Using Condenser and Evap Coil Software

[wzal]

Does anyone has rule of thumb regarding how to compensate for the oil in the refrigerant when sizing the condenser and evaporator?

Say I have a compressor has 3-6% oil carry over (% of mass).

Thanks,

 

[imok2]

wzal, you should have no problems with a 3-6% oil carryover on performance. The greatest problem is in the evaporator where oil will tend to accumulate if not properly returned.
When there is oil in the evaporator, three factors determine whether or not it stays there:
·the viscosity or fluidity of the oil
·the velocity of the refrigerant gas
·the geometry of the piping.
Oil itself is quite fluid and has a relatively low viscosity at ordinary temperatures. At lower temperatures, the oil becomes more viscous. It becomes more difficult to pour or flow. At very low temperatures, it becomes solid
You might assume that oil is most difficult to move in the coldest part of the evaporator, but viscosity measurements show that this is not so. Refrigerant gas is more soluble in oil at low temperatures than at high temperatures at the same pressure—or, in other words, near saturation conditions. In many systems, the point of highest viscosity may be outside the evaporator. In this case, it would help to the suction-line temperature quickly go up. So, regarding oil return, the principal factor is the viscosity of the oil.

 

[wzal]

Thanks,imok2.

I am trying to find the cause of the discrepancy between design capacity and test capacity for a new prototype we developped.We designed the system for 10 ton using coil calculation software. But when we put the sytem on the test stand we got about 8 ton. The evaporating temperature we measured during the test was ok. So I thought the 3-6% oil carry over could contributed to the discrepancy because when we calculate the evap performance, we assumed there was no oil in the refrigerant.

Maybe the evap performance software itself contributed to the error too.

Looks like the Heatcraft coil software is very popular in the industry. Anyone know how much is the accurary I could expect from the results of those software 2%, 5% or 10%?

Thanks,

 

[imok2]

Just a few thoughts on your problem; we know that the design of a unit cooler evolves around the fundamental formula for heat transfer shown in the following simplified formula for a finned coil is 1/U = R/hi + 1/ho. The major resistance to heat transfer is in the thin films of refrigerant and air, which form on the inside and outside of the coil surface. Thinner films result in less temperature drop, or in other words higher film coefficients,which results in better transfer rates. Thinner films can often be obtained with higher velocities of air and refrigerant because the faster flow scrubs the wall cleaner. Turning first to the air side performance, we note that the cfm (cubic feet per minute) of air does not appear in the basic heat transfer formula. However, the quantity of air is a most important item, and its effect on capacity is felt through its influence on the mean temperature difference or MTD.
It is possible to increase the air side coefficient by the use of spined fins or other arrangements which result in a high degree of air turbulence. The problem here is cleanliness, and it is usually not practical to use more than a few simple spacing fingers to induce air turbulence.

Next, let's look at the inside performance of the coil. Here the basic problem is the same as the outside. A high refrigerant velocity is desirable to give a high inside coefficient. For any given tubing size, the inside coefficient increases as the refrigerant velocity increases and as the tons per circuit increase.

However, this is limited by refrigerant side pressure drop. It is desirable to keep the refrigerant pressure drop low, because pressure drop means temperature drop and this reduces the coil capacity. Anyway here are a few things you might consider

 

[sterl]

Suugest you measure refrigerant Superheat at the Suction Side of the Evap Coil...Suspect you will find it quite high, which means you either aren't feeding refrigerant fast enough or you don't have enough refrigerant in the system.

If you have no on baord means of measuring the superheat, would suggest you refer to the Control Valve books for the process on setting Thermostatic Expansion valves...