).

2. What is the volume ratio (Vi)?

3. What is the compression ratio for an R-507 Booster operating at -50°F suction and +10°F

discharge (saturated)?



15

Answers to Exercises 1. What is the compression ratio for an ammonia compressor with suction 20°F and 95°F discharge

(both saturated).

20°F = 48.21 psia 95°F = 195.8 psia CR = 195.8/48.21 = 4.064 CR


= 4.064 0.7692 = 2.93 Vi

3. What is the compression ratio for an R-507 Booster operating at -50°F suction and +10°F

discharge (saturated)?

-50°F = 15.8 psia +10°F = 61 psia CR = 61/15.8 = 3.86 CR


3.86 .9090 = 3.41 Vi

sc 110 gas compression - fricktraining.blob.core.windows.net

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