Oil Air Coolers
Oil to Air Heat Exchangers and Oil to Water Heat Exchangers
For Mobile and Industrial Applications
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Tips and Tricks in Using Oil Air Coolers
1. Temperature Switch:
When using oil coolers, many customers, especially in the mobile equipment market, believe that there is a requirement to use thermostats. Don’t try to dissuade them, sell the thermostat, but make sure they are measuring the oil temperature that is flowing out of the oil cooler. This is important, otherwise the temperature re-switch difference of the thermostat (11°C at 47° and 60° thermostates) will occur ( 47°minus 11°=36°C operating temperature)!
For circulation cooling systems (kidney loop) don’t insert thermostats in the cooler element, but in the reservoir of the oil cooler. It will not have any effect in the element because the oil inside the oil cooler will not be warmed-up if the circulation pump is not started. The pump/fan will be switched on by the thermostat when temperature in the reservoir reaches the set point.
Thermostats must not be used without relays. Size the relays based on the higher power input ( 3 times normal power input "in rush current"). Customer complaints of relay failures are usually because the relay was dimensioned for normal power input from the catalog.
2. Check of Calculation:
The cooler calculation sometimes gives wrong results, if it is done according to the rules. There might be a intersection between the outlet temperatures of the fluids, especially in applications with water and glycol. Compared with oil application the heat dissipation rate is much higher ( up to 2 times).
Solution: Use of a larger oil cooler or cooler with the same size but higher airflow, higher number of revolutions on cooler with hydraulic motors, use of DC fan units with more power (see S-motors), use several oil coolers combined in series or parallel connection.
3. Series Connection of PHE:
The calculation is generally done with a computer program, which enables you to vary the input data and use different values to influence the result. Variation of the fins sometimes yields amazing results in size and cost.
Determination of the cooler design is done by means of the so-called "Operating Point". This point is the intersection between the characteristic lines of the element and the fan. Changes from this point result in a decrease in efficiency. The same is true if a second element is mounted in front of the first element (e.g. stationary motors). Exact data about power of the fan unit and the decrease in heat dissipation of the original element has to be taken into account.
If an adequate second element is mounted to a standard oil/air cooler you can expect approx. 40% additional heat dissipation. We recommend using combination elements (2 independent circuits within 1 element/fan unit).
4. Pressure drop on oil side, depending on viscosity:
All pressure drop curves in our catalog are calculated with 30cSt and checked. 30cSt means, that the oil used has, at 40°C a viscosity of 30cSt.
If the oil outlet temperature of the oil cooler is higher than 40°C, use the table on page 7 of the cooler catalog for calculation of the real pressure drop in the element. The present oil viscosity can be calculated exactly with the viscosity tables you received. For the transfer from cp into cSt use the following formula:
Recalculation of heat dissipation depending on the viscosity will be done only with oil of a high viscosity (ISO VG 150, 220). We have enough reserve, because the curves are very accurate, they are calculated first and then checked with our test installation. Most Italian producers have incorrect curves displayed in their catalogs.
5. Decrease of heat dissipation in dependence of the sea level:
The data for the fans was calculated for the complete fan-unit on the basis of sea level; air flow in kg/sec. If a fan-unit is not used on this level a correction factor must be used. The higher the altitude the thinner the air and therefore lighter. Less kg/sec air flows through the fan compared to the flow at sea level which is 1,225 kg/m3.
6. Cooling of water with use of aluminum elements:
Pure water without any additives cannot be used in an open circuit with aluminum elements. It causes oxidation inside the element, which will clog the oil channels. But you can use mixtures of water and glycol (at least 5% glycol). The glycol will adhere to the surface of the aluminum and prevent oxidation. In closed circuits the use of water without glycol is restricted. It depends on the supply of fresh water to the closed circuit. (supply of oxygen).
The calculation of the heat dissipation for water uses the factor 2.5, which means that the required specific heat dissipation has to be divided by 2.5 before choosing a cooler. Once you have converted the heat dissipation required you can use the curves in the catalog, which are designed for use with oil.
Attention, it is necessary to recalculate the outlet temperatures to avoid intersection!
Formula for difference of water temperature:
|t water =
14 x heat dissipation
7. Use of oil coolers in salty environment:
Salty air is very aggressive, permanent conservation is almost impossible, but there are some methods of temporary prevention of oxidation- especially for aluminum parts. The varnish coat of the elements cannot withstand the salt of the air, that is why a special coat for the cooler is necessary. Even the hub of the fan of all coolers with JEC motors are treated with that varnish. Parts made of steels, like housings, feet and connection parts, are powder coated or galvanized and do not need any further treatment. Electric motors should be applied with a damp room protection. More effort is not recommended.
Plate-heat exchanger and sea water or salt water as flow media will dissolve the material of the fins (1,4404) - battery effect.
8. Use of oil coolers in arctic environment:
The only component that might cause problems is the fan, which is made of polypropylene, in the arctic environment it must be made of glass-fibre reinforced polyamide. The electric motor uses temperature resistant grease and electric heating to avoid condensation.
A temperature switch should activate the oil cooler.
Practice showed that mobile vehicles in arctic environment are parked in halls during the night. And the engines run within the hall till operating temperature is reached. The danger of thermal shock is relatively low.
A bypass valve is advantageous during warm-up.
9. Use of bypass-valves:
Bypass-valves are needed to avoid over pressure in the system, watch the frequency of pressure shocks.
At high frequency and heavy pressure peaks a bypass-valve is not applicable.
Another possibility is the use of bypass-valves in applications with low heat dissipation and high oil flows or temporary oil flow peaks.
The cooler can be designed for the most favorable oil flow. The present pressure drop for the heat dissipation (see catalog) and the operating pressure for the bypass-valve with safety factor (appr. pressure drop x 1.5) have to be determined.
10. Maximum operating temperature:
The data of pressure can be applied for temperature up to 100°C.
If there is a need for higher temperatures, the following schedule is used:
||Maximum admissable pressure
|up to 100°C
|up to 150°C
|up to 200°C
11. Comparison with competitor:
Many producers have in their catalogs data about heat dissipation which does not meet reality. They take advantage of the fact that the data given by customers is not very exact. A test, which shows whether the data in the catalog meet the requirements of the customers, is rarely made. If the oil outlet temperature is 65°C instead of 60°C does not matter very much. Depending on the oil flow the difference can be up to 20%. This factor has to be taken into account as average at many competitors.
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