Cooling devices
The first device utilizes cooling fans (expressed FANS in this paper). Two fans are placed with ducts of plastic pipes in wearable harness (Fig. 1a). The weight is 700 g. FANS is located on the back and is intended not to be obstacles for body movement. It is detachable easily. There are three air outlets on the duct with the area of 1.5 cm2 for one outlet (Fig. 2a). The ends of the ducts were plugged. Fans are operated by rechargeable batteries and the volume of supplied air is estimated about 10 m3/h for one fan by measurements. Discharged air from the outlets mainly cools face and neck, bare parts of the body (Fig. 2b). The cooling effect is caused by high air velocity around the body, which leads to high convective heat transfer coefficients. It is effective both to sensible heat loss and to latent heat loss.
The second device is a cooling vest with PCMs (VEST, Fig. 1b). There are two kinds of phase change materials in this study. One is gels with water (referred to as ICE in this paper), which changes from solid to gel at 0 °C. The other is sodium sulfate, which changes from solid to gel at 28 °C (referred to as PCM). There are 11 pockets on the inner surface of VEST (Fig. 3) and each pocket can contains one or two cooling bags of PCMS. The weight of VEST with 22 bags is 2.5 kg. Those bags cool down the body mainly by direct contact and they work for sensible heat loss. VEST is mainly used to cool down the inner temperature of heat protective clothing in high temperature such as worn by fire fighters. The purpose of the measurement is the evaluation of effectiveness when used as the outer ware for construction workers.
Thermal manikin
Cooling devices mentioned above are effective to local parts of the body. The thermal effects by them have to be evaluated locally and for the whole body. A thermal manikin with 20 segments (Nimatic, Denmark) was used for the measurements. The shape of the manikin is an average Danish woman (Fig. 4). The height is 1.68 m and the total surface area is 1.483 m2. Temperature and heat loss for each segment are measured independently every one minute.
The material of the manikin’s temperature sensors is nickel. It is commonly used for temperature measurement. The manufacturer of the manikin recommends the calibration temperature with two levels, the lower temperature and the higher temperature. The manufacturer describes 20 and 30 °C as a set of example for calibration. The manikin was calibrated at 20 and 35 °C prior to the measurements for the evaluation. The higher calibration temperature of 35 °C was selected because the segment temperatures in the measurements were expected to rise to higher than 30 °C.
There are three logics for controlling the manikin. The first is constant temperature for each segment. The second is constant heat loss for each segment. The third is comfort control, which controls the relationship between the temperature and the heat loss for each segment by Eq. 1 (Tanabe et al. 1994). Comfort control was used when the measurements were carried out for the evaluation. In the measurements, temperatures and heat losses of each segment of the manikin were measured every 60 s.
$$t_{s,i} = 36.4 - 0.054Q_{t,i}$$
(1)
t
s
: surface temperature of thermal manikin [°C], Q
t
: sensible heat loss from the surface of manikin [W/m2], (Subscript i stands for body segment).
Climatic chamber
All the measurements were conducted in a climate chamber which has an inner room and five outer rooms. The inner room’s floor area is 40.5 m2 and its ceiling height is 2.8 m. Each room has its own air-conditioning system and temperatures in them are controlled independently. The air-conditioning systems for outer rooms, not the inner room, were operated for the measurements. As a result, the temperature of inner room was close to the average of the outer rooms and the air velocity was very low (less than 0.05 m/s) in the inner room. The manikin was located in the inner room and it was not affected by the air movement from the air-conditioning system of the inner room.
Protocol for evaluating FANS
As mentioned before, FANS changes the convective heat transfer coefficient around the body, which affects the sensible heat loss and the latent heat loss from the human body. The manikin used only simulates the sensible heat loss and it is difficult to evaluate the latent heat loss directly. To solve this problem, nominal change of clothing insulation was evaluated. There were evaluations for two clothing insulation. One was a typical clothing ensemble for outdoor workers in construction industry, which is referred to as REF (control). The other was REF with FANS. The temperature in the chamber was set to 25 °C for both measurements. The manikin was kept in the chamber at least 12 h before stating the measurement and had reached to steady state. The measurement was continued for 3 h after reaching at the steady state for REF but terminated for FANS when the fans stopped because of the end of power supplied by batteries.
Clothing insulations were evaluated at the temperature of 25 °C. The clothing insulation was calculated from the average value of the last 60 min. The clothing area factor was estimated by Eq. 2 (ASHRAE 2013).
$$f_{cl} = 1.0 + 0.3I_{cl}$$
(2)
f
cl
: clothing surface area [-], I
cl
: intrinsic clothing insulation [clo].
Protocol for evaluating VEST
As mentioned before, VEST is worn usually in heat protective clothing for workers in high temperature. Outdoor workers in construction sites don’t wear the protective clothing, so the vest would be worn on the daily clothing. All of heat absorption by cooling bags would not contribute to cool down the body when used in such ways. It is important to evaluate the cooling effect itself and the effective time period. The surrounding temperature was set to 31 °C. This temperature is selected to simulate the summer condition in Japan. 31 °C is not the highest temperature but is experienced daily in Japan. The manikin had reached to steady state before starting the measurement, similar to the evaluation of VEST. There was little dew condensation on cooling bags because the absolute humidity in the chamber was low without humidification. As there were two kinds of cooling bags for VEST (ICE and PCM), there were four possible combinations for the measurements. Selected cases for the measurement were PCM only (PCM), ICE only (ICE), combination of PCM for inner side and ICE for outer side (PCM + ICE).