Roof vs Cruise air

[Jon Wehrenberg]

Jack, I know the factory fill specs do not take into consideration the longer runs between the evaporator and condenser so we always need to add more. Have you measured the differential between the air out of the evaporator and the air in? I know these are very fussy and too much of a charge is as bad as too little.

I know the charge is close when I see vigorous foaming in the sight glass. Some bubbling is too little, clear liquid is too much. But I still see the temperature differential as the ultimate factor confirmed by pressures on the high and low side

[jack14r]

Each unit is different,I have a differential electronic thermometer and I have seen as much as 20 degrees and as little 14 degrees in different units.Johnny at AAP is very explicit about how to get max performance out of Cruise Airs and he will tell you that you must get the lowest possible temperature on the suction side at the condenser,I have followed his instructions and it works,he also has told me that if you get the pressures by the chart correct,you will need a little more freon to get the unit to max performance and I have found this to also be true.If I could not measure temperature I would adjust the freon pressures by the chart because I have found that is very close to max performance.The temperature of the suction tube at the condenser is obvious if you are holding it when the freon is just right,again(beer cold)is just right.

[johnklopp]

This has been an interesting discussion. Having never seen a cruse air unit I can only refer to the basics of vapor cycle cooling design and the affects typical conditions have on various components.

Keep in mind that a cruse air unit when designed was intended to be in a boat not a bus. Typical ambient temperatures would seldom exceed 95 degrees F. Most of the units probably had water cooled condensers that lowered the condenser cooling temperature to less than 85 degree F.

The compressor in the cruse air units is a standard off the shelf unit made by one of several manufactures. Little difference exists between compressors of similar capacity as the manufacturers will substitute an equivalent compressor if it has a cost benefit. If the compressor is a horizontal type the opportunity to substitute compressors will be limited. All compressors are rated based on an ARI rating standard that is 130 degrees F. condensing temperature and a 45 degree F. suction temperature.

If the condensing temperature is increased the compressor capacity will be decreased. Most compressors are rated up to a 150 degree F. condensing temperature. Unfortunately at that condition the cooling capacity will typically be reduced by approximately 20%
The condensing temperature is the point when the refrigerant condenses to a liquid in the last few rows of a condenser. A typical condenser design will have a 30 degree F. differential temperature between the air entering the condenser (ambient air) and the condensing temperature. That differential is seldom more than 40 degrees F. The condenser is also responsible for providing 5-10 degrees of sub cooling.

Sub cooing is required to ensure that the condensed refrigerant remains a liquid on its way to the metering device located on the inlet to the condenser. If the installer has used marginally sized liquid lines between the condenser and the evaporator, some of the refrigerant will flash to gas causing bubbles and reducing capacity.

If the installer has used a marginally sized suction line, the pressure drop will reduce the suction temperature at the compressor and reduce capacity.

What can be done to increase capacity?
Increase heat rejection at the condenser
lowering the ambient is not realistically possible.
Increase ambient air flow. Can be done with better fans and ducting.
Increase the size of the condenser. Most condensers can easily have an additional row added.
Increase the efficiency of the condenser. Late model condensers are usually micro channel instead of fin tube. Take a look at the condenser on the front of your car. It has this micro channels between the fins. The micro channels often have 5 or 6 holes or tubes in each section. Without a staggered tube design, the air pressure drop is reduced and the air flow increases with the same fans. Micro channel condensers will typically reject over 20% more heat in the same area.


Reduce or eliminate the critical charge
Increase the size of the liquid line receiver. Add one if the system does not have one. Acts somewhat like the water bellows tanks on our fresh water systems. The system should not be affected by the loss of 5 or 6 ounces of refrigerant

[jack14r]

John,you are right that the yacht units are water cooled,and the compressors are off the shelf Tecumseh #AKA8515 EXA they are rated at 15,000 BTU.I would like to find a condensing fan motor that was higher RPM and of course would have to be more HP that would fit in the same space,I think that this would help a lot.I don't think that Dometic really wants to improve the cruise air but just keep it as is and keep manufacturing it.

[garyde]

I am always amazed how much action this topic gets. Its crazy. This could go on to infinity. Lets agree that there is more than one way to cool a bus. Everything after that is just degrees. Pardon the pun.

[Jon Wehrenberg]

Actually this is a good discussion because if we pull out the opinions and personal preferences, this discussion has some excellent technical information such as what John has provided. As a result of the information being provided I suspect anyone with a Cruise Air can provide the information to an AC professional and get their CA performing to the max.

One point John makes suggests strongly the importance of maximizing air flow and it makes me wonder if all CA installations in our coaches have unrestricted air flow to and from the condensers.

[truk4u]

Great post John..

Tuga - When the ambient temps got to 90 and the asphalt is cooking, mine would shut down to over temp. Not sure if Liberty made any changes later in the years, but I believe through 2000, you had to remove the fridge to clean the filter. Maybe an XL2 owner will step in.