subcooling value and condensing split temperatures

[mrr]

Hi,

I have read that, provided we get coherent condensing pressures for a given refrigerant, high subcooling increases the refrigeration capacity. However I have also read "8 K" of subcooling is enough when charging the unit of refrigerant. On the other hand, I have read that high subcooling may indicate overcharge of refrigerant, but then I would get high condensing pressures.

I think I have an oversized condenser, since I do not get high condensing pressures but I get about 15 K of subcooling. Thus my question is:

- What is an advisable value of subcooling and what are the main causes it increases? (provided there is no overcharge and no heat plate exchanger at the condenser output for subcooling the refrigerant).


My second question is related with the condenser split temperature.
- Given different ambient temperatures, should I always decrease the condensing pressure to maintain the condensing split temperature?

If we have low ambient temperature, let's say 10 ºC, and considering R134a where "normal" condensing pressures would be about 10-13 barg. If I want to maintain the condensing split temperature I should lower it to 5 barg. Is it a common practice? (i'm assuming I have a condenser fan with an inverter I can control the condensing pressure).



Regards,

[Rob White]

.

We tend to work to rough rules of thumb which work for
most systems most of the time, but do not take into account
the differences between systems and setup's

So one common rule of thumb is "with an air cooled condenser,
the condenser temp is about 15K higher than ambient"

Your R134a at an ambient of 10 ish deg C would be condensing at
about 25 ish so that would equal at around the 5 -6 bar
mark.

Subcooling on the other hand or more important the actual liquid temp
cannot be colder than ambient (in normal air cooled condensers) because
there will always be a temperature difference and the typical subcooling
temp is about 4 - 7 K.

If the system has unusually high condensing temps / pressures for what
ever reason, but if the liquid is still cooled to say about 10 degs above ambient
the subcooling will appear to have increased, but in reality the system is worse off.

Typical temps would be condensing 15 degs above ambient with a 4 - 7K subcooling,
with air cooled condensers.

The other factor you need to think about then leads onto your second point.
You can't cool the discharge below ambient (not normally anyway) with air
cooled alone and therfore the condensing temp is governed by ambient temps.
Also most mechanical thermostatic expansion devices need a pressure difference
of at least 5 - 7 bar across them to work correctly.

If you lower the condensing temp too far and therefore lower the liquid line pressure
you run the risk of preventing your expansion device from working correctly
and as a result liquid will remain in the receiver and condenser.

This is often referred to condenser logging and tends to happen in the colder months
or when condenser capacity control is fitted incorrectly.

Lower condensing temps do improve efficiency and performance but you can lower
it too much and have a detrimental effect.

Remember nothing is ever free, we understand that we have to pay for the compressor
to run and suction superheat affects that performance, but we also have to pay
for subcooling, that might be an actual calculated cost of liquid line subcoolers or
just simply as an inefficiency that we tolerate and design for.

What goes in must come out and we have to pay for the privilege :)

Rob

.

[sterl]

Add a couple of things to discussion: Should the system layout require a large rise for the liquid line, the pressure in that line is actually reducing with the lift and a corresponding amount of subcooling will be required or the liquid line will have a bunch of bubbles in it. When this is observed at a sight glass upstream of the TXV a service tech might try to address by adding refrigerant. This may in fact solve it simply by flooding a portion of the condenser thus raising the condensing temperature and causing more subcooling within the condenser. However, if there is a "regulator" style of head pressure control that effectively mixes hot gas with the cold condensate to bring the liquid line pressure up, any subcooling in the condenser is going to be lost unless there is a pretty specific arrangement applied to keep the pressure high on a cold liquid, often effected at the high pressure receiver.

Assuming for a moment that form of CP control is not in place; and that you have an air cooled condenser of the tube back variety: If the opportunity becomes available to measure return bend temperatures with a "good" instrument, you can observe where the temperature starts to drop presumably as you work your way downstream on the bundle. This should yield some indication of what proportion of your condenser is actually condensing.