Heat Pumps and tube reduction

[PaulL]

HI there,

I have looked to previous threads but without finding direct references to this topic.

I am recently facing to more and more heat pumps.

What puzzles me is that, with respect of the past, these heat pumps utilize coils with tubes having reduced diameter with respect of traditional ones.
But at the same time, with respect of the past, I am dealing more and more are heat pumps with a lot of frost problems with respect of the past, ex coil partially frozen, clogging, bad distribution.
In shorts, they are not working suitably.
Heat pump producers justify this coil changing as a solution to save refrigerant, but what I am noticing is that because of tube diameter reduction there are more feedings, smaller diameter capillary tubes exc.
I think that this affects coil distribution and, at the end, the whole equipment operation.

Has anybody of you faced to similar experience ?
Do you think that this bad operation can be correlated to the coil tube diameter reduction or to other issues ?
I am becoming more and more convinced about this.

What do you think ?

Many thanks

[nike123]

Bad operation is correlated to producer know-how.
You have perfectly satisfactory operation even at -20°C outdoor temperature with producers which have required know-how.

Other issue is what is projected life span of equipment. That is something what every manufacturer deal in order to stay in businesses and in same time make money on new products.

I think that our fellow member DesA will give you more details if you send him Visitor Message.

[NoNickName]

PaulL, I don't comment concerning the bad distribution and frosting issue, but in terms of piping diameter, R410A is becoming more and more the refrigerant of choice for small heat pumps. Because of the inherent higher pressures of R410A, and the higher safety pressures required, it was necessary to reduce the diameter of the pipe for the same wall thickness. R410A also has got one of the highest mass flow compared to heat transfer, so a smaller pipe diameter is appropriate.

[PaulL]

Many thanks for your answers and opinions.

To Mr Nike 123

I agree perfectly with your point of view concerning producers, know how and sometimes also with experience of designers.
Please, I would be glad to contact Mr DesA but could you explain me how to contact him with Visitor Message ?

To Mr. NoNickName.
Your answer is totally correct.
Nevertheless my impression is that, because of this change of sections, clogging issues in refrigerant tubing is easier than in the past - lower section, increased number of feedings, different impact of bending, therefore easier - or more possible - clogging.
Secondly, I am not so sure that sections and lengths have been selected with same criteria - and background of testing - as in previous models.
Not least, I am quite concerned that at lower evaporating temperatures, because of lower sections, we would have higher pressure drop in the coil.
What surprises me is that R410A should be less sensitive with respect of R407C to distribution issues, but it seems not occurring.

May I ask you kindly your opinion about my thoughts ?

Many thanks

[nike123]

To Mr Nike 123

Please, I would be glad to contact Mr DesA but could you explain me how to contact him with Visitor Message ?


http://www.refrigeration-engineer.com/forums/member.php?u=27936

[NoNickName]

Nevertheless my impression is that, because of this change of sections, clogging issues in refrigerant tubing is easier than in the past - lower section, increased number of feedings, different impact of bending, therefore easier - or more possible - clogging.

If I was the designer of such a system, which actually I am, the clogging or fouling factor considered is ZERO. The inside of a refrigeration system should be cleaner than the dish you had lunch on. Therefore clogging is not a possible fault what I would consider when designing a heat pump or chiller system.



Secondly, I am not so sure that sections and lengths have been selected with same criteria - and background of testing - as in previous models.
Not least, I am quite concerned that at lower evaporating temperatures, because of lower sections, we would have higher pressure drop in the coil.
What surprises me is that R410A should be less sensitive with respect of R407C to distribution issues, but it seems not occurring.

R410A has got the lowest pressure drop for the same capacity than others refrigerants. Even if that would not be necessarily true, the ratio between pressure drops and increased operating pressures compared to other refrigerants, is still favouring R410A.
The real problem is that in cheap systems, the equalisation tube is not installed, and therefore TXV can't consider the inherent pressure drop of the evaporating coil.
One thing is sure: the newest designs are favouring safety criteria more than performance criteria.

[acnerd]

Talking about heat-pumps and performance criteria, I was told that as outdoor ambient approaches zero degrees celsius, the max performance drops off as well, and continues so the colder it gets. Is this true? I am assuming this is because there is less heat in the air to absorb by evaporation?

[NoNickName]

Talking about heat-pumps and performance criteria, I was told that as outdoor ambient approaches zero degrees celsius, the max performance drops off as well, and continues so the colder it gets. Is this true? I am assuming this is because there is less heat in the air to absorb by evaporation?

Exactly true. The performance is also negatively influenced by the amount of frost on the coil (exchange surface reduction)

[acnerd]

The performance is also negatively influenced by the amount of frost on the coil (exchange surface reduction)

Thought so, and the more time spent defrosting is wasted time that could be heating mode.

That's why I am watching the new domestic water heat pump with interest. When the snow hits in January :mad: I will prefer to stay with my environementally unfriendly boiler instead. At least I am guarenteed central heating. :D