Modulating Temperature and Humidity Control . t r o f m o C g n i d l i u B . y t i l a u Q g n i Modulating Temperature and Humidity Control n i f Temperature and humidity control are important to the quality of the e indoor environment. On-off type control has been prevalent for many D . years; however, fluctuations in temperature and humidity are inherent t r o with this type of control system. The technology of the on-off control f system has evolved and superior modulating control systems are now m o available. The following discussion demonstrates the improvements the C modulating compressor and modulating reheat systems provide to tem- g n i perature and humidity control. d l i u B . y Temperature Control t i l Temperature and humidity fluctuations are common in buildings with an on- a u off compressor and on-off reheat air conditioning equipment. When the air Q conditioning unit’s compressor turns on, it operates at full capacity. By provid- g n i ing cool air to the space, the unit begins to cool the area and the temperature n i f drifts downward toward, and then past, the setpoint. When the temperature in e D the area drops below the lower limit of the dead band, the unit turns off. After the . t r unit turns off, the sensible load in the space causes the temperature to drift up, o f past the setpoint, until the upper dead band is exceeded. When the upper limit of m o the temperature dead band is exceed, the unit’s compressor will turn on and the C g process will repeat. n i d l i u B Humidity Control . y When air is dehumidified, moisture is removed from the air. A common technique used t i l to remove the moisture is to condense the moisture onto a cold surface. Anyone a u who has poured a cold beverage on a hot and humid day knows that moisture will Q g condense on the glass. As air is cooled, it loses its ability to hold moisture; in the n i case of the cold glass, the moisture in the air condenses onto the glass. The n i f moisture in a building’s air is condensed in much the same way. A fan draws e D air from the space over the cooling coil of the unit to condense the moisture. t At the same time moisture is condensed from the air, the temperature is r o being decreased. The moisture condensing from the air is known as latent f m cooling; the temperature change of the air is known as sensible cooling. o C To increase the amount of moisture condensed from the air, and further g dehumidify the air, the temperature of the air must be decreased, some- n i times past the desired setpoint (subcooled). To prevent overcooling the d l i space, the air can be heated back up before it enters the space. This is u B done by passing the dry, cool air through a warm coil to reheat the air . y t back up to its desired temperature before it is returned to the space. De li fining Qua To understand how temperature and humidity are controlled using a de- humidifier and reheat system, it is important to realize the cooling coil, and corresponding compressor, affects both the temperature (sensible) and humidity (latent) control while the reheat coil affects only the temperature (sensible) control. Note: The temperature and humidity data shown in Figures 1-3 are taken from an actual job site installation in the central United States. The equipment was originally ordered with an on-off compressor and on- off reheat. The customer was not satisfied with the temperature and humidity swings associated with on-off control. Improvements were made to the system to include modulating humidity control and a modulating capacity compressor. The results of these enhancements can be seen graphically in the following illustrations. g n i C d o l m i f u o r B t . D . e y f i n t i n i g l Q u a a l i t u y . B u Q i l d i n g g C n o i m f n o i r f t . D e e f D i n i n g Q . t r o u f m a o C l g i n t i d y l i u . B . B y t i l u a u i l Q d g n i i n n i f g e D C . t o r o m f Figure 1: On-Off Hot Gas Reheat, Standard On-Off Compressor 11 AM ON 1 NO 0 AM 0 9 M P 9 1 0 A 8 M 0 7 M P 8 2 0 A 6 M Temperature 0 5 Humidity M P 0 7 4 3 A M 0 3 0 2 M P 6 0 4 A 1 M 90 80 0 7 0 M 6 0 1 2 0 3 0 P 5 4 0 0 5 5 0 0 4 5 A 0 3 0 2 1 0 6 0 M 7 0 8 0 9 0 M 1 4 P 0 A 6 M 2 0 3 0 M Note: The “turn-on” and 3 P 4 A 7 0 “turn-off” temperatures M 5 are deliberately made to 0 differ by a small amount, 6 M 0 known as the dead band, P 2 8 A M to prevent the unit from 7 0 frequently switching on and 8 M 0 off when the temperature is P 1 9 A M 9 0 near the setpoint. M M P I D 0 N 1 I G H T M P 1 1 On-Off Reheat & Standard On-Off Compressor Figure 1 displays the relationship of the on-off compressor and on-off reheat to the tem- perature and humidity values in the space. As the on-off compressor turns on, both the temperature and humidity decrease. To prevent the temperature of the dehumidified air from over-cooling the space, the reheat coil heats the air before it leaves the unit. With an on-off reheat coil, the leaving temperature cannot be modulated; instead, the tem- perature in the space will fluctuate up and down, as shown in the blue line of Figure 1, as the reheat coil cycles on and off. As the compressor turns on and off, the humidity will also fluctuate; while the compressor is on, the air is dehumidified and the space humid- ity drops. When the compressor is off, the latent load in the space causes the humidity to rise until the compressor turns on again and begins to dehumidify the air, as can be seen with the green line of data in Figure 1. 3 . t r o f m o C g n i d l i u B . y t i l a u Q g n i Modulating Reheat & Standard On-Off Compressor n i f Figure 2 displays the relationship of the on-off compressor and mod- e ulating reheat to the temperature and humidity values in the space. D . As the on-off compressor turns on, both the temperature and humid- t r o ity decrease, however, by adding modulating control to the reheat coil, f tight temperature control is achieved. This improved temperature control m o is accomplished by varying the amount of reheat applied to the dehu- C midified air so that temperature of the air leaving the air conditioning unit g n i matches the desired setpoint. However, while the modulating reheat control d l i improves the temperature variations, it does not affect the humidity swings. u B That is because the humidity oscillations are in response to the on-off opera- . y tion of the compressor. t i l a u At part load operation, the compressor must cycle off because of low suction Q temperature. Even a unit equipped with hot gas bypass will cycle its compressor g n i because the system will not bypass enough hot gas to operate continuously at n i f low part load conditions. e D . t r To control precisely both temperature and humidity one must be able to vary both o f the cooling coil temperature and the reheat coil temperature. In a direct expansion m o refrigeration system, the most efficient way to modulate coil temperature is with a C g modulating capacity compressor. AAON utilizes a compressor that can modulate its n i capacity from 10% to 100%. d l i u B . y t i Figure 2: Modulating Hot Gas Reheat, Standard On-Off Compressor l a u Q 11 AM g N NOO 10 A n M i 0 9 n M i P 9 f 1 0 A 8 M Temperature e D 0 Humidity 7 . M P t 8 2 r 0 A 6 M o f 0 5 m o M P 0 7 4 C 3 A M g 0 3 n i d 0 l 2 i M u P 6 0 4 A B 1 M . y 90 t i 80 D l e a 0 fin u 7 ing Q 0 M 6 0 1 2 0 3 0 P 5 4 0 0 5 5 0 0 4 5 A 0 3 0 2 1 0 6 0 M 7 0 8 0 9 0 M 1 4 P 0 A 6 M 2 0 3 0 M 3 P 4 A 7 0 M 5 0 6 M 0 P 2 8 A 7 M 0 8 M 0 P 1 9 A M 9 0 M M P I D 0 N 1 I G H T M P 1 1 g n i C d o l m i f u o r B t .