Problem Statement Calculate the power required by two

compressors in an ammonia system which

services a 250 kw evaporator at -250C. the

system uses two stage compression with

intercooling and removal of flash gas. The

condensing temperature is 350C.

Operating Conditions Evaporation Temprature -25 C

Condensation Temperature 35 C

Refrigerant R-717 , AMMONIA

Refrigeration Capacity 250 kW

Compressor Stages 2

Input power of low stage compressor 29.2kW

Input power of high stage compressor 40kW

Mass flow rate through low stage compressor 0.204kg/s

Mass flow rate through high stage comp. 0.255kg/s

Free Air Delivery for low stage compressor 10.56 m3/min

Free Air Delivery for high stage compressor 4.91 m3/min

Compressor Selection Criteria Evaporator Operating Conditions Refrigerant Type Ambient Conditions Required Refrigeration Capacity Compression Ratio Flow Rate through the compressors of the

designed system (Free Air Delivery, FAD) Compressor Cooling System Total Required Power Input

Sizing of Compressors Selection of an adequate sized compressor depends upon following factors:

1. The required Displacement 2. Required Input Power 3. Minimum Evaporator Flow rate 4. Minimum RPM of the available Compressor In order for the system to operate under the required

refrigeration capacity, there is a value of minimum flow rate that must be maintained.

Inadequate refrigerant flow as a result of the fitment of an undersized compressor will limit the performance at all speeds where the refrigerant flow through the evaporator is below the specified value.

COMPARISON WITH CONTEXT TO THE

PROBLEM STATEMENT For economical and justified application centrifugal compressor powers

should range from as low as 400 kW to more than 40 MW. As the compressor power for our case is much less than these limits, it will NOT be reasonable to use it in our case.

The reciprocating compressors cannot operate when there is moisture in the refrigerant. As in our problem the refrigerant enters the compressor at saturated vapor state in ideal case, so there is a significant possibility that the refrigerant might be in the wet region actually so superheating is required to be on the safer side which increases work input of the compressor. Therefore, we rule out the use of reciprocating compressor for this case.

Scroll Compressors have low scale applications such as residential and automobile air-conditioning because they have low pressure ratios and low refrigeration capacity. The maximum refrigeration capacity obtained using this type of compressors is 15KW. So in case of given example this compressor is ruled out.

Cont.. Comparison for Vane & Screw Compressor:

Out of reciprocating, vane, screw we must refer to the performance charts

corresponding to the operating conditions for this problem.

CHARACTERISTICS VANE SCREW

Efficiency Higher Lower

Maintenance Cost Lower Higher

Life period High Low

Operating Pressure Range (152-

1352 kPa)

0-1100 kPa 10-4000kPa

Running Hours without wear 100,000-200,000 >20,000

Why to opt Vane Compressor? It is evident from the above comparison that the vane compressors are the

best possible application for this particular problem Vane compressors have a low mass-to-displacement ratio, which, in

combination with compact size, makes them suitable for transport application. For Ammonia R-717 small compressors in the 2 to 40 kW range are single-staged for a saturated suction temperature range of −20 to 7°C at saturated condensing temperature up to 60°C. By employing a second stage, low-temperature applications down to −50°C are possible

Generally, vane compressors have a pressure ratio of upto 6:1, but in our

case the required pressure ratio is upto 9:1. In order to achieve this pressure ratio 2-stages of vane compressors must be employed

In order to achieve -25°C suction temperature and pressure ratio of upto 9:1 we employ

“Two-stage Vane Compressor System”

Required Compressor Rating

REQUIRED COMPRESSOR RATING

Suction Temperature & Pressure for low stage

compressor

248K , 152kPa

Mass Flow rate for low stage compressor 0.204 kg/s

CFM for low stage compressor 372.92 cfm =

10.56 m3/min

Power Input for “ 29 kW

Suction Temperature & Pressure for high stage

compressor

274.5K ,

453.33kPa

Mass Flow rate for high stage compressor 0.255 kg/s

CFM for high stage compressor 173.4 cfm = 4.91

m3/min

Power Input for “ 40 kW

Selected Compressor

Company: FLSmidth Market leader in the manufacturing & distribution of Vane type compressors

Low Stage Compressor

C50

High Stage Compressor

C100

Selected Compressor

CFM POWER

C50 LOW STAGE 275 40hp = 29.8kW

C100 HIGH STAGE 501 60hp = 44.7kW