ENHANCEMENT OF COP IN VAPOUR COMPRESSION REFRIGERATION SYSTEM WITH DC COMPRESSOR

: Vapour compression refrigeration system is a high dimensional thermodynamic coupling system in which the coefficient

of performance (COP) is the area of interest and tremendous researches are going to increase the coefficient of performance (COP). In

this project a Vapour compression refrigeration system using R600a refrigerant as a working fluid and working based on a vapour

compression cycle is designed and fabricated. In this vapour compression refrigeration system, a nozzle is placed between condenser

and expansion valve and a diffuser is placed between evaporator and compressor. The diffuser placed is used to increase the pressure

of refrigerant gas flowing into the compressor and thereby decreasing the power input given to compressor. And also replacing the AC

Compressor with DC Compressor to decrease the power consumption. And also replacing the AC Compressor with DC Compressor

to decrease the power consumption. The nozzle placed is used to increase the velocity of refrigerant liquid going from condenser to

expansion valve. An experimental investigation has been performed on this system with concentrating variables like coefficient of

performance (COP), compressor power consumption and cooling capacity. The performance of the simple vapour compression

refrigeration system and the system with nozzle, diffuser and DC compressor is compared and the best system is evaluated.

KEYWORDS - DC Compressor (Inverter), Nozzle, Diffuser and Coefficient of Performance (COP)

I. INTRODUCTION

Refrigeration is the science of the producing and maintaining temperatures below that of the surrounding atmosphere. This means

the removing of heat from a substance to be cooled. Heat always passes downhill, from a warm body to a cooler one, until both bodies

are at the same temperature. Not only perishables today many human work spaces in offices and factory buildings are air- conditioned

and a refrigeration unit is the heart of the system. Before the advent of mechanical refrigeration water was kept cool by storing it in

semiporous jugs so that the water could seep through and evaporate. The evaporation carried away heat and cooled the water. This system

was used by the Egyptians and by Indians in the Southwest. Natural ice from lakes and rivers was often cut during winter and stored in

caves, straw-lined pits, and later in sawdust insulated buildings to be used as required.

The Romans carried pack trains of snow from Alps to Rome for cooling the emperor's drinks. Though these methods of cooling all

make use of natural phenomena, they were used to maintain a lower temperature in a space or product and may properly be called

refrigeration. In simple, refrigeration means the cooling or removal of heat from a system. The equipment employed to maintain the

system at a low temperature is termed as refrigerating system and the system which is kept at lower temperature is called refrigerated

system.

Refrigeration is generally produced in one on the following three ways:

(1) By melting of a solid,

(2) By sublimation of a solid, and

(3) By evaporation of a liquid.

Most of the commercial refrigeration is produced by the evaporation of a liquid called refrigerant Mechanical refrigeration depends

upon the evaporation of liquid refrigerant and its circuit includes the equipment's naming evaporator, compressor, condenser and

expansion valve. It is used for preservation of food, manufacture of ice, solid carbon-dioxide and control of air temperature and humidity

in the air-conditioning system. All refrigeration systems must include at least four basic units as given below:

A low temperature thermal "sink" to which heat will flow from the space to be cooled.

Means of extracting energy from the sink, raising the temperature level of this energy, and delivering it to a heat receiver.

A receiver to which heat will be transferred from the high temperature high pressure refrigerant.

Means of reducing pressure and temperature of the refrigerant as it returns from the receiver to the "sink".

. METHODOLOGY

In the vapor compression refrigeration system, the refrigerant always undergoes the phase change from liquid to vapour and then

vapour to liquid during a closed cycle by absorbing the heat or warmth from the products to be refrigerated within the evaporator and

reject the heat or warmth at condenser. The coefficient of performance (cop), that may be a magnitude reaction of heat transfer rate at

the evaporator to the ability input to the compressor within the refrigeration system. The COP will be increased either by decreasing

the compressor work or by increasing the refrigeration effect. Completely different ways are tried for improving the cop of the VCR

system, as according to literature a setup was fabricated by placing a nozzle in between condenser outlet and expansion valve and

performance was measured in four cases: i) Simple VCR ii) VCR with nozzle iii) VCR with diffuser iv) VCR with nozzle and diffuse.

Nozzle

A nozzle is often a pipe or tube of varying cross-sectional area and it can be used to direct or modify the flow of a fluid (liquid or

gas). Nozzles are frequently used to control the rate of flow, speed, direction, mass, shape, and/or the pressure of the stream that

emerges from them. In a nozzle, the velocity of fluid increases at the expense of its pressure energy.

Diffuser

A diffuser is "a device for reducing the velocity and increasing the static pressure of a fluid passing through a system”. The fluid's

static pressure rises as it passes through a duct is commonly referred to as pressure recovery. In contrast, a nozzle is used to increase

the discharge velocity and lower the pressure of a fluid passing through it

The main working is as follows

The refrigerant enters the DC compressor at low temperature and low pressure .It is in a gaseous state. Here,

compression takes place to raise the temperature and refrigerant pressure. The refrigerant leaves the DC

compressor and enters to the condenser.

The vapor travels through part of the condenser which removes the superheat by cooling the vapor with constant

pressure

The saturated liquid refrigerant passes through the expansion valve and undergoes an abrupt decrease of pressure.

That process results in the adiabatic flash evaporation and auto-refrigeration of a portion of the liquid, the

adiabatic flash evaporation process is isenthalpic.

The cold and partially vaporized refrigerant travels through the coil or tubes in the evaporator where it is totally

vaporized by the warm air (from the space being refrigerated) that a fan circulates across the coil or tubes in the

evaporator. The evaporator operates at essentially constant pressure and boils off all available liquid vapour . The

required circuit is obtained by using the given valve configurations. Then the compressor is supplied.