Generator Fan Blower

[johnklopp]

Most liquid heat exchangers are designed for a 10 F degree drop at full load. The temperature drop will increase at light load due to the reduced flow caused by the modulating thermostat. The design point of all heat exchangers is the approach temperature which is a measurement of the difference between the outlet water temperature and the inlet air temperature acros the heat exchanger. Typical design conditions represent an approach temperature of 40 degrees F.

An impeller that has lost its vanes is rather unlikely. The most probable problem in my opinion was pointed out by Jon as a collapsed hose on the suction side of the pump. The suction hoses typically have a spring on the inside of the hose to prevent collapse when the engine cools and the water volume is reduced.

You should be able to determine if you have circulation and heat transfer by using your infrared thermometer If you don’t have one, you should, how else can you check your tires on those short rest stops?

[Jon Wehrenberg]

JUst as a point of information on some data that I do know, I observed my bus engine thermostat operate on a return trip from New Orleans this spring.

My thermostatic fan clutch had acted up so I had locked the fan. Prior to that my fan clutch controlled engine coolant temps and the temps would cycle between 200 and 205 as indicated on the gauge.. With only the engine thermostat to control engine temps the engine cycled between 190 and 195 as indicated by the gauge.

For the entire trip, including those portions where the bus was pulling hills and was operating at max power the thermostat cycled open and closed. This was consistent with the operation when the fan clutch was working normal, although the cycle temps when using the fan clutch were between 200 degrees indicated which I assume was when the fan clutch disengaged, and 210 under hill climb conditions which was when both clutch speeds were engaged. My point is that as long as a fan is fully engaged an engine is likely to cycle its engine coolant flow via the thermostat. With the thermostat removed as in Tuga's coach I would have expected to see temperatures in the 140 to 160 range at the radiator inlet side and lower at the suction side.

The reason I suggest they should be this low is because the cooling system is working with a generator that is lightly loaded and the system design should be capable of handling excessive ambient temps under full or heavy loads. I am having a problem agreeing with John on the 10 degree temperature drop, but until I can determine that is typical I have to accept his word because I doubt if any one else in the Prevost universe has a better understanding of the transfer of heat.

[phorner]

Jon,

I am sure that my generator radiator cools more than 10 degrees from inlet to outlet. When I get a chance, I'll run mine today (should be in the 90's here in Florida) under load and measure the actual temperature at inlet copper pipe and outlet copper pipe.

Last time I checked, the inlet pipe was too hot to hold yet the outlet pipe was not a problem at all. I was quite surprised.

[phorner]

Tuga,

For what it's worth, here are the operating stats from my generator this noon:

Generator running for 1 hour under load of 4 CruiseAir units plus whatever else was on, approximately 35 amps/leg.

Outside ambient temperature 91 degrees

Dash analog gauge reading 175 degrees

Top radiator tank is 157 degrees via IR thermometer (inlet copper piping too hot to hold)

Bottom radiator tank is 109 degrees via IR thermometer (outlet copper piping warm to touch, can hold without discomfort)

My radiator appears to provide nearly 50 degrees of cooling WITH the blower operating properly.

I don't know how much these numbers might change if I ran the generator all day or while under way on the highway.

Hope this helps some....

[phorner]

As an aside, it sure appears that the long run of copper coolant lines between the generator and the radiator provides for significant cooling, at least while parked...

[Jon Wehrenberg]

Those are numbers consistent with what my gut told me to expect. The spread between inlet and outlet temps through the radiator (heat exchanger) is going to be significantly impacted by ambient temps. If the OAT is lower than 90 then the difference between the OAT and the thermostatically maintained inlet temperature is going to be greater and thus the radiator outlet temps are going to be even lower. That in turn will result in greater periods of generator operation with the thermostat closed as a means of the generator operating at the desired temperature.

[merle&louise]

Tuga,

For what it's worth, here are the operating stats from my generator this noon:

Generator running for 1 hour under load of 4 CruiseAir units plus whatever else was on, approximately 35 amps/leg.

Outside ambient temperature 91 degrees

Dash analog gauge reading 175 degrees

Top radiator tank is 157 degrees via IR thermometer (inlet copper piping too hot to hold)

Bottom radiator tank is 109 degrees via IR thermometer (outlet copper piping warm to touch, can hold without discomfort)

My radiator appears to provide nearly 50 degrees of cooling WITH the blower operating properly.

I don't know how much these numbers might change if I ran the generator all day or while under way on the highway.

Hope this helps some....

Paul,

Thanks for running the "test" - I feel that the 50 difference you experienced is telling me that my system is either blocked or my water is not working.

Jon & John,

Thanks for your explanations and advice. I am going to remove my water pump and have a look at the impeller. I am also going to change the hoses.

[phorner]

Tuga,

I agree with Jon that you have nothing to lose by changing the hoses first and see if that corrects your problem.

Just might save you a lot of work....

[johnklopp]

I think everyone has presented excellent points taking into account the actual operation they are experiencing at that time. No one has presented opinions and data that I am in disagreement with.

Please note the following comments on various typical design conditions..

Temperature drop at full load is a design condition that will never be seen with normal operation. Heat exchangers are always going to be designed for the worst case you can imagine. The air temperature for a worst case design will likely be 20 degrees above the hottest day any place in the country. The extra 20 degrees of air temperature is due to heat radiated off the black pavement. This would suggest an entering temperature of approximately 150 degrees F.

The heat rejection of the engine is also rated at a higher amount than a properly operating engine will produce. Diesel engines are readily capable of exceeding maximum power ratings and heat rejection by nothing more than an improperly adjusted governor. Your 17.5Kw generator is probably capable of putting out close to 20Kw for a period of time without problems. This additional heat rejection is part of the full load design the engineer will use to select the heat exchanger, coolant flow rates and air flow rates for the system.

All fluids have a design standard called specific heat that is used to define the amount of energy or heat that can be transferred by a quantity of fluid. A fluid is either a liquid or a gas. Both use the same basic approach for calculating heat transfer. Air at sea level is equal to 1 and water is equal to 1. All of you pilots know how much the density of air changes as you go up in altitude. You will very quickly get to the point that you need to double the mass flow or volume of air to have the same heat transfer. Coolant has the same problem, you add antifreeze to increase the boiling point but it also reduces heat transfer at the same time. Your system was designed to have a 50/50 mix of antifreeze and water. One day your system is low so you just add undiluted antifreeze. Great, it won’t freeze now till you’re at 40 degrees below zero or some other temperature based on the dilution. But how many of you know that the additional antifreeze now requires you to move additional coolant thru the system to have the same heat transfer? Does anyone use a hydrometer to see what the coolant mix is?

The approach temperature differential between the leaving water temperature and the entering air temperature will be affected by a fouling factor caused by contaminants in the coolant and the cleanliness of the heat exchanger and the engine itself. The design engineer will take into account a moderate amount of fouling in his calculations. If a technician knows what the approach temperature is on a clean heat exchanger, heat can determine when a heat exchanger needs to be rodded out to return the heat transfer capability.

At anything less than full load, the coolant flow will be reduced causing the coolant to spend additional time in the heat exchanger. A greater temperature drop will take place due to the additional time the coolant spends in the heat exchanger.

[phorner]

John,

Thanks for the great post! Always informative and educational.

Your discussion regarding the design considerations reminds me a lot of some of the things we addressed while designing highways such as impact factors for bridges rather than just support of a static load and design vehicle speeds in considerable excess of posted speed limits when designing curve radii and superelevation.