Wet Bulb Temperature and Its Impact on Building Performance By: Kurmit Rockwell, PE, CEM, LEED AP and Justin Lee, PE, LEED, AP BD+C Energy Solution Services, AtSite, Inc. 1
What is Wet Bulb Temperature? Wet bulb temperature is an indication of the amount of moisture contained within the air. Wet bulb temperature differs from the temperature you feel when you go outside. The temperature you feel is called Dry Bulb temperature, and is independent of the moisture content of the air. Wet bulb temperature is the temperature you feel when water evaporates from the skin and can be measured by wrapping the bulb of a thermometer in a wet cloth. If you swing the thermometer quickly the resulting evaporation will drive the thermometer towards the wet bulb temperature. Unless the air is fully saturated (in other words, relative humidity is 100%), the web bulb temperature will be less than the ambient dry bulb temperature. 2 Since wet bulb is a measure of moisture levels within the air, the larger the differential between dry bulb and wet bulb temperatures the drier the air feels. For instance, take a look at the following example: Dry-Bulb Temperature Wet-Bulb Temperature Relative Humidity Day 1 80 F 70 F 61% Day 2 80 F 63 F 39% In the above example, both days have the same ambient dry bulb temperature, however Day 2 has a lower wet bulb and will therefore feel like a more pleasant day.
Wet Bulb Can be Used to Determine Other Physical Properties of Air By measuring the wet bulb and dry bulb temperatures, other physical properties of air, such as the relative humidity and the dew point temperature, can be determined. One means of determining the physical properties of air is by charting the known values on a psychrometric chart and interpolating the other values. 3
Wet Bulb Temperature How Does Wet Bulb Temperature Affect HVAC Systems? As we just learned, wet bulb temperature is the minimum temperature that can be attained by the evaporation of water. As a result, the wet bulb temperature is the lowest temperature that can be supplied by a cooling tower or an evaporative cooler. Cooling tower design supply temperatures are based on the worst case wet bulb temperatures for a particular region. The wet bulb temperature sets the cooling tower s lowest supply temperature limit. Cooling Tower Return: 95 F Cooling Tower Wet Bulb Temperature: 78 F Cooling Tower Supply: 85 F 4 The difference between the cooling tower return and the cooling tower supply temperatures is called the range temperature. Cooling tower range temperature is typically between 8 F - 15 F. The larger the cooling tower range temperature, the less pumping energy is required to move the cooling tower water through the HVAC system. The difference between the cooling tower supply temperature and the wet bulb temperature is called the approach temperature. Approach temperature is typically between 5 F - 10 F in the mid-atlantic region and can be higher in dry climates. As cooling tower approach temperature decreases, colder water is supplied to the HVAC system. The advantages of colder cooling tower supply temperature will be discussed in the next section.
Utilizing Wet Bulb to Increase Energy Efficiency Wet bulb temperature is typically at or above design conditions less than 1% of the year, so there is a significant opportunity to reduce cooling tower supply temperature throughout the year. This is accomplished by utilizing a control logic in the Building Management System (BMS) that varies the cooling tower supply temperature with the outside air wet bulb temperature. A common approach to cooling tower operation is to have two modes of operation; normal operation where the cooling tower supplies the design temperature to the mechanical HVAC equipment, and economizer operation where the cooling tower supplies the chilled water temperature that the HVAC system would typically supply. During economizer operation, the cooling tower supply temperature will typically range between 40 F - 50 F based on outdoor conditions. With the use of BMS controls a third operation can implemented when the cooling tower operation is in normal mode to allow the cooling tower supply temperature set point to vary (or reset) based on the wet bulb temperature. HVAC energy efficiency can typically increase upwards of 1%-2% for every degree that cooling tower supply temperature is reduced. The following graph is one example of the energy efficiency (KW/ton) of a centrifugal chiller at various entering condenser water temperatures at both 100% load and 60% load. 5 TRANE Engineers Newsletter: Vol. 25/No. 1
As indicated in the above graph, the KW/ton of the chiller decreases as the entering condenser water temperature decreases. This results in the chiller requiring less power (KW) to produce one ton of cooling, thereby increasing energy efficiency. The following is a list of several system types that would be potential candidates for condenser water supply reset strategies: Central chiller plant with centrifugal or double effect absorption chillers Floor by floor self-contained units Water cooled rooftop units Economizer Cooling When the ambient wet bulb temperature is low enough, cooling towers can produce chilled water without using mechanical cooling. Water side economizer saves energy by turning off the large compressors in the chillers and self-contained units within the building and providing chilled water straight to the units from the cooling tower. A common configuration for water side economizer is to provide cooling tower water to a plate and frame heat exchanger, which then provides cooling to the HVAC system. 6 All buildings that utilize cooling towers to reject heat are candidates for water side economizer.
Case Study: Economizer Cooling and Improving Building Performance Economizer cooling has been proven to significantly reduce building energy consumption and improve HVAC system efficiencies. For this case study we ll analyze one building where the building engineer was able to utilize economizer cooling during the morning and early afternoon hours, but had to use mechanical cooling later in the afternoon. Economizer Cooling Mechanical Cooling 7 Between 4am-1pm the average wet bulb temperature was 53 F, which allowed the economizer system to provide chilled water for free cooling. As the day progressed the chilled water temperature requirements changed due to the internal heat gains and between 1:00pm and 2:00pm the chiller was required to provide mechanical cooling to the building. The chiller activation is evident in the graph above by the large spike in electricity consumption. The electricity demand during peak economizer hours (6:00am-1:00pm) was reduced by approximately 200 KW, when compared to the 2pm to 5pm profile. This resulted in energy savings of approximately 1,400 KWh during the economizer hours. Economizer savings for this particular example were only realized during a portion of the day due to fact that this was a moderate day in mid-fall. During late fall through early spring, the building will operate almost exclusively in economizer mode. Therefore, potential savings for the building over the course of an entire year can be significant. Economizer savings for this particular example were only realized during a portion of the day due to fact that this was a moderate day in mid-fall. During late fall through early spring, the building will operate almost exclusively in economizer mode. Therefore, potential savings for the building over the course of an entire year can be significant.
Case Study: Diagnosing System Performance Issues In order to develop an optimized system performance strategy, AtSite energy managers developed a performance verification plan with the facility manger to establish a baseline of current system performance. After several weeks of diligently tracking wet bulb, dry bulb, cooling tower supply and cooling tower return temperatures the facility manger provided AtSite with the system performance results: Approach Range Two things were immediately noticed by AtSite engineers. First, the approach temperature is between 3 F-4 F consistently throughout the temperature readings. Second, the range temperature is between 15 F-18 F. These temperatures differed from the approach and range indicated on the construction documents (9 F and 8 F, respectively). The low approach and high range temperatures are indicative of a potential flow problem within the cooling tower system. 8 Based on this analysis, a cooling tower technician performed a detailed inspection of the cooling tower system and determined that the flow was restricted through the heat exchanger serving the closed loop heat pump system. Low flow through a heat exchanger can cause substantial losses in energy efficiency and impact system performance. Low velocity through a heat exchanger causes laminar flow which results in reduced heat transfer effectiveness and the build up of scale within the heat exchanger tubes. Low flow also causes the cooling tower fans to work harder to remove the heat from the condenser loop. In other words, instead of varying to meet the actual load the fans must operate at or near full capacity to satisfy cooling loads throughout all modes of operation. In addition to the issues cased by low flow, a blockage in the heat exchanger results in additional head losses through the system which causes the cooling tower pumps to work harder to produce flow. At the direction of the building owner, the technician was able to clean the heat exchanger to mitigate the flow issues and restore system performance. By diligently tracking wet bulb temperature and comparing actual system performance, AtSite engineers and the facility manager were able to diagnose and rectify an issue with the building HVAC system that was adversely impacting building energy performance. Without this process, the issue may have continued unabated until the problem reached critical failure and building systems went offline. By addressing the issue prior to system failure, AtSite engineers and energy managers improved building performance and protected valuable building assets from additional damage which may have shortened useful life.