Efficient Humidity Control with Heat Pipes 1

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1 Fact Sheet EES-75 December 1991 Efficient Humidity Control with Heat Pipes 1 Roy Johannesen and Michael West 2 Building moisture is usually controlled by air conditioning (AC), but some installed systems cannot control the extreme moisture load encountered in Florida. One efficient approach to removing this excess moisture is the heat pipe. A heat pipe can greatly increase the moisture removal ability of an AC system and save energy at the same time. Another advantage is that heat pipes have no moving parts and are essentially maintenance free. An AC system that doesn t control humidity can induce a variety of health and comfort problems. This fact sheet explains how adding a heat pipe system to some air conditioners will control the humidity in a home or business. Information is also presented on possible energy savings, and on installation and operating costs. The key to understanding what a heat pipe system will do is to know how an AC system removes moisture. The next section will explain this and why some systems are inadequate. HUMIDITY CONTROL BY AIR CONDITIONING A building s air conditioning system is responsible for removing moisture from the air in order to provide for both human comfort and mold-and-mildew control. Inside the air conditioner, warm moist air is blown through a cooling coil. In the coil, the air is cooled below its dew point temperature. The dew point temperature is defined as the temperature of the air when the relative humidity is 100 percent. Relative humidity is defined as the amount of moisture in the air relative to the most moisture the air can hold at the same temperature. As air is cooled it loses its ability to hold moisture. So, relative humidity is increased by cooling the air, as well as by adding moisture to it. For example, as the air cools on a muggy night the relative humidity increases. When the relative humidity reaches 100%, the air has been cooled to its dew point and dew forms on surfaces. Similarly for the air conditioner, once the air is cooled below the dew point, the air releases moisture which collects in a drain pan, and drains out of the system. The cooled and dried air is delivered to the building. The air now has a lower dew point called the exit dew point. Many air conditioning systems do not remove adequate amounts of moisture for Florida s climate. Most AC systems are designed to handle peak load conditions -- the hottest afternoon of the summer. Accordingly, they work best during the hottest times of the year but not so well at other times. 1. This document is Fact Sheet EES-75, a series of the Florida Energy Extension Service, Florida Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida. Publication date: December Roy Johannesen, Former Energy Extension Specialist; Michael West, Assistant Energy Extension Specialist, Mechanical Engineering Dept., Cooperative Extension Service, Institute of Food and Agricultural Sciences, University of Florida, Gainesville FL The Florida Energy Extension Service receives funding from the Florida Energy Office, Department of Community Affairs and is operated by the University of Florida s Institute of Food and Agricultural Sciences through the Cooperative Extension Service. The information contained herein is the product of the Florida Energy Extension Service and does not necessarily reflect the views of the Florida Energy Office. The Institute of Food and Agricultural Sciences is an equal opportunity/affirmative action employer authorized to provide research, educational information and other services only to individuals and institutions that function without regard to race, color, sex, age, handicap, or national origin. For information on obtaining other extension publications, contact your county Cooperative Extension Service office. Florida Cooperative Extension Service / Institute of Food and Agricultural Sciences / University of Florida / Christine Taylor Stephens, Dean

2 Efficient Humidity Control with Heat Pipes Page 2 AC systems are designed to remove a certain amount moisture at peak conditions. This is called the latent heat ratio of the system. Sensible heat is heat in terms of degrees one reads on a common thermometer. Latent heat, the other kind of heat, is heat in terms of moisture removed. Sensible plus latent is the total heat removed. The latent heat ratio of an AC system is the portion of latent heat it can remove out of the total heat it can remove. It is typically around 30 percent at peak conditions (95 F outdoors), and a few percent higher at night conditions (75 F outdoors). The building load also has a latent heat ratio: it is the portion of latent heat that needs to be removed from the building out of the total heat that needs to be removed. At peak conditions there is much more sensible heat than latent heat. At night and on cooler days the amount of sensible heat shrinks but the amount of latent heat does not. And on wet days, the amount of latent heat grows. The latent heat ratio may rise to 50 or more. During humid and/or cool weather the AC system indeed cools the building, but it can t dehumidify adequately. To make things even worse, SOME new air conditioning units have sacrificed latent (moisture removal) capacity in order to increase their nameplate SEER ratings. (SEER stands for Seasonal Energy Efficiency Ratio and is a measure of energy efficiency.) One way manufacturers increase SEER is to raise the cooling coil temperature. Unfortunately, this means that the air blown through the coil does not reach a low dew point temperature. Some of these high efficiency units have a latent heat ratio of 15 percent or less at design conditions. INCREASING MOISTURE REMOVAL Ideally, an AC system is properly sized and designed before it is installed. A complete and accurate building heat load analysis, with special attention paid to internal moisture sources and infiltration, allows the selection of the proper equipment. Unfortunately, equipment sizing is often done using time-saving rules-of-thumb. For example, residential moisture load is rarely calculated: it is assumed to be 30 percent of the sensible heat load! Furthermore, building usage and occupancy often change, and AC system requirements change with them. Traditional methods of increasing the moisture removal capacity of AC units include undersizing, reducing fan speed, and adding reheat with either hot gas bypass or electrical strip heating. Reheat methods result in substantial increases in electrical consumption, especially electric strip reheat which will triple the required energy input. If new AC equipment is being selected, remember that cooling coils with more tube rows or greater number of fins have a higher latent heat ratio. Conventional AC equipment that can handle high latent loads may have a low SEER. Make comparisons between different manufacturers. An undersized AC unit runs longer since it does not easily satisfy the thermostat. Longer run cycles allow the system to remove more moisture from the air, but indoor temperature may rise 4 to 7 degrees during the late afternoon. Reducing the indoor fan speed causes the coil temperature to drop, and also allows the air to remain in the coil longer. This lowers the dew point of the exit air, but system efficiency is reduced. Reheat methods heat the air after it passes through the cooling coil. This allows the removal of moisture without over-cooling the air. Hot gas bypass uses hot refrigerant to reheat the air, and electric strip reheat uses resistance coils. When reheat is used, the cooling ability of the AC system goes to waste. Electric reheat uses over twice the electricity to reheat the air as it took to cool it! Dehumidifiers use a form of reheat. The heat removed from the air to cool it to its dew point is put back before the air is blown out of the unit. Again, potential cooling ability is totally wasted. HEAT PIPE HUMIDITY CONTROL Heat pipe technology offers enhanced moisture removal for very little additional energy input. Since heat pipes have no moving parts and are sealed units, they offer reliability equal to the air conditioning system to which they are fitted. Heat pipes are a simple yet elegant way to move heat from one point to another (Figure 1). If one end of a small heat pipe is dipped into a cup of hot coffee, the other end becomes very hot very rapidly. The sealed pipe is filled with certain amount of a refrigerant. The refrigerant is in a saturated state (that is, there is liquid and vapor at the same temperature and pressure; for example, water boiling in a pot and the steam above it are both at 212 F and at atmospheric pressure). When heat is applied to one end of the heat pipe some of the

3 Efficient Humidity Control with Heat Pipes Page 3 To help understand how a heat pipe is applied to air conditioning, the process has been divided into three principle steps (Figure 2). In step 1, the air is pre-cooled by the heat pipe system. In step 2, moisture and heat is removed from air by the air-conditioning cooling coil. (The cooling coil has refrigerant or chilled water flowing through it.) In step 3, heat is added to the air by the heat pipe system. Figure 1. A simple heat pipe moves heat from candle to air very quickly. There are two fluid streams that are being affected by the process, namely air (which passes over the cooling coil and heat pipe) and refrigerant (which flows within the heat pipe). Only the effects on the air stream were described in the three-step process mentioned above. A more in-depth heat exchange explanation follows in the next paragraph. An air temperature graph that corresponds to the system drawing shown directly above it is also depicted in Figure 2. As shown in the system drawing, the heat pipe is fitted around an air conditioning cooling coil. One end of the heat pipe is placed in front of the coil and the other end is placed after coil. Oncoming air transfers heat into the heat pipe (consequently dropping air temperature), and causes the refrigerant in the heat pipe to boil. The pre-cooled air next travels across the cooling coil where heat and moisture are removed. The refrigerant vapor in the heat pipe travels to the condensing end of the heat pipe. Finally, air exiting the coil absorbs heat from the heat pipe causing the refrigerant in the heat pipe to condense, completing the heat pipe cycle. Figure 2. Top diagram shows heat pipe installed around a conventional AC coil. Bottom graph compares drop in air temperature. liquid refrigerant boils into a vapor. This vapor quickly rises to the higher end of the pipe and condenses, thereby releasing heat into the air at that end of the pipe. Condensed liquid refrigerant returns to the boiling end of the heat pipe by gravity. The overall effect is that heat is transferred from the flame to the other end of the pipe very quickly. The temperature graph shows how the heat-pipe fitted cooling coil compares with a conventional cooling coil. Compared to the conventional coil, the air entering the heat-pipe fitted coil is now at a lower temperature and therefore closer to its dew point. In both the heatpipe fitted and conventional coil, the temperature drop across the coil is nearly the same. That means that the heat-pipe fitted coil will chill the air to a lower temperature than the conventional coil. Since the air is cooled further below its initial dew point, the heat-pipe fitted coil removes more moisture from the air. When the air leaves the heat-pipe fitted cooling coil, it is too cold. However, as it passes over the other (condensing) end of the heat pipe the refrigerant vapor in the heat pipe transfers heat into the cold air and warms the air to a tolerable temperature. The condensed refrigerant is returned to the boiling end of the heat pipe. The air is now conditioned by the system to be at the right temperature and humidity to meet a building s moisture load.

4 Efficient Humidity Control with Heat Pipes Page 4 The overall effect of this process, theoretically, is to almost double the moisture removal capacity of the cooling coil (at the same indoor temperature and humidity) while reducing total cooling capacity by only a few percent. Because total air conditioning system capacity is not significantly affected, heat pipes are an attractive retrofit solution for a humidity problem. HEAT PIPE PERFORMANCE It s valuable to know how the heat pipe system works and, to help predict the performance of a heat pipe system, it is also useful to know why it works. The heat pipe system takes advantage of an inherent characteristic of all cooling coils: (1) The cooler the air entering the coil, the dryer it exits. For example, if the thermostat is lowered from 80 (at 50 percent rh) to 75 F, a typical AC s latent heat ratio increases from 25 percent to 40 percent. The heat pipe precools the air, so the coil "thinks" the thermostat is lowered, and it "gives" dryer air. Unfortunately, AC coil behavior is more complicated than (1) suggests. There is another coil characteristic which limits heat pipe performance: (2) The dryer the air entering the coil, the less moisture is removed. Due to (1) alone, the increased latent capacity would reduce the humidity to about 50 percent (at 75 F). However, due to (2) the latent capacity of a typical AC is actually less at 75 F-50 percent rh than at 80 F-50 percent rh (it decreases from 25 percent to 18 percent ). So the humidity will actually be somewhere between 50 percent and 60 percent. To make matters more confusing a third characteristic comes into play: (3) The hotter it is outside, the less moisture is removed -- about a 2 to 3 percent lower latent heat ratio for each 10 degrees F higher outdoor temperature. So, the true latent capacity of an installed system is at a balance point between the capacity increase due to the heat pipe, the capacity decrease due to the resulting dryer air, and capacity changes due to thermostat setting, outdoor conditions, and moisture generation in the building. The bottom line is that humidity will decrease. How much it decreases depends on many interrelated factors. ENERGY SAVINGS There is an additional benefit to heat-pipe moistureremoval systems. It is possible to save energy and money with such a system. Here s why: For humans to remain comfortable, both temperature and humidity must be at tolerable levels. People cool themselves by evaporating moisture from their skin. If the air has too much moisture in it, evaporation is limited and not enough cooling occurs. If the air is dry, like the Arizona desert at 110 F and 3 percent rh, a person can still be comfortable in extreme heat. Traditionally, an indoor temperature of 75 F and 50 percent relative humidity is considered an ideal state (which is difficult to achieve in Florida). Actually, there is a range of humidity and temperature which is quite comfortable. Within this comfort range, the lower the humidity, the higher the tolerable temperature. If one can lower building humidity, then one can raise the thermostat setting and remain comfortable. For example, let s say that the indoor relative humidity is 65 percent. According to extensive studies done on people in rooms at different temperatures and humidities, the average person could be comfortable at a temperature of about 75 F. If we lower the humidity to 40 percent (possible in Florida conditions by retrofitting with heat pipes), then we can increase indoor temperature to 80 F and still stay within the comfort zone. This represents a 5 degree F increase in AC thermostat setting. A conservation rule of thumb says that for every degree increase in set point temperature, a two percent reduction in air conditioning energy consumption occurs. After subtracting 4 percent (because of loss in total system efficiency due to heat pipe installation), this results in a 6 percent savings in energy cost for this simple case. In an actual application with a conventional thermostat, savings will be less. This is because increased latent capacity comes at the expense decreased sensible capacity. The system will have to run longer before it satisfies the thermostat, and it will run correspondingly more hours over the course of the cooling season. One way to maximize savings is to install a humidistat so that the AC cycles on and off based on comfort, not simply temperature.

5 Efficient Humidity Control with Heat Pipes Page 5 Table 1. An energy efficiency comparison of three reheat methods. Reheat Options Where Moisture Load is 40% of the Total Cooling Load Reheat Type Energy Consumption Relative to a Conventional System* Heat Pipes 1.06 Hot gas bypass 1.60 Electrical reheat 3.04 The following studies were used in determining relative energy consumption: 1991 air conditioning load and energy consumption calculations performed at the University of Florida Mechanical Engineering Department by Michael West. A case study of a well insulated, block construction, 1300 sq. ft., 3 bedroom, 2 bath home in the Gainesville, FL area was conducted. The calculated moisture load is 4700 Btuh (26% of total at 95 F outdoor temperature). An 18,000 Btuh nominal high efficiency (EER=9.75) heat pump without heat pipes had a latent capacity of 2250 Btuh (12% of total capacity). With heat pipes, latent capacity increased to 4300 Btuh (23% of total capacity). Indoor relative humidity decreased from 65% to 50%. Annual electricity cost ($0.078/kWh) for cooling increased from $304 to $347. An increase in thermostat setting from 75 F to 80 F decreased annual cost to $322. Calculations conducted in accordance with standard ASHRAE procedures test data of a 1.5 ton heat pipe installation. Test conducted by Applied Research Laboratories of Miami, FL. Results show the heat pipes increased latent capacity by a factor of 1.9, decreased EER by 4.2%, and decreased total capacity by 4.3%. Cromer, J.C. "Desiccant Moisture Exchange for Dehumidification Enhancement of Air Conditioners." Fifth annual Symposium on Improving Building Energy Efficiency in Hot and Humid Climates *For our purposes, a conventional system is considered to be an air conditioning system which has the same total (sensible + latent) heat removal capacity but no reheat capability. Note that, under the 40% latent cooling load condition stated for this comparison, this conventional system would not be able to remove the required amount of moisture. Air conditioning loads are in actually affected by many interrelated factors so this example should not be considered definitive, but merely illustrative. In fact, most residential heat pipe installations have resulted in small energy savings. It seems that most home owners who install heat pipes do experience dryer and more comfortable conditions but are reluctant to change the thermostat to a higher setting. In commercial applications there is more potential for saving money, especially in any building which must employ a reheat system to achieve adequate moisture removal capacity. Reheat works like this: The air conditioning system is sized to remove the required amount of moisture. This results in excess capacity to remove sensible heat. If nothing were done, the building would be overcooled and uncomfortable. To compensate for overcooling, heat is added to the conditioned air. This heat can be supplied in a number of ways. One is the heat pipe system. Another is called hot gas bypass. In a hot gas bypass system, some of the hightemperature refrigerant which leaves the compressor is routed through a bypass line to a heat exchanger located at the air exit of the cooling coil. There, this hot gas adds heat to the over-cooled air. The most common method of adding reheat to conditioned air is also the least energy efficient. This method is called electrical strip reheat and uses electrical resistance heating elements located in the air exit side of the cooling coil. An energy efficiency comparison of these three reheat methods shows the benefits of the heat pipe (Table 1).

6 Efficient Humidity Control with Heat Pipes Page 6 Table 2. Heat pipe installation costs based on size and type required. Cost of Various Types of Heat Pipe Installations Type Flat: (Installed in supply and return ducting) Split: (Heat input remote from heat output) Fitted Coil: (Integrated coilheat pipe unit) Air Handler: (Fitted coil with blower and housing) Capacity (tons) COST Installed Cost Range (dollars) Note: Manufacture s retail price data suggest an 18% average increase in price per additional ton of capacity a 40% average increase in price per type upgrade The total cost for buying and installing a heat pipe system was estimated for a range of sizes (Table 2). The flat heat pipe (the first option in Table 2) is rectangular in shape and is the cheapest of the heat pipe options. It is installed in ducting in a place where the supply and return ducts run up and down (perpendicular to the ground) and are side-by-side. If the ducts do not have a section like this, consider a split heat pipe installation. Since heat pipe systems have a high initial cost, it pays to try to reduce the building moisture (and sensible cooling) load first, before purchasing additional moisture removal capacity. To reduce moisture loads, take these simple and inexpensive steps: 1) Call your local utility and arrange to have them conduct a free energy audit of your home or a no cost/low cost energy audit of your business. Tell the auditor that you are especially concerned about identifying AC duct leaks and other areas where moist outside air might be infiltrating into your building. 2) Follow up on the low-cost weather stripping and sealing actions recommended in the audit report. 3) Increase ventilation to problem areas such as bathrooms and closets. Conditioned air must be allowed to circulate in these areas. Installing louvers in doors or increasing the clearance at the bottoms of doors are two low cost ways of increasing air flow. Exhaust fans, box fans, and paddle fans also help aid circulation. 4) Reduce internal moisture generation by covering cooking pots and using cooler water for washing. Use an exhaust fan to remove moist air from the bathroom and kitchen before it dissipates The following Extension publication offers additional information on low-cost moisture and mildew control: Say Goodbye to Mildew and Save Energy (EES 65) To obtain this free publication and other energy and humidity related publications, contact your local county Extension office. Dew Point - Relative Humidity - GLOSSARY The temperature of air when it is saturated with moisture. The ratio of the amount of moisture in the air to the amount in air saturated at the same temperature. Sensible Heat - Energy that changes the air s temperature. Latent Heat - EER - Energy that is released when moisture in the air condenses. Energy Efficiency Ratio, the cooling capacity of an AC system in Btuh, divided by its power consumption in kw at a standard set of operating conditions.

7 Efficient Humidity Control with Heat Pipes Page 7 SEER - Seasonal Energy Efficiency Ratio, the EER averaged over a typical cooling season. This takes changes in efficiency at night and on cooler days into account, giving a more realistic prediction of actual operating cost. Btuh - British Thermal Units per hour. The Btu is a unit of heat, it takes 1 Btu to raise the temperature of 1 pound of water by 1 degree Fahrenheit. Ton - A unit of AC cooling capacity, equal to 12,000 Btuh. This unit originated as the amount of refrigerating capacity required to freeze one ton of ice in 24 hours.