Yes, I think you could say that. one of the functions of expansion valve is to reduce the temperature of refrigerant, so it could evaporate under a relatively low temperature and cool down the other medium, such as water.  The another function is to control the flow rate, and further control the cooling capacity. so, your word that "....reduce the pressure of a fluid by controlling the flow rate of the fluid.." might be inaccurate.

Thank you for your answer.

Can I say expansion valve is used to regulate the pressure of the refrigerant as it controls the drop in pressure for refrigerant evaporation & cold generation?

In principle no. I depende on the lay out of the plant. If you have a by bass regulating valve on the compressure suction side this will control the pressure at expansion.

You have to first draw the process flowsheet, including piping and instrumentation, to understand what is the primary function of the refrigeration system (which should be treated as a Utility).  My view is that the prime objective is NOT to produce refrigerant, but to enable some kind of manufacturing process.  It is the process needs for cooling that must be satisfied.  How you do it mechanically is a secondary issue.  Once you clarify the primary objective, the answer to you questions will be come obvious.

Allow me to comment your text : 

The main function of the expansion valve in a refrigeration system is to reduce the pressure of a fluid by controlling the flow rate of the fluid.

well, I would rather instead of "pressure of fluid" use term  "pressure inside evaporator"

and use also term controlling the   mass flow rate  (kg/s)  instead of flow rate. 

it is hardly to imagine pressure of fluid thrown by the expansion valve inside the evaporator...  this pressure is also influenced by the total internal volume of evaporator, by the side of the mass flow rate. 

When the pressure of the fluid decrease, so thus the temperature of the fluid

thus , when the pressure inside evaporator decreases, temperature of  refrigerant in evaporator will drop down also 

(according to the properties of the fluid, the pressure reduces and the enthalpy might remain unchanged, the temperature will drops). So, is it correct if I say I use expansion valve to reduce the temperature of the fluid/ refrigerant.

 Imagine , when you do not have operating compressor, and there is no suction pressure active , which sucting heated gases from evaporator away.

What happens, when you have  A/ closed  expansion valve ?   almost nothing, or the bubled or liquid refrigerant inside evaporator may evaporize, but it will stay inside (no compressor is running, remember)

B/ you under these circumstances open the expansion valve : result will be the pressure increase, refrigerant (no matter its state) volume increase. if on the other side of heat exchanger (evaporator) will not be higher temperature, pressure on refrigerant side will be set according the temperatures equilibrium on both sides of evaporator .  That means, there will not be temperature drop (no suction, no evaporating, no temperature and pressure decreasing) .... 

if you consider these relationships, I would tell, no, expasion valve cannot reduce temperature of refrigerant, and reduce pressure inside evaporator on its own. 

On its own, in cooperation with temperature sensor (or pressure pipe) on the exit from evaporator, it can only regulate mass flow of refrigerant flowing inside evaporator. 

With the help of compressor suction force, it can regulate the pressure thus temperature inside evaporator. Not without it. 

It has some purposes, including converting the fluid to gas, a separation point between high and low-pressure parts. During passes the refrigerant, a temperature drop will occur too, which is useful.

You have to be clear on the system's objective. If the objective is cold production, then by using EV reducing the pressure will meet the objective by the consequent reducing temperature. I believe in not fixing a specific strict function on an instrument but rather on fulfilling the objective of the system. Thus you may have also capillary tube, throttling valve, or any forced convection 'tool' to reduce the working fluid temperature.

Check the Joule -Thomson coefficient

Dear all,

Noted. Thank you for your answer.

I should say yes ... because the lowering of pressure is necessary for the lowering of the temperature. The main reason to run an inverse cycle is actually to move heat from a low temperature source to a high temperature sink ... and using a compressor is necessary ... we'd prefer not to use a compressor ... but we have to.

The answer is yes, if there is a phase change, say from liquid to gaseous (vapor) phase.

The use of the expansion valve is precisely to drop the pressure to that of evaporation and control the refrigeration flow.