Virtual controller for mixed air low temperature protection of HVAC systems

Abstract

A method for protecting an HVAC system of an air handling unit from low temperature outdoor ventilation air. The air handling unit may include an outdoor fresh air region, a return air region, a supply air region, and a damper situated in or adjacent to the fresh air region to regulate the flow of outdoor air into the air handling unit. The air handling unit mixes the outdoor fresh air and the return air to provide a mixed air stream to the HVAC system. In one illustrative embodiment, one or more sensors are used to measure the temperature and flow rate of the air entering or passing through the outdoor fresh air region, the temperature of the air entering or passing through the return air region and the flow rate of the air passing through the supply air region. The temperature of the mixed air stream, which is provided to the HVAC system, is then calculated using a controller or the like. In some cases, when the temperature of the mixed air stream falls below a threshold value, the controller may instruct the damper to close and reduce the fresh outdoor air that is brought into the air handler unit. Various other embodiments and algorithms are disclosed.

1 . A method for determining a characteristic of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes at least two flow inputs that are mixed to provide the mixed airflow stream, and a flow output, the method comprising: 
 providing a measure of a first characteristic and a second characteristic of air in a first flow input;    providing a measure of the first characteristic of air in a second flow input;    providing a measure of the second characteristic of air in the flow output; and    calculating the first characteristic of the mixed air stream in the AHU as a function of the first and second characteristics of the first flow input, the first characteristic of the second flow input, and the second characteristic of the flow output.    
     
     
         2 . The method of  claim 1  wherein the first characteristic is temperature and the second characteristic is air flow.  
     
     
         3 . The method of  claim 1  wherein the first characteristic is moisture content and the second characteristic is air flow.  
     
     
         4 . The method of  claim 1  wherein the first characteristic is carbon dioxide concentration and the second characteristic is air flow.  
     
     
         5 . The method of  claim 2  wherein the first input is a fresh outdoor air input.  
     
     
         6 . The method of  claim 5  wherein the second input is a return air input.  
     
     
         7 . The method of  claim 6  wherein the flow output is a supply air output.  
     
     
         8 . The method of  claim 1  wherein the AHU includes a heating, ventilation, and air conditioning (HVAC) system.  
     
     
         9 . The method of  claim 1  wherein the measure of the first characteristic and the second characteristic are provided by one or more sensors.  
     
     
         10 . A method for determining a characteristic of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes at least two flow inputs that are mixed to provide the mixed airflow stream, and a flow output, the method comprising: 
 providing a measure of a first characteristic and a second characteristic of air in a first flow input;    providing a measure of the first characteristic and the second characteristic of air in a second flow input;    calculating the first characteristic of the mixed air stream in the AHU as a function of the first and second characteristics of the first flow input and the first and second characteristics of the second flow input.    
     
     
         11 . The method of  claim 10  wherein the first characteristic is temperature and the second characteristic is air flow.  
     
     
         12 . The method of  claim 10  wherein the first characteristic is moisture content and the second characteristic is air flow.  
     
     
         13 . The method of  claim 10  wherein the first characteristic is carbon dioxide concentration and the second characteristic is air flow.  
     
     
         14 . The method of  claim 11  wherein the first input is a fresh outdoor air input.  
     
     
         15 . The method of  claim 14  wherein the second input is a return air input.  
     
     
         16 . The method of  claim 15  wherein the flow output is a supply air output.  
     
     
         17 . The method of  claim 10  wherein the AHU includes a heating, ventilation, and air conditioning (HVAC) system.  
     
     
         18 . The method of  claim 10  wherein the measure of the first characteristic and the second characteristic are provided by one or more sensors.  
     
     
         19 . A method for determining the temperature of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes an outdoor fresh air input port, a return air input port, a return air exhaust port, and a supply air port, wherein the mixed airflow stream is a mixture of the air passing through the outdoor fresh air input port and the return air input port, the method comprising: 
 providing a measure of temperature and air flow for air passing through the outdoor fresh air input port;    providing a measure of temperature for air passing through the return air input port;    providing a measure of air flow through the supply air port; and    calculating a temperature for the mixed air stream in the AHU as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow through the outdoor fresh air input port and the supply air port.    
     
     
         20 . The method of  claim 19 , wherein the AHU further includes a damper for controlling the air flow through the outdoor fresh air input port, the method further comprising the steps of: 
 adjusting the damper to reduce the air flow through the outdoor fresh air input port if the temperature of the mixed air stream falls below a predetermined threshold temperature value.    
     
     
         21 . The method of  claim 20  wherein the adjusting step only occurs if the temperature of the mixed air stream falls below the predetermined threshold temperature value for a predetermined time period.  
     
     
         22 . A method for determining the temperature of a mixed airflow stream in a air handling unit (AHU), wherein the AHU includes an outdoor fresh air input port, a return air input port, a return air exhaust port, and a supply air port, wherein the mixed airflow stream is a mixture of the air passing through the outdoor fresh air input port and the return air input port, the method comprising: 
 providing a measure of temperature for air passing through the outdoor fresh air input port;    providing a measure of temperature and air flow for air passing through the return air input port;    providing a measure of air flow for air passing through the return air exhaust port;    calculating a temperature for the mixed air stream in the AHU as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow flowing through the return air input port and the return air exhaust port.    
     
     
         23 . An air handling unit (AHU) having an outdoor fresh air input port, a return air input port, and a supply air port, wherein the AHU mixes the air passing through the outdoor fresh air input port with the return air input port to produce a mixed air stream, the AHU comprising: 
 a first temperature sensor positioned in the outdoor fresh air input port for providing a measure of the temperature of the air passing through the outdoor fresh air input port;    a first air flow sensor positioned in the outdoor fresh air input port for providing a measure of the air flow of the air passing through the outdoor fresh air input port;    a second temperature sensor positioned in the return air input port for providing a measure of the temperature of the air passing through the return air input port;    a second air flow sensor positioned in the supply air port for providing a measure of air flow through the supply air port; and    a controller coupled to the first temperature sensor, the second temperature sensor, the first air flow sensor and the second air flow sensor, the controller adapted to calculate a temperature for the mixed air stream as a function of the temperature of the air flowing through the outdoor fresh air input port and the return air input port, and the air flow passing through the outdoor fresh air input port and the supply air port.    
     
     
         24 . The air handling unit (AHU) of  claim 23  further comprising a damper for controlling the air flow through the outdoor fresh air input port.  
     
     
         25 . The air handling unit (AHU) of  claim 24  wherein the controller is adapted to adjust the damper to reduce the air flow through the outdoor fresh air input port if the temperature of the mixed air stream falls below a predetermined threshold temperature value.  
     
     
         26 . The air handling unit (AHU) of  claim 25  wherein the controller is further adapted to only adjust the damper if the temperature of the mixed air stream falls below the predetermined threshold temperature value for a predetermined time period.  
     
     
         27 . A method for determining the mixed air temperature in a air handling unit (AHU), the air handling unit including a fresh air region, a return air region, and a supply air region, the method comprising: 
 determining the supply airflow rate in the supply region;    sensing the outdoor air temperature in the fresh air region;    sensing the outdoor airflow rate in the fresh air region;    sensing the temperature of the return airflow in the return air region; and    calculating the mixed airflow temperature as a function of the supply airflow rate, the outdoor air temperature, the outdoor airflow rate, and the temperature of the return airflow.    
     
     
         28 . The method of  claim 27 , wherein the supply airflow is calculated.  
     
     
         29 . The method of  claim 27 , wherein the supply airflow is measured.  
     
     
         30 . The method of  claim 27  further comprising the steps of: 
 comparing the outdoor air temperature and the mixed air temperature to a threshold temperature; and    adjusting an outdoor air damper to regulate the outdoor airflow into the AHU if the outdoor air temperature and/or the mixed air temperature fall below the threshold temperature.    
     
     
         31 . A method for protecting a HVAC system of an air handling unit from low temperatures wherein the air handling unit includes a fresh air region, a return air region, a supply air region, and a damper situated in or adjacent to the fresh air region to regulate the flow of outdoor air into the HVAC, the method comprising: 
 determining the flow rate of the supply air in the supply air region;    sensing the outdoor air temperature in the fresh air region;    sensing the outdoor airflow rate in the fresh air region;    sensing the temperature of the return airflow in the return flow region;    calculating the temperature of the mixed air in the air handling unit as a function of the supply airflow rate, the outdoor air temperature, the outdoor airflow rate, and the temperature of the return airflow;    comparing the outdoor air temperature and the mixed air temperature to a threshold temperature; and    closing the position of the damper to decrease the flow of outdoor air into the air handling unit when the outdoor air temperature and the mixed air temperature are less than the threshold temperature.    
     
     
         32 . The method of  claim 31  further comprising issuing a warning signal, an alarm, and/or a message when the outdoor air temperature and the mixed air temperature are less than the threshold temperature.  
     
     
         33 . The method of  claim 31  further comprising the steps of: 
 counting the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature with a timer;    comparing the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature to a predetermined limit of time allowable for the mixed air temperature and outdoor air temperature to be below the threshold temperature; and    closing the position of the damper when the length of time that the mixed air temperature and outdoor air temperature are below the threshold temperature is greater than the predetermined limit of time allowable for the mixed air temperature and outdoor air temperature to be below the threshold temperature.