A restaurant roof is a harsh environment for equipment. It can be particularly severe on HVAC equipment due to heat, grease, exhaust fumes, and dirt. These pictures are from a three month old restaurant in North Texas. (click on any of the photos to view a larger version)

The Problem

It is difficult to keep enough distance between rooftop air conditioning units (RTU’s) and exhaust fans on a small roof. The heat from the exhaust is little trouble compared to the grease. The grease forms an aerosol and coats everything on the roof. The picture below is a close-up of the RTU’s condenser coil.

The relatively clean band of coil has a structural element behind it blocking the airflow through this portion of the coil. The remainder is clearly getting filthy as the grease-laden air passes through the fins. The grease adheres to the fins which in turns collects dust, cottonwood, etc. This has a pernicious effect by reducing both the airflow through the coil and the heat transfer rate from the aluminum fins to the remaining airflow.

The Cost

This causes the RTU’s compressor to draw more electrical power to raise the refrigerant pressure in order to reject enough heat from inside the restaurant. Since it was 103 F on the roof the day these photos were taken, we expected to measure 20.4 kW of power consumption and 268 MBH of gross cooling. Instead, the RTU was consuming 23.3 kW and only providing 250 MBH of cooling. This is as though the roof temperature was actually between 115 and 120 F.

Besides using substantially more electricity because of higher power consumption and longer runtime due to lower cooling capacity, the raised compressor pressures (~ 15% higher) and increased motor temperatures (~ 30% higher) will reduce the compressor life.

A few weeks after our tests, this RTU shut down on high gas pressure and would not restart until service technicians arrived on an emergency call to clean the condenser coils, a messy and time-consuming process. This RTU will use an average of 8950 kWh more electricity per year than a clean unit. But this cost pales compared to the cost of emergency repairs and inconvenience and discomfort to customers. There is a straightforward solution.

The UltraMister™

The UltraMister™ evaporative precooler is fabricated with two large sets of fiberglass filters that create a “misting chamber” between them. The modular precoolers are attached to the outside of the RTU’s condenser coils. The outer set of filters effectively captures dirt and debris along with grease droplets. It also settles turbulent airflow often found on roofs. Inside the chamber, water is atomized or misted towards the incoming airflow. The airflow is rapidly cooled by flash evaporation. Any mist that doesn’t evaporate is captured by the inner set of filters, preventing it from depositing minerals on the coils. The UltraMister™ precooler not only cools the condenser air, but it also captures grease and dirt very effectively.

Large 2-Module UltraMister™ Precooler installed on 50-ton RTU

Inside UltraMister™ (air flows right to left)

The filters have a large flow area and very low pressure loss (< 0.05″ w.c.) even when dirty. The standard 24×24 filters can be easily and quickly replaced from the hinged access door by your regular maintenance provider or by a bus-boy.

The following photos show how effectively the fiberglass filters capture airborne grease on the roof. In just six days, the filters went from clean and blue to filthy gray. Though the pressure drop had not changed measurably.

7-26-05: Clean prototype filter installed

8-01-05: Filter is filthy from Grease Exhaust

Payback Summary

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