crankcase heater thought

[philjd26]

hi all just a thought,and wonderin am i right in saying that a chiller is running 247 and the set point is 10c on the return,if i use a ambient stat to enable crankcase heaters to come on when outsde temp drops below 15c, i will not get liquid migration in crankcase on the off cycle as the compressor is exposed to ambient temp resulting in liquid going to to the coldest area the evap..

cheers

[lana]

Hi philjd26,

Basically you are right.

Some points worth mentioning though.

1- Is the chiller (with compressor) located outside, i.e. exposed to ambient conditions?
If so, you can do what you proposed.
2- Does the system pump-down before cut off?
If so, then you would not need heater.
3- Crankcase heater must be de-energized when compressor is running.
4- Refrigerant migration happens based on temperature difference between the evaporator and compressor OR between compressor and condenser. You have to evaluate the situation when there is temperature difference. Don't judge based on one temperature only.


Hope this helps.
Cheers

[philjd26]

thanx for cvomment iana,yes the compressors are at ambient temp,so what is the result of a temp differance between condenser and compressor apart from liquid sitting on valves,and what are the effects of this? could liquid make its way into crankcase if all reeds are good..

[monkey spanners]

Hi philjd26

We run our crankcase heaters off an auxiliary contact on the compressor contactor, its a normaly closed (nc) contact so it powers the cch whenever the compressor is off.

On the dx milk tanks that i work on the milk vessel (evaporator) is washed with water and chemical at up to 70c so the compressor is always going to be colder. Also the ones that i install are on pumpdown.

Jon

[LRAC]

Don't worry about it just fit the crankcase heater on the normally closed auxillary contact on the contactor.

Lrac

[lana]

Hi philjd26,

Compressor discharge has no connection to crankcase, only suction side is connected to crankcase and this is why the suction pressure is the same as crankcase pressure (except in double-stage compressors).
So if liquid refrigerant goes into the compressor from the condenser, then it goes directly into the cylinders. If the refrigerant stays as liquid there, then at the start up, the compressor may be damage. This is why we use inverted traps at the condenser inlet.

The general rule is :

Direct the oil towards the compressor in the suction side, and direct the oil towards the condenser in the discharge side.
But there must be no liquid refrigerant going into the compressor from both sides.

Hope this helps.
Cheers
LANA

[philjd26]

nice one,so if compressor discharge is not connected to crankcase,presssure equailises on both sides on a non pump dwn system letting discharge valves on a recip sit well allowing nothing into cylinder.....???another question i have is,would i be write in saying temp of your oil is more or less heat from windings and and suction super heat, provided all oil is in order and mechanical components????

cheers

[mohamed khamis]

Hi philjd26,

4- Refrigerant migration happens based on temperature difference between the evaporator and compressor OR between compressor and condenser. You have to evaluate the situation when there is temperature difference. Don't judge based on one temperature only.


Hope this helps.
Cheers

Hi Lana

Welcome back....The refrigerant migration happens based on the refrigerant pressure difference then temperature difference during off cycle. In the shutdown period, the refrigerant tends to flow from high pressure side to pressure side until the two pressure are stabilized (unless there is a pump down system). The low pressure side is the evaporator and compressor. If the compressor is outdoor and is subjected to a cold weather the refrigerant tends to migrate to the compressor at this moment even though the evaporator and compressor in the same level. If the compressor in cold weather the cylinder wall is cooled and so the accumulative refrigerant inside and in turn its pressure is decreased "for a constant volume process for closed system". Therefore, the refrigerant in warm region with respect to that in a compressor tends to migrate from high pressure (as a result of a relative high temperature) to low pressure in a compressor. Therefore, the crank heater should be operated to increase the accumulative refrigerant inside the compressor at least 10 k above the other point in the system.

The problem of entering refrigerant liquid in the compressor firstly is the refrigerant dilutes the oil viscosity and excessive amount can make a severe degradation in its viscosity and very excessive amount of it will wreak the suction valves.

Cheers

[mohamed khamis]

Hi philjd26,

So if liquid refrigerant goes into the compressor from the condenser, then it goes directly into the cylinders. If the refrigerant stays as liquid there, then at the start up, the compressor may be damage. This is why we use inverted traps at the condenser inlet.


Hope this helps.
Cheers
LANA

Also if liquid refrigerant goes into the compressor from the condenser during off cycle, it could not go into cylinders because the discharge valve will prevent that it is in reverse direction of returning refrigerant flow and so it normally closed on off cylce. However the problem of migration of refrigerant from condenser to the compressor head (not cylinder) is the excessive load on the discharge valve to be opened on the compression stroke...So to shun that the inverted trap at inlet condenser is installed to mitigate the refrigeration flow back to the compressor head and by the way prevents soft solders to the discharge valve.

Cheers

[philjd26]

cheers mohamad,so iverted trap eliminates liquid from sitting on heads and the event unloading of valves at strart up, if liquid were to enter the cylinder, the valves are spring loaded...... so would agree in saying that i could use an ambient stat to control the crankcase heater, the reson i ask is i came to a site and found eight heaters faulty at 180 a piece and the system is only in 3 years,heaters do go off when compressors are in demand but i feel the system is oversized so crankcase heaters are on more than normal

cheers

[lana]

Hi Mohamed,

Thanks for correcting my words. "Cylinder" to " Cylinder head"


About the refrigerant migration : This happens based on temperature difference if there is no pressure difference. If pressure difference exists then this is the driving force no question about that. But if there is no pressure difference then the driving force is the temperature.
This is called "Watt Cold Surface". Refrigerant gas will migrate to colder area and condenses there as liquid.

Cheers

[US Iceman]

Lana,

Can you provide more detail on the phrase "Watt Cold Surface" please? This is a new term for me.

On the migration issue I agree. The migration of refrigerant is caused by a difference in vapor pressures in the system. The vapor pressure difference is due to temperature difference between components.

This is one area where I think the use of traps is a fallacy. Traps only collect liquid, not gas. An inverted trap prevents liquid from flowing in reverse when the system shuts down.

Traps do not prevent refrigerant migration!

I'm sure this will get a discussion started soon.:rolleyes:

[lana]

Hi US Iceman,

I agree with what you said.

About the "Watt Cold Surface" I read it somewhere but I don't recall it where:confused:.

The explanation goes like this :
There are two cylinders one with liquid refrigerant in it and the other one empty. These two are connected to each other. One is placed in a colder place than the other. After a while the empty cylinder will be filled with liquid and the other cylinder will be empty.

If I remember the reference then I shall provide it here.

Cheers

[mohamed khamis]

Lana,

This is one area where I think the use of traps is a fallacy. Traps only collect liquid, not gas. An inverted trap prevents liquid from flowing in reverse when the system shuts down.

Traps do not prevent refrigerant migration!

I'm sure this will get a discussion started soon.:rolleyes:

Hi US Iceman

Yes it is correct the inverted trap prevents partially the refrigerant liquid not gas. The reason is if u make force analysis for the refrigerant in the inverted trap u find out there two driving forces want to pull the refrigerant to flow and there are two forces resist that and wants to brake the refrigerant flow. The two driving forces are the buoyancy force (is due to the difference in temperature) and the other is generated due to the difference of vapor pressure. The opposing forces are the gravity force and friction force which depends on the material roughness and the length of refrigerant path. Therefore, two competitive effects are at a work. In liquid form, the gravity force is higher than that for gas form as a result of the big difference in two densities. On the other hand, the buoyancy force for gas form (as a result of the higher volume expansion coefficient for the gas) is higher than that Therefore, in liquid form the gravity force reinforces significantly the position of the resistive side with provision of a weak buoyancy force and this assist the braking force to prevail the competing action. On the other hand, in a gaseous form, the buoyancy force has a paramount decision with existence of a weak gravity force, hence the driving force dominates the conflict. In a short, the inverted trap has no authority on the refrigerant migration in case
of the gaseous form but it is an effective in case of liquid and mitigate the refrigerant migration in reverse motion to compressor cylinder head. Sorry for long poast
I wish it could help

Best regards:)

[mohamed khamis]

Hi US Iceman,

The explanation goes like this :
There are two cylinders one with liquid refrigerant in it and the other one empty. These two are connected to each other. One is placed in a colder place than the other. After a while the empty cylinder will be filled with liquid and the other cylinder will be empty.

If I remember the reference then I shall provide it here.

Cheers

Hi Lana

Yes it is totally correct if here are two cylinders one with GAS refrigerant in it and the other one empty. These two are connected to each other. One is placed in a colder place than the other. After a while the gas in a warm cylinder will flow to the empty cold cylinder and fill it until equilibrium has been achieved. This method is typically used in purging system to eliminates the noncondensible gases in either HVAC system or steam power plant condensers.

Best regards:)

[mohamed khamis]

so would agree in saying that i could use an ambient stat to control the crankcase heater, the reson i ask is i came to a site and found eight heaters faulty at 180 a piece and the system is only in 3 years,heaters do go off when compressors are in demand but i feel the system is oversized so crankcase heaters are on more than normal

cheers

Hi philjd26

Firstly as i understood from ur sentences that u want to activate the crankcase heater during the running time and shutdown time, am I right or wrong. If I am right, the heater is used only during the off time to protect the compressor oil from dilution particularly in cold location but in running time the using of the heaters can overheat the motor windings and raise up the discharge temperature affecting badly on oil viscosity. The main purpose of the heater is to boil off the accumulative restful refrigerant liquid during the shut down time only but on the operation time the refrigerant enters the compressor into gaseous form (unless there is a detrimental flood back) and at this is the oil is not diluted. Regarding to the faulty heaters, it sounds the heaters were selected with improper size to can boil off effectively the amount of the accumulative liquid or the shut down period is very short and impede the heaters to malfunction and complete its job. And i think the second reason is the dominator where u mentioned the system is oversized that means the cycling rate of the compressor is very rapid especially if the cooling load is very small relatively to the system capacity making the off cycle is very short and the heater can not complete its function leaving diluted oil in the crankcase. So it may be suggested use a timer delay to de-energize the compressor contractor to restart by adding more 3 to 5 minutes. If u need further discussion i'd like to be in ur service.

Best Regards:)

[lana]

Hi there,

I found the reference. It's in Kotza book called RefRepair Manual page 193.
I quote the exact words :



Watt's cold surface experiment :

An "empty" cylinder (containing only vapour and no liquid refrigerant) is placed in a cold room at, say 0°C.
Then a second cylinder located outside the cold chamber is connected to the first cylinder. The second cylinder holds liquid R22 at 20°C.
Since both cylinders are connected at their highest points, no flow of liquid under gravity is possible.
The gauges fitted to each cylinder show a pressure of 8 bar, which agrees with the pressure-temperature-relationship for R22 vapour at 20°C.
After a certain time (which principally depends on the amount of liquid present, the temperature difference and the diameter of the connecting pipe work), all the liquid has migrated into the cold cylinder, and both gauges show the same reading of 4 bar (according to the pressure-temperature relationship for R22, this is the pressure of vapour at 0°C!).


Cheers

[mohamed khamis]

Hi there,

I found the reference. It's in Kotza book called RefRepair Manual page 193.
I quote the exact words :



Cheers

Thanks Lana for this information

Cheers

[US Iceman]

But wouldn't the gas condensing in the tank at 0°C cause the tank to warm up?

I think the statement made is only valid if you assume the tank wall is the control volume with no heat transfer to the space, right?

[mohamed khamis]

But wouldn't the gas condensing in the tank at 0°C cause the tank to warm up?



As i deduced from this example the empty "with gas" cylinder is located in cold chamber which will be maintained
at 0°C by using refrigeration unit regardless to the imposed cooling load. so the rejected heat of gas condensing is absorbed directly by the refrigeration unit causes the tank temperature is kept at 0°C.


I think the statement made is only valid if you assume the tank wall is the control volume with no heat transfer to the space, right?


No there is a heat transfer from the tank to the space during the gas condensing and refrigerant migration which is temperature is 20°C. It might be asked if the migrated refrigerant at 20°C and condensed gas at 0°C and they are mixed the net product temperature should be of a middle value between 20°C and 0°C (let us say 8 or 12..ect) and the example reveals the net product after mixing is also at 0°C. The expansion is as aforementioned the cold space is maintained at 0°C and of course the tank volume is very small comparing to the cold space volume thus the existence of temperature of 20°C has not meaningful to change the space temperature. For instance , if someone throw a ball of 20°C in a sea water of 0°C, there is a heat transfer will be occurred between the ball and the sea but Intuitively the sea water will not change its temperature for the sake of the ball..;)

Cheers

[lana]

Hi US Iceman,

You are right. Of course the temperature changes.

Mohamed explained it.

The whole idea is that refrigerant migration happens by either pressure or temperature difference.

Cheers

[US Iceman]

...so the rejected heat of gas condensing is absorbed directly by the refrigeration unit causes the tank temperature is kept at 0°C.


Ahhh, that's assuming a refrigeration system is used to maintain 0°C in that space.

I think we are all saying the same thing. The point I make is we have to list our assumptions on the validity of the statements. This is where a control volume helps to list those assumptions.

[philjd26]

cheers mohamad for advice, the chiller uses a buffer tank and have never seen the system running at full load,it seems to hold temp well on one stage out of six stages.....heaters do drop out when compressor runs so i think i could use ambient hold off for heaters when temp reaches 20c+ but as i said its just a thought!

[mohamed khamis]

cheers mohamad for advice, the chiller uses a buffer tank and have never seen the system running at full load,it seems to hold temp well on one stage out of six stages.....heaters do drop out when compressor runs so i think i could use ambient hold off for heaters when temp reaches 20c+ but as i said its just a thought!

U welcome and I agree with u about ur thought but there are two points i wish there are clarified:

1. If the ambient temperature are kept below 20°C for more than 5 hours this means the heater is energized along with this time and it inevitably raises up significantly the oil temperature affecting badly on the compressor performance. So why don not use the oil temperature itself as an indicator i.e. add 10 K to the ambient temperature of 20°C (as u mentioned) and immerse a thermostat (biometal or whatever...) in the oil itself and if the thermostat reaches to 30°C it hods off the heater without harming the compressor performance and so u have to check on the ordinary conditions what definitely the optimal oil temperature (30 or 35 or ...ect) and adapt the thermostat settling on it. Once the oil heater is cooled below this value the thermostat sensor probes this value and open the thermostat contact and in turn the heater contact is closed to be activated, once the oil temperature reaches the optimal value the hearer contact is opened. This protect ur compressor from overheating problem with heater working the ambient sensing based.

1. U mentioned that u see around 8 faulty heaters, what does it means by faulty heaters they got spoiled quickly or did not work properly?

Cheers

[LRAC]

Be careful on new chiller systems especially Carrier units that employ the Prologic controller. They won't run without a working crankcase heater being present, the amount of times we've been called out on breakdowns just too find the heater has packed up, stupid design where a whole machine shuts down just because of the above.

regards
Lrac

[Lowrider]

Be careful on new chiller systems especially Carrier units that employ the Prologic controller. They won't run without a working crankcase heater being present, the amount of times we've been called out on breakdowns just too find the heater has packed up, stupid design where a whole machine shuts down just because of the above.

regards
Lrac

Not all do! Was at one last week where the heater was burned out but no fault what so ever!

Since where on the prologic, what's your opinion abouth it? I'm wondering if you think they are reliable and what you come across most as a fault!