sizing up an orifice

[frosty74]

am i correct in saying if i was to size up an orifice i would have to take into consideration

evap size
capacity
compresor size
refrigerant type
pipe sizes suction and liquid line

this is to clear up an argument at work with another mechanic.

if i am incorrect please advise.

[Samarjit Sen]

In sizing the orifice which I presume is for the selection of the TEV, you have to know the following details.

Refrigerant,Evaporator Size, Liquid temperature approaching the TEV, Suction Temperature. Pipe sizes have nothing to do with the selection of the TEV.

[Iced-up]

you need to know the evaporating temperature, duty and the refrigerant type for normal selection of a valve

[Peter_1]

Needed capacity, refrigerant, evaporating pressure, DP over the valve and amount of subcooling.

[ntfreezer]

orifice diameter =0.5X(F/N)^(1/2)
F is evaporate area m^2
N is the number of evaporate's tube

[Peter_1]

You don't mean this, don't you?

[Andy W]

If you are on service call and you have a faulty expansion valve on a previously working piece of equipment you dont always have the evaporator data so what I do is obtain all the information from the condensing unit, identify gas type, operating temperature of room, evaporating temperature and identify the duty of the condensing unit, with that same duty I then look down the selection chart for the expansion valves, once the valve is fitted you will soon see by what your gauges are doing if this selection is satisfactory.

We had a grade 1 engineer on the last company that I worked for who could not size up a valve, instead he always insisting that it was a design engineers job and not a service engineers job, absolute nonesense. Too many people dabble with refrigeration equipment who shouldn't.

[Andy]

If you are on service call and you have a faulty expansion valve on a previously working piece of equipment you dont always have the evaporator data so what I do is obtain all the information from the condensing unit, identify gas type, operating temperature of room, evaporating temperature and identify the duty of the condensing unit, with that same duty I then look down the selection chart for the expansion valves, once the valve is fitted you will soon see by what your gauges are doing if this selection is satisfactory.

.

You should always size the valve using the duty of the condensing unit NOT the evaporator. That way you will never over load the condensing unit.

Kind Regards Andy

[Andy W]

You should always size the valve using the duty of the condensing unit NOT the evaporator. That way you will never over load the condensing unit.

Kind Regards Andy

Very true as the evaporator can have a lot of variables that as a fact a lot of engineers would not know or understand unless you have experience in selecting evaporators, just look at an evaporator chart for example, the same evap will give numerous duties at different evaporating temperatures and different td's.

[DANIEL ARYEE]

In sizing the orifice which I presume is for the selection of the TEV, you have to know the following details.

Refrigerant,Evaporator Size, Liquid temperature approaching the TEV, Suction Temperature. Pipe sizes have nothing to do with the selection of the TEV.

DEAR FRIEND, I THINK THE MOST IMPORTANT THING IS THE EVAPORATOR SIZE. IF YOU CHOOSE AN OVERSIZE ORIFFICE, THEN LIQUID WILL ENTER THE COMPRESSOR AT ALL COST. IT WILL EVEN BE BETTER IF THE EVERPORATOR IS A BIT BIGGER THAN THE ORIFFICE. THIS WILL ALLOW WELL SATURATED VAPOUR TO ENTER THE COMPRESSOR. REMEMBER THE COMPRESSOR IS NOT A LIQUID PUMP, BUT A VAPOUR PUMP.

[Andy]

DEAR FRIEND, I THINK THE MOST IMPORTANT THING IS THE EVAPORATOR SIZE. IF YOU CHOOSE AN OVERSIZE ORIFFICE, THEN LIQUID WILL ENTER THE COMPRESSOR AT ALL COST. IT WILL EVEN BE BETTER IF THE EVERPORATOR IS A BIT BIGGER THAN THE ORIFFICE. THIS WILL ALLOW WELL SATURATED VAPOUR TO ENTER THE COMPRESSOR. REMEMBER THE COMPRESSOR IS NOT A LIQUID PUMP, BUT A VAPOUR PUMP.

Sorry Daniel I would size the oriface to suit the compressor NOT the evaporator.If your evaporator does 6kW at the TD (Say 6K) and your compressor does 5kW at same TD, you need to fit an oriface to suit 5kW not 7kW, either that or the compressor is overloaded OR you evaporation is too high and your loose a large proportion of your Latent cooling AND you overload the compressor(more often on a condensing unit where the condenser is one fits all).

Kind Regards Andy

[Samarjit Sen]

Dear Andy,

For selection of the orifice of the TEV . I feel that the size of the Evaporator plays the important role. TEV is a part of the whole system. While designing and selecting the refrigeration system, we have to balance all the components such as the Compressor, Condenser ( based on the Total heat Rejection of the system ) the Evaporator and the Controls. Some people tend to not to calculate the details and install units which do not match. This causes failures in the systems and one tend to blame either the Compressor or the TEV. This is wrong. Condensing unit is important as when you size the TEV and the orifice , among the other factors as stated by me , the temperature of liquid approaching the TEV is also considered. But then the capacity of the Evaporator has to be taken into consideration. I have been following this system of selecting the components and ( God Willing ) till now have never faced any problems.

With best wishes,

[TXiceman]

If you are designing the coil to start with, you need to first determine the proper circuiting. Now you neeed to look distributor tube length and diameter. To select the proper orifice, you need to first balance the condensing unit with the coil and get the load and ET at the balance point. Use this load and ET as well as CT for the design point and select an orifice and TXV to all match. Now comes the trick, rerun the selction and allow the CT and liquid temperasture to drop to the lowest ambinet conditoins you pan to run. The pressure differentila is lower, but the subcooling is much higher, especially if you are flooding the condenser coil. You nay find the orifice too large due to the subcooling and have to take a bit more PD at the orifice when you are at design.

Get a copy of the SDporland program and run a few cases at design and at low ambient design.

Ken

[Andy]

Dear Andy,

For selection of the orifice of the TEV . I feel that the size of the Evaporator plays the important role. TEV is a part of the whole system. While designing and selecting the refrigeration system, we have to balance all the components such as the Compressor, Condenser ( based on the Total heat Rejection of the system ) the Evaporator and the Controls. Some people tend to not to calculate the details and install units which do not match.
With best wishes,

I would say any system is a compromise. But the balance point in the system is acheived by basing the TEV selection on the condensing unit capacity. The evaporator has to be selected for the correct duty, or as near as possible

The governing factor is the condenser/ compressor capacity not the evaporator


Kind Regards Andy