Emerson VSSG, VSG manual Installation, Hot Air Recirculation

Section 3 • Installation

HOT AIR RECIRCULATION

There are two situations where hot recirculation could occur.

•Intake velocity of the cooler is higher than the dis- charge velocity

•Environmental issues such as strong cross winds which is installation specific

We addressed how to minimize the opportunity for hot air recirculation involving the operation of the air cool- ers under the guidelines for the Free Flow section above. However, environmental hot air recirculation is some- thing that is outside of the control of the cooler manu- facturer. If the site has the potential for strong cross winds or tunneling then you may want to consider the following items to minimize the potential of hot air recir- culation due to the environment.

•All air coolers should discharge at the same elevation.

•If feasible all of the air coolers in a common area should be placed as close to the next cooler as possible.

Unfortunately this comes with a cost. By reducing the in- take perimeter available, the leg height increases, which is a cost. In addition there is no guarantee that hot air recirculation can be eliminated due to environmental issues.

Discharge Elevation

In order to minimize the potential for hot air recircula- tion, it is recommended to install the air coolers so that they all discharge at the same elevation. This is impor- tant because not all air coolers have the same plenum heights. You can satisfy the leg height requirements by the calculation above but not satisfy the requirements for discharge elevation. Based on the size of the cool- ers above, cooler 1 has a plenum height of 36”, cooler 2 has a plenum height of 42”, and cooler 3 has a plenum height of 54”. Plenum heights are calculated from the standard of a 45 degree dispersion angle in API 661. If you would like more details on this calculation please contact engineering.

As you can see in Figure 3-13 in the not preferred con- figuration. the leg height requirement in the first cal- culation is satisfied. However, the coolers are not at the same discharge elevation. This could be an issue if there is a cross wind from left to right. Cooler 3 could block the discharge air flow from coolers 1 and 2 and recirculate it back down to the intake of the coolers. The best way to address this is to increase the leg heights on coolers 1 and 2 as shown in the preferred configuration.

This is especially critical if all of the air coolers are not place as close together as possible.

Cooler Placement and Spacing

The concern about cooler placement as far as proximity from one another has to do with the potential for hot air recirculation due to cross winds. Let’s look at only having coolers 1 and 3 in the same area. The leg height requirement of cooler 1 is 3.23ft so we would use a 4ft leg height. The leg height requirement for cooler 3 is 4.61ft but we would put a 7 foot leg height on the cooler due to size and for serviceability. Figure F. shows this installation.

In this case, if you get a strong cross wind from left to right there is the potential that the discharge coming from the top of cooler 1 could easily be pushed into the intake of cooler 3. Thus, in Figure 3-14, the not preferred configuration is more susceptible to the potential of hot air recirculation due to the environment and is not rec- ommended. The preferred configuration is the recom- mended installation of the coolers with a space between them.

All of the information provided is a general guideline for installing multiple air coolers in a common area. If you minimally follow all of the Free Flow requirements and keep the air coolers discharging at the same elevation then you have a good basis for site layout.

Sometimes due to extenuating circumstances, the above guidelines cannot be followed. Please contact en- gineering to discuss remedies to accommodate different configurations.

3 – 14 VSG/VSSG • Installation, Operation and Maintenance Manual •Vilter/Emerson • 35391SSG