Kooltronic Design Guide - Air Conditioner Selection

Air Conditioner Selection

DETERMINE THE COOLING CAPACITY REQUIRED:

Air conditioners for cooling electrical enclosures should be sized to provide adequate cooling for the anticipated worst case conditions. This is usually when the ambient is the highest, and also when the electrical loads through the enclosure are at the maximum. However the air conditioner should not be over-sized, as this could result in compressor short cycling. This might cause wide swings in enclosure temperatures.

The total cooling capacity required of the air conditioner includes the:

(A) INTERNAL HEAT LOAD, (B) SOLAR HEAT LOAD and (C) HEAT LOAD TRANSFER.

(A)

The INTERNAL HEAT LOAD is the heat generated by the components within the enclosure.

(B)

The SOLAR HEAT LOAD is the additional heat due to the sun’s rays.
NOTE: Unfortunately the calculation required to properly identify the true Solar Heat Load is too extensive to provide here. Therefore we recommend you call the Kooltronic Sales Department. They have access to a computer program that will provide an accurate answer after a few simple questions. If you have an outdoor application, do not ignore the solar heat load. It can be substantial.

(C)

The HEAT LOAD TRANSFER is the heat that is lost through the walls of the enclosure. (This statement assumes the outside ambient is cooler than the air inside the enclosure.) NOTE: Refer to the Glossary/Technical Comments section for explanations of technical terms and more information about Engineering issues.

STEP 1: Calculate the Internal Heat Load by using the Incoming / Outgoing Power Test Method

The Internal Heat Load can be determined by measuring the electrical energy that stays inside the enclosure. It is assumed that this energy is eventually transformed into waste heat. To measure this electricity, the current going In and Out must be measured in amps. The voltage of this current is also important. It is critical that all wires entering and leaving the enclosure must be included. Typically, a voltmeter and a clamp-on type ammeter should be used. The data must be recorded during the time when the current flow is the highest. A qualified technician is recommended for safety and accuracy reasons. The Internal Heat Load = 3.413 x Voltage (Current IN - Current OUT.) NOTE: This equation is derived from: 3.413 BTU = 1 Watt and watts = volts x amps. For example, if you measured 220 volts, 40 amps IN, 35 amps OUT, the Internal Heat Load = 3.413 x 220 x (40 - 35) = 3754 BTU/H. Consult with an Electrical Engineer if 3 phase power or a very complicated circuit is involved.

STEP 2: Calculate the Heat Load Transfer

The heat load transfer is the additional heat added to the enclosure through the walls from the surrounding ambient. This is identified by the formula: Heat Load Transfer = (Max. Outside Ambient - Max. Allowable Internal Enclosure Temperature) x Surface Area x 1.25 HLT = (MOA - MAIET) x SA x 1.25

NOTE: 1.25 is a constant for metal enclosures. Use 0.8 for a plastic enclosure or 0.6 for an insulated enclosure.

The Maximum Outside Ambient (MOA) is the warmest room temperature surrounding the enclosure that might happen all year long. The MOA might be as high as 130ºF in an industrial equipment room. The Maximum Allowable Internal Enclosure Temperature (MAIET) should not exceed the heat tolerance specification of the most sensitive component in your system. The MAIET might not be allowed to go over 90ºF per the enclosure’s component specifications.
The Surface Area (SA) is calculated as follows:
Surface Area = (H x W) + (H x W) + (H x D) + (H x D) + (W x D)
H = height in feet, W = width in feet, D = depth in feet
For example; H = 4, W = 2, D = 3: Surface area = (4 x 2) + (4 x 2) + (4 x 3) + (4 x 3) + (2 x 3) = 46sq.ft.
Therefore in our example, the HLT = (130ºF - 90ºF) x 46 x 1.25 = 2300 BTU/H

STEP 3: Calculate the Total Cooling Capacity Required

The total cooling capacity required to cool your equipment is equal to:
Internal Heat Load + the Heat Load Transfer. The example: 3754 + 2300 = 6054 BTU/H

AIR CONDITIONER PERFORMANCE CHART EXPLANATION:

In the air conditioner section there are detailed specifications for each cooling system and a performance chart similar to the one shown below. Use the charts to verify that the correct unit, with the proper cooling capacity, has been selected.

Example:

The maximum ambient temperature is 110ºF.
The maximum enclosure air temperature is 100ºF.
The minimum air conditioner capacity is 8,000 BTU/H.

1. Locate the 110ºF requirement at the bottom of the example chart.
2. Follow the vertical line up until it intersects with the 100ºF maximum enclosure air temperature.
3. Follow the horizontal line to identify the actual cooling capacity that the unit will deliver at these conditions.
4. In this case, the unit will deliver approximately 8,000 BTU/H which is acceptable. If the unit’s capacity under this

condition was below 8,000 BTU/H, select another model with more capacity. If the capacity was significantly above the 8,000 BTU/H requirement, the next size smaller model should be selected.

SELECT THE APPROPRIATE AIR CONDITIONER PRODUCT LINE: